ALGAE. The Latin word alga seems to have been the equivalent of the English word "seaweed" and probably stood for any or all of the species of plants which form the "wrack" of a seashore.
When the word "Algae" came to be employed in botanical classification as the name of a class, an arbitrary limitation had to be set to its signification, and this was not always in keeping with its original meaning. The absence of differentiation into root, stem and leaf which prevails among seaweeds, seems, for example, to have led Linnaeus to employ the term in the Genera Plantarum for a sub-class of Cryptogamia, the members of which presented this character in a greater or less degree. Of the fifteen genera included by Linnaeus among algae, not more than six-viz. Chara, Fucus, Diva and Conferva, and in part Tremella and Byssus-would to-day, in any sense in which the term is employed, be regarded as algae. The excluded genera are distributed among the liverworts, lichens and fungi; but notwithstanding the great advance in knowledge since the time of Linnaeus, the difficulty of deciding what limits to assign to the group to be designated Algae still remains. It arises from the fact that algae, as generally understood, do not constitute a homogeneous group, suggesting a descent from a common stock. Among them there exist, as will be seen hereafter, many well-marked but isolated natural groups, and their inclusion in the larger group is generally felt to be a matter of convenience rather than the expression of a belief in their close inter-relationship. Efforts are therefore continually being made by successive writers to exclude certain outlying sub-groups, and to reserve the term Algae for a central group reconstituted on a more natural basis within narrower limits.
It is perhaps desirable, in an article like this, to treat of algae in the widest possible sense in which the term may be used, an indication being at the same time given of the narrower senses in which it has been proposed to employ it. Interpreted in this way, the place of algae in the vegetable kingdom may be shown by means of a
| | Thallophyta | Fungi
| Cryptogamia | | Algae
The Vegetable | | Bryophyta
Kingdom | | Pteridophyta
| | Gymnosperms
| Phanerogamia | Angiosperms
Algae in this wide sense may be briefly described as the aggregate of those simpler forms of plant life usually devoid, like the rest of the Thallophyta, of differentiation into root, stem and leaf; but, unlike other Thallophyta, possessed of a colouring matter; by means of which they are enabled, in the presence of sunlight, to make use of the carbonic acid gas of the atmosphere as a source of carbon. It is true that certain Bryophyta (Marchantiaceae, Anthoceroteae) possess a thalloid structure similar to that of Thallophyta, and are at the same time possessed of the colouring matter of the Green Algae. Their life-cycle, however, the structure of the reproductive organs and their whole organization proclaim them to be Bryophyta (q.v..) On the other hand, certain undoubted animals (Stentor, Hydra, Bonellia) are provided with a green colouring matter by means of which they make use of atmospheric carbonic acid. A more important consideration is the occasional absence of this colour in species, or groups of species, with, in other respects, algal affinities. Such aberrant forms are to be regarded in the same light as Cuscuta and Orobanchaceae, for example, among Phanerogams. As these non-green plants do not cease to be classed with other Phanerogams, so must the forms in question be retained among algae. In all cases the loss of the colouring matter is associated with an incapacity to take up carbon from so simple a compound as carbonic acid
It might be mentioned here that the whole group of the Fungi (q.v.),with its many thousands of species, is now generally regarded as having been derived from algae, and the system of classification of fungi devised by Brefeld is based upon this belief. The similarity of the morphological characters of one group of fungi to those of certain algae has earned for it the name Phycomycetes or alga-fungi.
Further discussion of the general characters of algae will be deferred in order to take a brief survey of the subdivisions of the group. For this purpose there will be adopted the classification of algae into four sub-groups, founded on the nature of the colouring matters present in the plant:- I. CYANOPHYCEAE, or Blue-green Algae. II. CHLOROPHYCEAE, or Green Algae. III. PHAEOPHYCEAE, or Brown Algae. IV. RHODOPHYCEAE, or Red Algae. The merits and demerits of this system will appear during the description of the characters of the members of the several subdivisions.
I. CYANOPHYCEAE.-This group derives its name from the circumstance that the cells contain in addition to the green colouring matter, chlorophyll, a blue-green colouring matter to which the term phycocyanin has been applied.
To the eye, however, members of this group present a greater variety of colour than those of any other-yellow, brown, olive, red, purple, violet and variations of all these being known. They undoubtedly represent the lowest grade of algal life, and their distribution rivals that of the Green Algae. They occur in the sea, in fresh water, on moist earth, on damp rocks and on the bark of trees. Certain species are regularly found in the intercellular spaces of higher plants; such are species of Nostoc in the thallus of Anthoceros, the leaves of Azolla and the roots of Cycads. Many of them enter into the structure of the lichen-thallus, as the so-called gonidia. It is remarkable that species belonging to the Oscillatoriaceae are known to flourish in hot springs, the temperature of which rises as high as 85 deg. C.
The thallus may be unicellular or multicellular. When unicellular, it may consist of isolated cells, but more commonly the cells are held together in a common jelly (Chroococcaceae) derived from the outer layers of the cell-wall. The multicellular species consist of filaments, branched or unbranched, which arise by the repeated divisions of the cells in parallel planes, no formation of mucilage occurring in the dividing walls. Such filaments may not give rise to mucilage on the lateral surface either, in which case they are said to be free; when mucilage does occur on the lateral wall, it appears as the sheath surrounding either the single filament, or a sheaf of filaments of common origin. The mucilage may also form an embedding substance similar to that of Chroococcaceae, in which the filaments lie parallel or radiate from a common centre (Rivulariaceae). The cells of the filament may be all alike, and growth may occur equally in all parts (Oscillatoriaceae); or certain cells (heterocysts) may become marked off by their larger size and the transparency of their contents; in which case growth may still be distributed equally throughout (Nostoc), or the filament may be attached where the heterocyst arises, and grow out at the opposite extremity into a fine hair (Rivulariaceae). An African form (Camptothrix), devoid of heterocysts and hair-like at both extremities, has recently been described. Branching has been described as "false" and "true." The former arises when a filament in a sheath, either in consequence of growth in length beyond the capacity of the sheath to accommodate it, or because of the decay of a cell, becomes interrupted by breaking, and the free ends slip past one another. "True" branching arises only by the longitudinal division of a cell of a filament and the lateral outgrowth of one of the cells resulting from the division (Sirosiohonaceae).
The nature of the contents of the cells of Cyanophyceae has given rise to considerable controversy. The cells are for the most part exceedingly minute, and are not easy to free from their colouring matters, so that investigation has been attended with great difficulty. Occupying as these algae do perhaps the lowest grade of plant life, it is a matter of interest to ascertain whether a nucleus or chromatophore is differentiated in their cells, or whether the functions and properties of these bodies are diffused through the whole protoplast. It is certain that the centre of the cell, which is usually non-vacuolated, is occupied by protoplasm of different properties from the peripheral region; and A. Fischer has further established the fact that the peripheral mass, which is a hollow Sphere in spherical cells, and either a hollow cylinder or barrel-shaped body in filamentous forms, must be regarded as the single chromatophore of the Cyanophyceous cell. But what precisely is the nature of the central mass is still uncertain. Some investigators, such as R. Hegler, F. G. Kohl and E. W. Olive, claim that this body is a true nucleus comparable with that of the higher plants. It is said to undergo division by a mitosis essentially of the same character, with the formation of a spindle and the differentiation of chromosomes. It is further stated by Olive that the chromosomes undergo longitudinal fission, and that for the same species the same number of chromosomes appear at each division. H. Wager speaks with greater reserve, acknowledging, however, the central body to be a nucleus of a rudimentary type, but devoid of nuclear membrane and nucleolus. He thinks it may possibly originate in the vacuolization of the central region, and the accumulation of chromatin granules therein. He finds no spindle fibres or true chromosomes, and considers the division direct, not indirect. With reference to the existence of a chromatophore, he with others finds the colouring matter localized in granules in the peripheral region, but does not consider these individually or in the aggregate as chromatophores. Among other contents of the cell, fatty substances and tannin are known. A curious adaptation seems to occur in certain floating forms, in the presence of a gas-vacuole, which may be made to vary its volume with varying pressure. There is evidence that the dividing wall of filamentous forms is deeply pitted, as is found to be the case in red algae. Reproduction is chiefly effected by the vegetative method. Asexual reproductive cells are not infrequent, but sexual reproduction even in its initial stages is unknown. Nor is motility by means of cilia known in the group. In the unicellular forms, cell-division involves multiplication of the plant. In all the multicellular plants of this group which have been adequately investigated, vegetative multiplication by means of what are known as hormogonia has been found to occur. These are short segments of filaments consisting of a few cells which disengage themselves from the ambient jelly, if it be present, in virtue of a peculiar creeping movement which they possess at this stage. After a time they come to rest and give rise to new colonies. True reproduction of the asexual kind occurs, however, in the formation of sporangia, particularly in the Chamaesiohonaceae. Here the contents of certain cells break up endogenously into a great number of spores, which are distributed as a fine dust. Resting spores are also known. In these cases, certain cells of a colony of unicellular plants or of the filaments of multicellular plants enlarge greatly and thicken their wall. When unfavourable external conditions supervene and the ordinary cells become atrophied, these cells persist and reproduce the plant with the return of more favourable conditions. The Oscillatoriaceae are capable of a peculiar oscillatory movement, which has earned for them their name, and which enables them to move through considerable distances. It is not clear how the movement is effected, though it has frequently been the subject of careful investigation.
With the Cyanophyceae must be included, as their nearest allies, the Bacteriaceae (see BACTERIOLOGY.) Notwithstanding the absence of chlorophyll, and the consequent parasitic or saprophytic habit, Bacteriaceae agree in so many morphological features with Cyanophyceae that the affinity can hardly be doubted.
A census of the Cyanophyceae with their two main groups is given below:-
1. Coccogoneae-2 families, 29 genera, 253 species. 2. Hormogoneae-6 families, 59 genera, 701 species. (Engler and Prantl's Pflanzenfamilien, 1900)
II. CHLOROPHYCEAE.-This group includes those algae in which the green colouring matter, chlorophyll, is not accompanied by a second colouring matter, as it is in other groups. It consists of three subdivisions-Conjugatae, Euchlorophyceae and Characeae. Of these the first and last are relatively small and sharply defined families, distinguished from the second family, which forms the bulk of the group, by characters so diverse that their inclusion with them in one larger group can only be justified on the ground of convenience. Chlorophyceae include both marine and freshwater plants.
Euchlorophyceae in their turn have been until recently regarded as made up of the three series of families-Protococcales, Confervales and Siphonales. As the result of recent investigations by two Swedish algologists, Bohlin and Luther, it has been proposed to make a re-classification of a far-reaching nature. Algae are withdrawn from each of the three series enumerated above and consolidated into an entirely new group. In these algae, the colouring matter is said to be yellowish-green, not strictly green, and contained in numerous small discoid chromatophores which are devoid of pyrenoids. The products of assimilation are stored up in the form of a fatty substance and not starch. A certain inequality in the character of the two cilia of the zoospores of some of the members of the group has earned for it the title Heterokontae, from the Greek kontos, a punting-pole. In consonance with this name, its authors propose to re-name the Conjugatae; Akontae and Oedogoniaceae with a chaplet of cilia become Stephanokontae, and the algae remaining over in the three series from which the Heterokontae and Stephanokontae are withdrawn become Isokontae. Conjugatae, Protococcales and Characeae are exclusively freshwater; Confervales and Siphonales are both freshwater and marine, but the latter group attains its greatest development in the sea. Some Chlorophyceae are terrestrial in habit, usually growing on a damp substratum, however. Trentepohlia grows on rocks and can survive considerable desiccation. Phycopeltis grows on the surface of leaves, Phyllobium and Phyllosiphon in their tissues. Gomontia is a shell-boring alga, FIG. 2.-Chlorophyceae, variously magnified.
A. Chlamydomonas sp., unicellular; chr., chromatophore; p., pyrenoid; n., nucleus; p.v., pulsating vacuoles; e.s., eyespot.
B1. Volvox sp., with a, antheridia, and o, oogonia.
B2. Volvox sp., surface view of a single cell showing connexions.
C. Pandorina sp., a 16-celled colony.
D. Hydrodictyon, a single mesh surrounded by 6 cells.
E. Microspora sp., showing H-pieces in the wall.
F. Entoderma sp., endophytic in Ectocarpus.
G. Coleochaete sp., growing as a plate.
H. Oedogonium sp., intercalated growth by insertion of new piece (a) leaving caps.
K. Struvea sp., showing branches forming a net-work.
L. Caulerpa sp., showing portion of axis with leaf-like and root- like appendages.
M1. Chara sp., axis with leaf-like appendages and a branch.
M2. Chara sp., apical region.
N. Botrydium, a simple siphonaceous alga with root-like attachment.
O. Acetabularia Mediterranea, mushroom-like calcareous siphonaceous alga.
(A, C, E, F, G, H, K, M1, M2, from from Engler and Prantl,
Pflanzenfamilien, by permission of Wilhelm Engelmann; B1,
N, from Vines, Student's Text Book of Botany, by permission of
Swan Sonnenschein and Co.; B2, D, O from Oltmanns, Morphologie
u. Biologie der Algen, by permission of Gustav Fischer.)
Dermatophyton grows on the carapace of the tortoise and Trichophilus in the hairs of the sloth. Certain Protococcales and Confervales exist as the gonidia of the lichenthallus.
The thallus is of more varied structure in this group than in any other. In the simplest case it may consist of a single cell, which may remain free during the whole of the greater part of its existence, or be loosely aggregated together within a common mucilage, or be held together by the adhesion of the cell-walls at the surface of contact. These aggregations or colonies, as they are termed, may assume the form of a plate, a ring, a solid Sphere, a hollow sphere, a perforate Sphere, a closed net, or a simple or branched filament. It is not easy in all cases to draw a distinction between a colony of planes and a multicellular individual. in a Volvox Sphere, for example, there is a marked protoplasmic continuity between all the cells of the colony. The Ulvaceae, the thallus of which consists of laminae, one or more cells thick, or hollow tubes, probably represent a still more advanced stage in the passaae of a colony into a multicellelar plant. Here there is some amount of localization of growth and distinction of parts. It is only in such cases as Volvox and Ulvaceae that there is any pretension to the formation of a true parenchyma within the limits of the Chlorophyceae. In the whole series of the Confervales, the thallus consists of filaments branched or unbranched, attached at one extremity, and growing almost wholly at the free end. The branches end in fine hairs in Chaetophoraceae. In Coleochaetaceae the branches are often welded into a plate, simulating a parenchyma. In all Conjugatae and most Protococcales, and in the bulk of the Confervales, the thallus consists of a cell or cells, the Protoplast of which contains a single nucleus. In Hydrodictyaceae, Cladophoraceae, Sphaeropleaceae and Gomontiaceae this is no longer the case. Instead of a single relatively large nucleus, each cell is found to contain many small nuclei, and is spoken of as a coenocyte. This character becomes still more pronounced in the large group of the Siphonales. Valoniaceae and Dasycladaceae are partially septate, but elsewhere no cellulose partitions occur, and the thallus is more or less the continuous tube from which the group is named. Yet the siphonaceous algae may assume great variety of form and reach a high degree of differentiation. Protosiphon and Botrydium, on the one hand, are minute vesicles attached to muddy surfaces by rhizoids; Caulerpa, on the other, presents a remarkable instance of the way in which much the same external morphology as that of cormophytes has been reached by a totally different internal structure. Many Siphonales are encrusted with lime like Corallina among Red Algae. Penicillus is brush-like, Hallimeda and Cymopolia are jointed, Acetabularia has much the same external form as an expanded Coprinus, Neomeris simulates the fertile shoot of Equisetum with its densely packed whorled branches, and in Microdictyon, Anadyomene, Struvea and Boodlea the branches, spreading in one plane, become bound together in a more or less close network. Characeae are separated from other Chloroohlceae by a long interval, and present the highest degree of differentiation of parts known among Green Algae. Attached to the bottom of pools by means of rhizoids, the thallus of Characeae grows upwards by means of an apical cell, giving off whorled appendages at regular intervals. The appendages have a limited growth; but in connexion with each whorl there arise, singly or in pairs, branches which have the same unlimited growth as the main axis. There is thus a close approach to the external morphology of the higher plants. The streaming of the protoplasm, known elsewhere among Chlorophyceae, is a conspicuous feature of the cells of Characeae.
The Chlorophyceae excel all other groups of algae in the magnitude and variety of form of the chlorophyll-bodies. In Ulva and Mesocarpus the chromatophore is a single plate, which in the latter genus places its edge towards the incident light; in Spirogyra they are spiral bands embedded in the primordial utricle; in Zygnema they are a pair of stellate masses, the rays of which branch peripherally; in Oedogonium they are longitudinally-disposed anastomosing bands; in Desmids plates with irregular margins; in Cladophora polyhedral plates: in Vaucheria minute elliptical bodies occurring in immense numbers. Embedded in the chromatophore, much in the same way as the nucleus is embedded in the cytoplasm, are the pyrenoids. Unknown in Cyanophyceae and Phoeophyeeae, known only in Bangiaceae and Nemalion among Rhodophyceae, they are of frequent occurrence among Chlorophyceae, excepting Characeae. Sometimes several pyrenoids occur in each chloroplast, as in Mesocarpus and Spirogyra; sometimes only an occasional chloroplast contains pyrenoid at all, as in Cadophora. The pyrenoid seems to be of proteid nature and gelatinous consistency, and to arise as a new formation or by division of pre-existing pyrenoids. When carbon-assimilation is active, starch-granules crowd upon the surface of the pyrenoid and completely obscure it from view.
Special provision for vegetative multiplication is not common among Chlorophyceae. Valonia and Caulerpa among Siphonales detach portions of their thallus, which are capable of independent growth. In Caulerpa no other means of multiplication is as yet known. In Characeae no fewer than four methods of vegetative reproduction have been described, and the facility with which buds and branches are in these cases detached has been adduced as an evidence of affinity with Bryophyta, which, as a class, are distinguished by their ready resort to vegetative reproduction.
With regard to true reproduction, which is characterized by the formation of special cells, the group Euchlorophyceae is characterized by the production of zoospores ; that is to say, cells capable of motility through the agency of cilia. Such ciliary motion is known in the adult condition of the cells of Volvocaceae, but where this is not the case the reproductive cells are endowed with motility for a brief period. The zoospore is usually a pyriform mass of naked protoplasm, the beaked end of which where the cilia arise is devoid of colouring matter. A reddish-brown body, known as the eyespot, is usually situated near the limits of the hyaline portion, and in the protoolasm contractile vacuoles similar to those of lower animals have been occasionally detected. The movement of the zoospore is effected by the lashing of the cilia and is in the direction of the beak, while the zoospore slowly rotates on Botrydium and Hydrodictyon only one is present; in certain species of Cladophora four; in Dasycladus a chaplet, and in Oedogonium a ring of many cilia. The so-called zoospore of Vaucheria is a coenocyte covered over with paired cilia corresponding in position to nuclei lying below. In all other cases, zoospores are uninucleate bodies. Zoospores arise in cells of ordinary size and form termed zoosporangia. In unicellular forms (Sphaerella) the thallus becomes transformed into a zoosporangium at the reproductive stage. In the zoosporangia of Oedogonium, Tetraspora and Coleochaete the contents become transformed into a single zoospore. In most cases repeated division seems to take place, and the final number is represented by some power of two. In coenocytic forms the zoospores would seem to arise simultaneously, probably because many nuclei are already present. The escape of zoospores is effected by the degeneration of the sporangial wall (Chaetophora), or by a pore (Cladophora), a slit (Pediastrum ), or a circular fracture (Oedogonium). Zoospores are of two kinds: (1) Those which come to rest and germinate to form a new plant; these are asexual and are zoospores proper. (2) Those which are unable to germinate of themselves, but fuse with another cell, the product giving rise to a new individual; these are sexual and are zoogametes . When two similar zoogametes fuse, the process is conjugation, and the product a zygospore . Usually, however, only one of the fusing cells is a zoogamete, the other gamete being a much larger resting cell. In such a case the zoogamete is male, is called an antherozoid or spermatozoid, and arises in an antheridium; the larger gamete is an oosphere and arises in an oogonium. The fusion is now known as fertilization, and the product is an oospore. Reproduction by conjugation is also known as isogamy, by fertilization as oogamy. When zoospores come to rest, a new cell is formed and germination ensues at once. When zygospores and oospores are produced a new cell-wall is also formed, but a long period of rest ensues. All investigation goes to show that an essential part of sexual union is the fusion of the two nuclei concerned. It is interesting to know, on the authority of Oltmanns, that when the oosphere is forming in the oogonium of Vaucheria, there is a retrocession of all the included nuclei but one. that the antherozoid of Vaucheria contains a single nucleus had been inferred before.
From a comparison of those Euchlorophyceae which have been most closely investigated, it appears probable that sexual reproductive cells have in the course of evolution arisen as the result of specialization among asexual reproductive cells, and that in turn oogamous reproduction has arisen as the result of differentiation of the two conjugating cells into the smaller male gamete and the larger male gamete. It would further appear that oogamous reproduction has arisen independently in each of the three main groups of Euchlorophyceae, viz. Ptotococcales, Siphonales and Confervales. Thus among Volvocaceae, a family of Protococcales, while in some of the genera (Chloraster, Sphondylomorum) no sexual union has as yet been observed, in others (Pandorina, Chlorogonium, Stephanosphaera, Sphaerella) conjugation of similar gametes takes place, in others still (Phacotus, Eudorina, Volvox) the union is of the nature of fertilization. No other family of Protococcales has advanced beyond the stage of isogamous reproduction. Again, among Siphonales only one family (Vaucheriaceae) has reached the stage of oogamy, although an incipient heterogamy is said to occur in two other families (Codiaceae, Bryopsidaceae). Elsewhere among Siphonales, in those cases where reproductive cells are known, the reproduction is either isogamous or asexual. Among Confervales there is no family in which sexual reproduction-isogamy or oogamy-is not known to occur among some of the component species, and as many as four families (Cylindrocapsaceae, Sphaeropleaceae, Oedogoniaceae, Coleochaetaceae) are oogamous. On these, as well as other grounds. Confervales are regarded as having attained to the highest rank among Euchlorophyceae. Although the phenomena attending isogamous and oogamous reproduction respectively are essentially the same in all cases, slight variations in both instances appear in different families, attributable doubtless to the independent origin of the process in different groups. Thus, although isogamy consists in typical cases of a union of naked motile gametes by a fusion which begins at the beaked ends, and results in the formation of an immotile spherical zygote surrounded by a cell-wall, in Leptosira it is noticeable that the fusion begins at the blunt end; in a species of Chlamydomonas the two gametes are each included in a cell-wall before fusion; and in many cases the zygote retains for some time its motility with the double number of cilia. Again, in oogamous reproduction, while in general only one oosphere is differentiated in the oogonium, in Sphaeroplea several oospheres arise in each oogonium; and while the oospheres usually contract away from the oogonial wall, acquiring for themselves a new cell-wall after fertilization, in Coleochaete the oosphere remains throughout in contact with the oogonial wall. The oosphere is in all cases fertilized while still within the oogonium, the antherozoids being admitted by means of a pore. There is usually distinguishable upon the surface of the oosphere an area free from chlorophyll, known as the receptive spot, at which the fusion with the antherozoid takes place; and in many cases, before fertilization, a small mucilaginous mass has been observed to separate itself off from the oosphere at this point and to escape through the pore. In Coleochaete the oogonial wall is drawn out into a considerable tube, which is provided with an apical pore, and this tube has a somewhat similar appearance to the imperforate trichogyne of Florideae to be hereafter described. In certain species of Oedogonium minute male plantlets, known as dwarf males, become attached to the female plant in the neighbourhood of the oogonia, thus facilitating fertilization. Indeed the genus Oedogonium exhibits a high degree of specialization in its reproductive system, considering that its thallus has not advanced beyond the stage of an unbranched filament.
Many Euchlorophyceae are endowed with both asexual and sexual reproduction. Such are Coleochaete, Oedogonium, Cylindrocapsa, Ulothrix, Vaucheria, Volvox, etc. In others only the asexual method is yet known. When a species resorts to both methods, it is generally found that the asexual method prevails in the early part of the vegetative period and the sexual towards the close of that period. This is in consonance with the facts already mentioned that zoospores germinate forthwith, and that the sexually-produced cell or zygote enters upon a period of rest. It is known that zoogametes, which usually conjugate, may, when conjugation fails, germinate directly (Sphaerella.) In rare cases the oosphere has been known to germinate without fertilization (Oedogonium, Cylindrocapsa.) The germination of a zygospore or oospore is effected by the rupture of an outer cuticularized exosporium; then the cell may protrude an inner wall, the endosporium, and grow out into the new plant ( Vaucheria), or the contents may break up into a first brood of zoospores. It is held that in Coleochaetea parenchyma results from the division of the oospore, from each cell of which a zoospore arises.
Reproduction is also effected among Euchlorophyceae by means of aplanospores and akinetes. Aplanospores would seem to represent zoospores arrested in their development; without reaching the stage of motility, they germinate within the sporangium. Akinetes are ordinary thallus cells, which on account of their acquisition of a thick wall are capable of surviving unfavourable conditions. Both aplanospores and akinetes may germinate with or without the formation of zoospores at the initial stage.
Among Conjugatae reproduction is effected solely by means of conjugation of what are literally aplanospores. Among those Desmidiaceae which live a free life, two plants become surrounded by a common mucilage, in which they lie either parallel (Closterium) or crosswise (Cosmarium.) Gaps then appear in the apposed surfaces, usually at the isthmus; the entire protoplasts either pass out to melt into one another clear of the old walls, or partly pass out and fuse without complete detachment from the old walls. Among colonial Desmidiaceae, the break-up of the filament is a preliminary to this conjugation; otherwise the process is the same. The zygospore becomes surrounded with its own wall, consisting finally of three layers, the outer of which is furnished with spicular prominences of various forms. In Zygnemaceae there is no dissolution of the filaments, but the whole contents of one cell pass over by means of a conjugation-tube into the cavity of a cell of a neighbouring filament, where the zygospore is formed by the fusion of the two
Protoplasts. In these cases the activity of one of the gametes, and the passivity of the other, is regarded as evidence of incipient sex. In Strogonium there is cell-division in the parent-cell prior to conjugation; and as two segments are cut off in the case of the active gamete, and only one in the case of the passive gamete, there is a corresponding difference of size, marking another step in the sexual differentiation. In Zygogonium, although no cell-division takes place, the gametes consist of a portion only of the contents of a cell, and this is regularly the case in Mesocarpaceae, which occupy the highest grade among Conjugatae. Some Zygnemaceae and Mesocarpaceae form either a short conjugating tube, or none at all, but the filaments approach each other by a knee-like bend, and the zygospore is formed at the point of contact, often being partially contained within the walls of the parent-cell. It would seem that in some cases the nuclei of the gametes remain distinct in the zygospore for a considerable time after conjugation. It is probable that in all cases nuclear fusion takes place sooner or later. In Zygnemaceae and Mesocarpaceae the zygospore, after a period of rest, germinates, to form a new filamentous colony; in Desmidiaceae its contents divide on germination, and thus give rise to two or more Desmids. Gametes which fail to conjugate sometimes assume the appearance of zygospores and germinate in due course. They are known as azygospores.
The reproduction of Characeae is characterized by a pronounced oogamy, the reproductive organs being the most highly differentiated among Chlorophyceae. The antheridia and oogonia are formed at the nodes of the appendages. The oogonium, seated on a stalk cell, is surrounded by an investment consisting of five spirally-wound cells, from the projecting ends of which segments are cut off, constituting the so-called stigma. The oosphere is not differentiated within the wall of the oogonium, but certain cells known as wendungszellen, the significance of which has given rise to much speculation, are cut off from the basal portion of the parent-cell during its development. The antheridia are spherical orange-coloured bodies of very complex structure. The antherozoid is a spirally-coiled thread of protoplasm, furnished at one end with a pair of cilia. It much more resembles the antherozoids of Bryophyta and certain Pteridophyta than any known among other algae. The fertilized egg charged with food reserves rests for a considerable period, surrounded by its cortex, the whole having assumed a reddish-brown colour. On germination it gives rise to a row of cells in which short (nodal) and long (internodal) cells alternate. From the first node arise rhizoids; from the second a lateral bud, which becomes the new plant. This peculiar product of germination, which intervenes between the oospore and the adult form, is the proembryo. It will be remembered that in Musci, the asexual spore somewhat similarly gives rise to a protonema, from which the adult plant is produced as a lateral bud. The proembryonic branches of Characeae, one of the means of vegetative reproduction already referred to, are so called because they repeat the characters of the proembryo.
Before leaving the Chlorophyceae, it should be mentioned that the genus Volvox has been included by some zoologists (Butschli, for example) among Flagellata; on the other hand, certain green Flagellata, such as Euglena, are included by some botanists (for example, van Tieghem) among unicellular plants. A similar uncertainty exists with reference to certain groups of Phaeophyceae, and the matter will thus arise again.
A census of the Chlorophyceae is furnished below:-
1. Confervoideae--12 families, 77 genera, 1021 species.
2. Siphoneae-9 families, 26 genera, 271 species.
3. Protococcoideae-2 families, 90 genera, 342 species.
4. Conjugateae-2 families, 33 genera, 1296 species. (De Toni's Sylloge Algarum, 1889.)
5. Characeae-2 families, 6 genera, 181 species. (Engler and Prantl's Pflanzenfamilien, 1897)
III. PHAEOPHYCEAE.-The Phaeophyceae are distinguished by the possession of a brown colouring matter, phycophaein, in addition to chlorophyll. They consist of the following groups:-Fucaceae, Phaeosporeae; Dictyotaceae, Cryptomonadaceae, Peridiniaceae and Diatomaceae. Of these the first three include multicellular plants, some of them of great size; the last three are unicellular organisms, with little in common with the rest excepting the possession of a brown colouring matter. Fucaceae and Phaeosporeae are doubtless closely allied, and to these Dictyotaceae may be joined, though the relationship is less close. They constitute the Euphaeophyceae, and will be dealt with in the first place.
Euphaeophyceae are almost exclusively marine, growing on rocks and stones on the coast, or epiphytic upon other algae. In tidal seas they range from the limits of high water to some distance beyond the low-water line. On the British coasts zones are observable in passing from high to low water mark, characterized by the prevalence of different species, thus:--Pelvetia canaliculata, Fucus platycarpus, Fucus vesiculosus, Ascophyllum nodosum, Fucus serratus, Laminaria digitata. Some species are minute filamentous plants, requiring the microscope for their detection; others, like Lessonia, are of considerable bulk, or, like Macrocystis, of enormous length. In Fucaceae, Dictyotacea, and in Laminariaceae and Sphacelariaceae, among Phaeosporeae, the thallus consists of a true parenchyma; elsewhere it consists of free filaments, or filaments so compacted together, as in Cutleriaceae and Desmarestiaceae, as to form a false parenchyma. In Fucaceae and Laminariaceae the inner tissue is differentiated into a conducting system. In Laminariaceae the inflation of the ends of conducting cells gives rise to the so-called trumpet-hyphae. In Nereocystis and Macrocystis a zone of tubes occurs, which present the appearance of sieve-tubes even to the eventual obliteration of the perforations by a callus. While there is a general tendency in the group to mucilaginous degeneration of the cell-wall, in Laminaria digitata there are also glands secreting a plentiful mucilage. Secondary growth in thickness is effected by the tangential division of superficial cells. The most fundamental external differentiation is into holdfast and shoot. In Laminariaceae secondary cylindrical props arise obliquely from the base of the thallus. In epiphytic forms the rhizoids of the epiphyte often penetrate into the tissue of the host, and certain epiphytes are not known to occur excepting in connexion with a certain host; but to what extent, if any, there is a partial parasitism in these cases has not been ascertained. In filamentous forms there is a differentiation into branches of limited and branches of unlimited growth (Sphacelaria.) In Laminariaceae there is a distinction of stipe and blade. The blade is centrally-ribbed in Alaria and laterally-ribbed in Macrocystis. It is among the Sargassaceae that the greatest amount of external differentiation, rivalling that of the higher leafy plants, is reached. A characteristic feature of the more massive species is the occurrence of air-vesicles in their tissues. In Fucus vesiculosus they arise in lateral pairs; in Ascophyllum they are single and median; in Macrocystis one vesicle arises at the base of each thallus segment; in Sargassum and Halidrys the vesicles arise on special branches. They serve to buoy up the plant when attached to the sea-bottom, and thus light is admitted into the forest-like growths of the gregarious species. When such plants are detached they are enabled to float for great distances, and the great Sargasso Sea of the North Atlantic Ocean is probably only renewed by the constant addition of plants detached from the shores of the Caribbean Sea and Gulf of Mexico.
Growth in length is effected in a variety of ways. In Dictyota Sphacelariaceae and Fucaceae there is a definite apical cell. In the first it is a biconvex lens, from which segments are continually cut off parallel to the posterior surface; and in the second an elongated dome, from which segments are cut off by a transverse wall. While, however, in Dictyota the product of the subsequent division in the segment enlarges with each subdivision, the divisions in the cylindrical segment of Sphacelariaceae are such that the whole product after subdivision, however many cells it may consist of, does not exceed in bulk the segment as cut off from the apical cell. In Dictyotaceae the apical cell occasionally divides longitudinally, and thus the dichotomous branching is provided for. In some Sphacelariaceae branches may appear at their inception as lateral protuberances of the apical cell itself. In Fucaceae an apical cell is situate at the surface of the thallus in a slit-like depression at the apex. From this cell segments are cut off in three or four lateral oblique planes.
A peculiar manner of growth in length is that to which the term trichothallic has been applied. It may readily be observed that in the hair-like branches of Ectocarpaceae, the point at which most rapid division occurs is situate near the base of the hair. In Desmarestia and Arthrocladia, for example, it is found that the thallus ends in a tuft of such hairs, each of them growing by means of an intercalated growing point. In these cases, however, the portions of the hairs behind the growing region become agglutinated together into a solid cylindrical pseudo-parenchymatous axis. In Cutleria the laminated thallus is formed in the same way. The intercalated growing region of Laminaria affords an example of another variety of growth in Phaeophyceae. While the laminated portion of the thallus is being gradually worn off in our latitudes during the autumnal storms, a vigorous new growth appears at the junction of the stipe and the blade, as the result of which a new piece is added to the stipe and the lamina entirely renovated.
Both asexual and sexual reproduction occur among Euphaeophyceae. Fucaceae are marked by an entire absence of the asexual method. The sexual organs-oogonia and antheridia--are borne on special portions of the thallus in cavities known as conceptacles. Both organs may occur in one conceptacle, as in Pelvetia, or each may be confined to one conceptacle or even one plant, as in Fucus vesiculosus. The oogonia arise on a stalk cell from the lining layer of the cavity, the contents dividing to form eight oospheres as in Fucus, four as in Ascophyllum, two as in Pelvetia, or one only as in Hallidrys. It would seem that eight nuclei primarily arise in all Fucaceae, and that a number corresponding to the number of oospheres subsequently formed is reserved, the rest being discharged to the periphery, where they may be detected at a late stage. On the maturation of the oospheres the outer layer of the oogonial wall ruptures, and the oospheres, still surrounded by a middle and inner layer, pass out through the mouth of the conceptacle. Then usually these layers successively give way, and the spherical naked oospheres float free in the water. The antheridia, which arise in the conceptacular cavity as special cells of branched filaments, are similarly discharged whole, the antherozoids only escaping when the antheridia are clear of the conceptacle. The antherozoids are attracted to the oospheres, round each of which they swarm in great numbers. Suddenly the attraction ceases, and the oosphere is fertilized, probably at that moment, by the entry of a single antherozoid into the substance of the oosphere; a cell-wall is formed thereupon, in some cases in so short an interval as five minutes. Remarkable changes of size and outline of the oosphere have recently been described as accompanying fertilization in Hallidrys. Probably the act of fertilization in plants has nowhere been observed in such detail as in Fucaceae. Dictyotaceae resemble Fucaceae in their pronounced oogamy. They differ, however, in being also asexually reproduced. The asexual cells are immotile spores arising in fours in sporangia from superficial cells of the thallus. In Dictyota the oospheres arise singly in oogonia, crowded together in sori on the surface of the female plant. The antheridia have a similar origin and grouping on the male plant. Until the recent discovery by Williams of motility, by means of a single cilium, of the antherozoids of Dictyota and Taonia, they were believed to be immotile bodies, like the male cells of red seaweeds. in Dictyota the unfertilized oosphere is found to be capable of undergoing a limited number of divisions, but the body thus formed appears to atrophy sooner or later.
Of the small family of the Tilopteridaceae our knowledge is as yet inadequate, but they probably present the only case of pronounced oogamy among Phaeosporeae. They are filamentous forms, exhibiting, however, a tendency to division in more than one plane, even in the vegetative parts. The discovery by Brebner of the specific identity of Haplospora globosa and Scaphospora speciosa marks an important step in the advance of our knowledge of the group. Three kinds of reproductive organs are known: first, sporangia, which each give rise to a single tetra-, or multi-nucleate non-motile, probably asexual spore; second, plurilocular sporangia, which are probably antheridia, generating antherozoids; and third, sporangia, which are probably oogonia, giving rise to single uni- nucleate non-motile oospheres. No process of fertilization has as yet been observed.
The Cutleriaceae exhibit a heterogamy in which the female sexual cell is not highly specialized, as is in the groups already described. From each locule of a plurilocular sporangium there is set free an oosphere, which, being furnished with a pair of cilia, swarms for a time. In similar organs on separate plants the much smaller antherozoids arise. Fertilization has been observed at Naples; but it apparently depends on climatic conditions, as at Plymouth the oospheres have been observed to germinate parthenogenetically. The asexual organs in the case of Cutleria multifida arise on a crustaceous form, Aglaozonia reptans, formerly considered to be a distinct species. They are unilocular, each producing a small number of zoospores.
The possession of two kinds of reproductive organs, unilocular and plurilocular sporangia, is general among the rest of the Phaeosporeae. Bornet, however, called attention in 1871 to the fact that two kinds of plurilocular sporangia occurred in certain species of the genus Ectocarpus-somewhat transparent organs of an orange tint producing small zoospores, and also more opaque organs of a darker colour producing relatively larger zoospores. On the discovery of another such species by F. H. Buffham, Batters in 1892 separated the three species, Ectocarpus secundus, E. fenestratus, E. Lebelii, together with the new species, into a genus, Giffordia, characterized by the possession of two kinds of plurilocular sporangia. The suspicion that a distinction of sex accompanied this difference of structure has been justified by the discovery by Sauvageau of undoubted fertilization in Gidordia secunda and G. fenestrata. The conjugation of similar gametes, arising from distinct plurilocular sporangia, was observed by Berthold in Ectocarpus siliculosus and Scytosiphon lomentarius in 1880; and these observations have been recently confirmed in the case of the former species by Sauvageau, and in the case of the latter by Kuckuck. In these cases, however, the potential gametes may, failing conjugation, germinate directly, like the zoospores derived from unilocular sporangia. The assertion of Areschoug that conjugation occurs among zoospores derived from unilocular sporangia, in the case of Dictyosiphon hippuroides, is no doubt to be ascribed to error of observation. It would thus seem that the explanation of the existence of two kinds of sporangia, unilocular and plurilocular, among Phaeosporeae, lies in the fact that unilocular sporangia are for asexual reproduction, and that plurilocular sporangia are gametangia-potential or actual. It must, however, be remembered that so important a generalization is as yet supported upon a somewhat narrow base of observation. Moreover, for the important family of the Laminariaceae only unilocular sporangia are known to occur; and for many species of other families, only one or other kind, and in some cases neither kind, has hitherto been observed. The four species-Ectocarpus siliculosus, Giffordia secunda, Cutleria multifida and Haplospora globosa-may be taken to represent, within the phaeosporeae, successive steps in the advance from isogamy to oogamy.
The Peridiniaceae have been included among Flagellata under the title of Dinoflagellata. The majority of the species belong to the sea, but many are found in fresh water. The thallus is somewhat spherical and unicellular, exhibiting a distinction between anterior and posterior extremities, and dorsal and ventral surfaces. The wall consists of a basis of cellulose, and in some cases readily breaks up into a definite number of plates, fitting into one another like the plates of the carapace of a tortoise; it is, moreover, often finely sculptured or coarsely ridged and flanged. Two grooves are a constant feature of the family, one running transversely and anoiher longitudinally. In these grooves lie two c;lia, attached at the point of meeting on the dorsal surface. The protoplast is uninucleate and vacuolate, and contains chromatophores of a brownish colour. It is not clear that FIG. 4.-Phaeophyceae, variously magnified.
A. Halopteris, apical region.
B. Chordaria sp., apical region showing so-called trichothallic growth.
C. Dictyota sp., apical cells immediately after dichotomy.
D. Cutleria sp., margin of thallus showing trichothallic growth.
E. Halidrys, apical depression with leading cell.
F. Macrocystis sp., tubular elements from the medulla, with sieve-like transverse walls.
G. Laminaria sp., hyphae with trumpet-like ends also from medulla.
H. Elachistea sp., Plurilocular sporanges.
K. Ectocarpus sp., unilocular sporange.
L. Ectocarpus siliculosus, female gamete surrounded by male gametes a, b, c, d, e, stages of conjugation.
M. Cutleria multifida. a, antherozoids, b, a female gamete.
N1. Fucus vesiculosus, young oogonium.
N2. Fucus vesiculosus, discharge of eight oospheres from oogonium
O. Laminaria sp., sporanges among paraphyses.
P. Dictyota dichotoma, a sorus of oogonia.
Q. Dictyota dichotoma, part of a sorus of antheridia.
(A, B, C, D, E, H, L, M, P, from Engler and Prantl, by permission of Wilhelm Engelmann; F, G, K, O, from Oltmanns, by permission of Gustav Fischer; Q. from The Annals of Botany, by permission of the Clarendon Press; N1, N2, from Hauck, Meeresalgen, by permission of Eduard Kummer.)
the brown colouring matter which is added to chlorophyll is identical with phycophaein: two varieties of it have been termed phycopyrrin and peridinine. Certain species, such as Gymnodinium spirale, are colourless and therefore saprophytic in their method of nutrition. Multiplication takes place in some cases by the endogenous formation of zoospores, the organism having come to rest; in others by longitudinal division, when the organism is still motile. No method of sexual reproduction is known with certainty.
The Cryptomonadaceae also are unicellular, and live free or in colonies. Each cell contains a flattened chromatophore of a brown or yellow colour. Hydrurus forms a branched gelatinous colony attached to stones in mountain streams. Chromophyton forms an eight-celled colony. Both plants multiply solely by means of zoospores. The Cryptomonadeae and Chromulineae are motile through the greater part of their life. Cryptomonas, when dividing in a mucilage after encystment, recalls the condition in Gloeocystis. In Synura and Chromulina the cells form a spherical motile colony, recalling Volvocaceae. Chromulina is uniciliate, and is contained in a hyaline capsule. Like the Peridiniaceae, the Cryptomonadaceae have been included among Flagellata. They have no close affinity with Euphaeophyceae. Such colonial forms as Hydrurus and Phaeocystis are supposed, however, to indicate a stage in the passage to the multicellular condition.
Diatomaceae have long been recognized as plants. Together with Peridiniaceae they constitute the bulk of marine plankton, and thus play an important part in the support of marine animal life. They exhibit striking adaptations in these circumstances to the floating habit. (See DIATOMACEAE.)
A census of Phaeophyceae is given below:-
(1) Cyclosporinae (Fucaceae)-4 families, 32 genera, 347 species.
(2) Tetrasporinae (Dictyotaceae)- 1 family, 17 genera, 130 species.
(3) Phaeozoosporineae (Phaeosporeae)-24 families, 143 genera, 571 species. (De Toni's Sylloge ALgarum.)
(4) Peridiniales--3 families, 32 genera, 167 species.
(5) Cryptomonadaceae (including Chrysomonadaceae) -2 families, 28 genera, 50-60 species.
(6) Bacillariales (Diatomaceae)-about 150 genera and 5000 species, fossil and recent.
(Engler and Prantl's Pflanzenfamilien)
IV. RHODOPHYCEAE, or FLORIDEAE.-The members of this group are characterized by the possession of a red colouring matter, phycoerythrin, in addition to chlorophyll. There is, however, a considerable amount of difference in the shades of red which mark different species. The brightest belongs to those species which grow near low-water mark, or under the shade of larger algae at higher levels; species which grow near high-water mark are usually of so dark a hue that they are easily mistaken for brown seaweeds. Rhodophyceae are mostly marine, but not exclusively so. Thorea, Lemanea, Tuomeya, Stenocladia, Batrachospermum, Balbiania are genera belonging entirely to fresh water; and Bangia, Chanitransia, Caloglossa, Bostrychia and Delesseria contain each one or more freshwater species. Most of the larger species of marine Rhodophyceae are attached by means of a disc to rocks, stones or shells. Many are epiphytic on other algae, more especially the larger Phaeophyceae and Rhodophyceae. As in the case of epiphytic brown seaweeds, the rhizoids of the epiphyte often penetrate the substance of the supporting alga. Some Red Algae find a home in the gelatinous substance of Flustra, Alcyonidium and other polyzoa, only emerging for the formation of the reproductive organs. Some are perforating algae and burrow into the substance of molluscan shells, in company with certain Green and Blue-green Algae. Some species belonging to the families Squamariaceae nnd Corallinaceae grow attached through their whole length and breadth, and are often encrusted with lime. The forms which grow away from the substratum vary greatly in external configuration. In point of size the largest cannot rival the larger Brown Algae, while the majority require the aid of the microscope for their investigation.
No unicellular Rhodophyceae are known, although a flagellate organism, Rhodomonas, has recently been described as possessed of the same red colouring matter. If the sub-group, Bangiaceae, be excluded, they may be said to consist exclusively of branched filaments. Growth in these cases takes place by means of an apical cell, from which successive segments are cut off by means of a transverse wall. The segment so cut off does not usually divide again by means of a transverse wall, nor indeed by a longitudinal wall which passes through the organic axis of the cell. New cells may be cut off laterally, which become the apical cells of branches. When the new cells grow no further, but constitute a palisading round the central cell covering its whole length, the condition is reached which characterizes the species of Polysiphonia, the "siphons" of which may be regarded as one-celled branches. To the law that no subsequent transverse division takes place in segments cut off from the apical cell, there seem to be two exceptions: first, the calcareous genus Corallinia, in the pliable joints of which intercalated division occurs; and, second, the Nitophylleae, in which, moreover, median longitudinal division of axial cells is said to occur. Like the Fungi, therefore, the Red Algae consist for the most part of branched filaments, even where the thallus appears massive to the eye, and, as in the case of Fungi, this fact is not inconsistent with a great variery of external morphology. In the great majority the thallus is obviously filamentous, as in some species of Cillithamnion. In other species of that genus an apparent cortication arises by the downward growth of rhizoids, which are retained within the gelatinous wall of the axial cells. in Batrachospermum the whole system of branches are retained within a diffluent gelatinous substance derived from the outer layers of the cell-walls. In other cases the mucilage is denser and the branches more closely compacted Helminthora.) In such cases as Lemanea, the terminal cells of the lateral branches form a superficial layer which has all the appearance of a parenchyma when viewed from the surface. In Champia and allied genera, the cylindrical axis is due not to the derivatives of one axial filament, but of several, the growth of which is co-ordinated to form a septated tube. The branching of the thallus, which meets the eye in all these cases, is due to the unlimited growth of a few branches. When such a lateral branch overtops the main axis whose growth has become limited, as in Plocamium and Dasya, a sympodium is formed. For the most part the branching is monopodial. Besides the differentiation into holdfast and shoot, and into branches of limited and branches of unlimited growth, there appear superficial structures of the nature of hairs. These are for the most part long, thin-walled, unicellular and colourless, and arise from the outer cells of the pseudo-cortex, or from the terminal cells of branches when the filaments are free. Among Rhodomelaceae, hair-like structures of a higher order are known. These arise from the axial cell, and are multicellular and branched. They soon fall off, and it is from the persistent basal cell that the branches of unlimited growth arise. Upon them also the reproductive organs arise in this family. It is not surprising, therefore, that they have been regarded as the rudiments of leaves. In Iridaea the thallus is an entire lamina; in Callophyllis a lobed lamina; in Delesseria it is provided with midrib and veins, simulating the appearance of a leaf of the higher plants; in Constantinea the axis remains cylindrical, and the lateral branches assume the form of leaves. In the compact thalli a secondary development often takes place by the growth of rhizoid-like internal filaments. They present a hypha-like appearance, running longitudinally for considerable distances. It is not difficult in such compact species to distinguish between superficial cells, whose chief function is assimilation, subjacent cells charged with reserve material, and a core of tissue engaged in the convection of elaborated material from part to part.
An interesting feature of the minute anatomy of Euflorideae, as the Red Algae, exclusive of the Bangiaceae, have been termed, is the existence of the so-called Floridean pit. When a cell divides it is found that there remains in the middle of the new wall a single large circular pit, which persists throughout the life of the cells, becoming more and more conspicuous with the progress of the thickening of the wall. These pits serve to indicate the genetic relationship of adjacent cells, when they form a compact pseudo-parenchyma, notwithstanding the fact that somewhat smaller secondary pits appear later between any contiguous cells. Protoplasmic continuity has been observed in the delicate membrane closing the pit.
Vegetative multiplication occurs only sparingly in Rhodophyceae. Melobesia callithamnioides gives rise to multicellular propagula; (Griffithsia corallina is said to give rise to new individuals, by detaching portions of the thallus from the base of which new attachment organs have already arisen. The spores of Monospora are by some regarded as unicellular propagula. Reproduction is both asexual and sexual. It is noteworthy that although all the members of the group are aquatic no zoospores are produced, a negative character common to them and the Blue-green Algae. As a rule the asexual cells, and the male and female sexual cells arise upon different plants, so that the species may be said to be trioecious. Numerous exceptions, however, occur. Thus in Lemaneaceae asexual spores are unknown; in Batracho-spermum, Bonnemaisonia and Polysiphonia byssoides both kinds of sexual cells appear on the same plant; and in some cases the asexual cells may occur in conjunction with either the male or female sexual cells. The asexual cells are termed tetraspores on account of the usual occurrence of four in each sporangium. What may be termed monospores, bispores and octospores, however, are not unknown. The sporangia may be terminal or intercalated. When they are confined to special branches such branches are spoken of as stichidia. The tetrasrores may arise by the simultaneous division of the contents of a sporangium, when they are arranged tetrahedrally, or they may arise by two successive divisions, in which case the arrangement may be zonate when the spores are in a row, or cruciate when the second divisions are at right angles to the first, or tetrahedral when the second divisions are at right angles to the first and also to one another. Tetraspores are at first naked, but soon acquire a cell-wall and germinate without a period of rest. The male sexual cells are produced singly in the terminal cells of branches. They are spoken of as spermaria. Great numbers of antheridia are usually crowded together, when the part is distinguishable by the absence of the usual red colour. In Polysiphonia they cover the joints of the so-called leaves; in Chondria they arise on flattened dishs; in the more massive forms they arise in patches on the ordinary surface; in a few cases (Gracilaria, Corallina, Galaxaura) they line the walls of conceptacle-like depressions. The female sexual cell is represented by the contents of a cell which is terminal on ordinary or specialized branches. This is the carpogonium: it consists of a neutral portion which contains a nucleus, but in which no oosphere is differentiated, and an elongated tubular portion known as the trichogyne, into which the cytoplasm extends. Fertilization is effected by the passive convection of a spermatium from the antheridium to the trichogyne, to which it adheres, and to which it passes over its nucleus through an open communication set up at the point of contact. The nucleus then passes down the trichogyne and fuses with that of the eeg. This fusion has been observed by Wille in Nemalion multifidim, and by Schmidle in Batrachospermum. It is singular that in the last-named species two nuclei occur regularly in the spermatium. The ventral portion of the carpogonium may be imbedded deep in the thallus in the massive species; the trichogyne, however, always reaches the surface. The first effect of fertilization is the occlusion of the trichogyne from the fertilized carpogonium. The subsequent course of development is characteristic of the Florideae. The carpogonium germinates forthwith, drawing its nourishment almost wholly from the parent plant. The ultimate product in all cases is a number of carpospores, but before this stage is reached the development is different in different subgroups. In Batrachospermum filaments arise from the carpogonium on all sides; in Chantransia and Scinaia on one side only; in Helminthora the filaments are enclosed in a dense mucilage; in Nemalion, prior to the formation of the filaments, a sterile segment is cut off below. In all these cases, however, the end-cells of the filaments each give rise to a carpospore, and the aggregate of such sporiferous filaments is a cystocarp. Again, in the family of the Gelidiaceae, the single filament arising from the carpogonium grows back into the tissue and preys upon the cells of the axis and larger branches, after which the end-cells give rise to carpospores and a diffused cystocarp is formed. In the whole group of the Cryptonemiales the parasitism becomes more marked still. The filaments arising from the carpogonia grow into long thin tubes, which fuse with special cells rich in protoplasm contents; and from these points issue isolated tufts of sporogenous filaments, several of which may form the product of one fertilized female cell. In Naccaria, one of the Gelidiaceae, it is observable that the ooblastema filament, as the tube arising from the fertilized carpogonium has been called, fuses completely with a cell contiguous to the carpogonium before giving rise to the foraging filaments already referred to. This is also the case among Cryptonemiales. In a whole series of Red Algae, the existence or a highly specialized auxiliary cell in the neighbourhood of the carpogonium is a characteristic feature. In the Gigartinales it is already differentiated previous to fertilization; in Rhodymeniales it arises subsequent to fertilization. In the Gigartinales, the filaments which arise from the auxiliary cell may spread and give rise to isolated tufts of sporogenous filaments, as in the Cryptonemiales. In the Rhodymeniales a single tuft arises directly from the auxiliary cell. The carpospores are in all cases bright red naked masses of protoplasm when first discharged. They soon acquire a cell-wall, and germinate without a period of rest. When the cystocarps or segments of cystocarps are formed in the substance of a thallus, the site is marked merely by a swelling of the substance. When the cystocarp is produced externally, it may form a berry-like mass without an envelope, in which case it it known as a favella. In Rhodomelaceae there is a special urn-shaped envelope surrounding the sporogenous filaments. This is a ceramidium.
The attachment of the cell of an ooblastema filament to a cell of the thallus may be effected by means of a minute pore, or the two cells may fuse their contents into one protoplasmic mass. In the latter case, and especially where the union is with a special auxiliary cell, it is of importance to know what happens to the nuclei of the fusing cells. Schmitz was of opinion that in the cases of open union there occurred a fusion of nuclei similar to that which occurs in the sexual union of two cells. He founded his generalization to a large extent upon the observation that in Gloeosiphonia capillaris two cells completely fuse, and that only one nucleus can be detecteo in the fused mass. Oltmanns has recently re-investigated the phenomena in this plant, among others, and has shown that the nucleus of the cell which is being preyed upon recedes to the wall and gradually atrophies. The nucleus of the ooblastema filament dominates the FIG. 5.-Rhodophyceae, variously magnified.
A. Polysiphonia sp., apical region showing leading cell and cutting off of pericentral cell.
B. Polysiphonia sp., transverse section through a branch, and at a, mother-cell of tetraspores.
C. Lomeittaria sp., apex showing growth in length through coordinated growth of many filaments.
D. Delesseria sp., showing apical region with leading cell.
E. Chrysymenia uvaria, axis with swollen leaf-like appendages.
F. Polyzonia sp., branch with leaf-like branches of limited growth.
G. Collithamnson sp., tetrasporangium with spores arranged in a tetrad.
H. Corallina sp., tetrasporangia with zonate arrangement of tetraspores.
K. Nemalion sp., carpogonial and antheridial branches.
L. Batrachospermum sp., trichogyne with spermatia attached; carpospores arising from fertilized carpogonium.
M. Polysiphonia sp., antheridium.
N. Constantinea sp., with flattened leaf-like appendages.
O. Dudresnaya coccinea, fusion of ooblastema filaments with auxiliary cells; a is an axial cell in transverse section with four appendages.
P. Callithamnion corymbosum, a joint cell with carpogonial branch and a, b, two auxiliary cells.
Q. Callithamnion corymbosum, fusion of products of fertilization with auxiliary cells, the nuclei of which a and b retire to the wall.
R. Polysiphonia sp., section through young cystocarp.
(A, C, D, E, F, G, H, K, L, M, P, Q, from Oltmanns, by permission of Gustav Fischer; B, N, O, R, from Engler and Prantl, by permission of Wilhelm Engelmann.)
mass and from it all the nuclei of the carpospores are thus derived. There thus seems to be no justification for believing, as Schmitz taught, that a second sexual act occurs in the life-cycle of these Florideae.
The Bangiales are a relatively small group of Red Algae, to which much of the description now given does not apply. Structurally they are either a plate of cells, as in Porphyra, or filaments, as in Bangia. There is no exclusive apical growth, and the cells divide in all directions. The characteristic pit is also absent. Sexual and asexual reproduction prevail. The male cell is a spermatium, but the female cell bears no such receptive trichogyne as occurs in other Rhodophyceae. After fertilization the equivalent of the oospore divides directly to form a group of carpospores. There is thus a certain resemblance to Euflorideae, but sufficient difference to necessitate their being grouped apart. Fertilization by means of non-motile spermatia and a trichogyne are known among the Fungi in the families Collemaceae any Laboulbeniaceae.
A census of Rhodophyceae is furnished below:-
(1) Bangiaceae-4 families, 9 genera, 58 species.
(2) Nemalioninae-4 families, 33 genera, 343 species.
(3) Gigartininae-3 families, 54 genera, 409 species.
(4) Rhodymeninae-4 families, 92 genera, 602 species. (De Toni's Sylloge Algarum, 1897.)
Limits of the algae.
After this survey of the four groups comprised under Algae it is easier to indicate the variations in the limits of the class as defined by different authorities. To consider the Cyanophyceae first, either the marked contrast in the method of nutrition of the generally colourless Bacteriaceae to that of the blue-green Cyanophyceae is regarded as sufficient ground for excluding Bacteriaceae from algae altogether, notwithstanding their acknowledged morphological affinity with Cyanophyceae, or, in recognition of the incongruity of effecting such a separation, the whole group of the Schizophyta -that is to say, the Cyanophyceae in the narrow sense, together with Bacteriaceae, is included or excluded together. Again, while Conjugatae may be shut out from Chlorophyceae as an independent group co-ordinate with them in rank, the Characeae constitute so aberrant a group that it has even been proposed to raise them as Charophyta to the dignity of a main division co-ordinate with Thallophyta. Similarly, while Diatomaceae may be excluded from among Phaeophyceae, though retained among algae, the Cryptomonadaceae and Peridiniaceae, like Euglena and other Chlorophyceae, may be excluded from Thallophyta and ranged among the flagellate Protozoa. It is doubtful, however, whether the conventional distinction between plants and animals will continue to be urged; and the suggestion of Haeckel that a class Protista should be established to receive the forms exhibiting both animal and plant affinities has much to recommend it on phylogenetic grounds. To adopt a figure, it is probable that the sources from which the two streams of life-animal and vegetable-spring may not be separable by a well-defined watershed at all, but consist of a great level upland, in which the waterways anastomose. Finally, while Chlorophyceae and Phaeophyceae exhibit important affinities, the Rhodophyceae are so distinct that the term "algae" cannot be made to include them, except when used in its widest sense.
It has been well said that the attempt to classify plants according to their natural affinities is an attempt to construct for them the genealogical tree by which their relationships can be traced. Algae are, however, so heterogeneous a class, of which the constituent groups are so inadequately known, that it is at present futile to endeavour thus to exhibit their pedigree. A synoptical representation of the present state of knowledge would be expressed by a network rather than by a tree. The following table is an adaptation of a scheme devised by Klebs, and indicates the inter-relationships
Volvocaceae Chlorosphaeraceae CONGUGATAE SIPHONALES
of the various constituent groups. The area included in the thick boundary line represents algae in the widest sense in which the term is used, and the four included areas the four main subdivisions. A continuous line indicates a close affinity, and a dotted line a doubtful relationship.
Alternation of generations.
In comparing algae with the great archegoniate series which has doubtless sprung from them, it is natural to inquire to what extent, if any, they present evidence of the existence of the marked alternation of generations which dominates the life-history of the higher plants. Turning first to the Rhodophyceae, both on account of the high place which they occupy among algae and also the remarkable uniformity in their reproductive processes, it is clear that, as is the case among Archegoniatae, the product of the sexual act never germinates directly into a plant which gives rise to the sexual organs. Even among Bangiaceae the carpospores arise from the fertilized cell by division, while in all other Rhodophyceae the oospore, as it may be called, gives rise to a filamentous structure, varying greatly in its dimensions, epiphytic, and to a large extent parasitic upon the egg-bearing parent plant, and in the end giving rise to carpospores in the terminal cells of certain branches. There is here obviously a certain parallelism with the case of Bryophyta, where the sporogonium arising from the oospore is epiphytic and partially parasitic upon the female plant, and always culminates in the production of spores. Not even Riccia, with its rudimentary sporogonium, has so simple a corresponding stage as Bangia, for, while there is some amount of sterile tissue in Riccia, in Bangia the oospore completely divides to form carpospores. Excluding Bangiaceae, however, from consideration, the Euflorideae present in the product of the development of the oospore like Bryophyta a structure partly sterile and partly fertile. There is, nevertheless, this important difference between the two cases. While the spore of Bryophyta on germination gives rise to the sexual plant, the carpospore of the alga may give rise on germination to a plant bearing a second sort of asexual cells, viz. the tetraspores, and the sexual plant may only be reached after a series of such plants have been successively generated. It is possible, however, that the tetraspore formation should be regarded as comparable with the prolific vegetative reproduction of Bryophyta, and in favour of this view there is the fact that the tetraspores originate on the thallus in a different way from carpospores with which the spores of Bryophyta are in the first place to be compared; moreover, in certain Nemalionales the production of tetraspores does not occur, and the difficulty referred to does not arise in such cases. Altogether it is difficult on morphological grounds to resist the conclusion that Florideae present the same fundamental phenomenon of alternation of generations as prevails in the higher plants. It is by means of the cytological evidence, however, that this problem will finally be solved. As is well known, the dividing nuclei of the cells of the sporophyte generation of the higher plants exhibit a double number of chromosomes, while the dividing nuclei of the cells of the gametophyte generation exhibit the single number. In a fern-plant, for example, which is a sporophyte, every karyokinesis divulges the double number, while in the prothallium, which is the gametophyte generation, the single number appears. The doubling process is provided by the act of fertilization, where an antherozoid with the single number of chromosomes fuses with an oosphere also with the single number to provide a fertilized egg with the double number. The reduction stage, on the other hand, is the first division of the mother-cell of the spore. From egg to spore-mother-cell is sporophyte; from spore-mother-cell to egg is gametophyte. And since this rule has been found to hold good for all the archegoniate series and also for the flowering plants where, however, the gametophyte generation has become so extremely reduced as to be only with difficulty discerned, it is natural that when alternation of generation is stated to occur in any group of Thallophyta it should be required that the cytological evidence should support the view. The genus Nemalion has been recently investigated by Wolfe with the object of examining the cytological evidence. He finds that eight chromosomes appear in karyokinesis in the ordinary thallus cells, but sixteen in the gonimoblast filaments derived from the fertilized carpogonium. Eight chromosomes appear again in the ultimate divisions which give rise to the carpospores. Upon the evidence it would seem therefore that so far as Nemalion is concerned an alternation occurs comparable with that existing in the lower Bryophyta where the sporophyte is relatively small, being attached to and to some extent parasitic upon the gametophyte. Nemalion is, however, one of those Florideae in which tetraspores do not occur. What is the case with those Florideae which have been described as trioecious? If the sporophyte generation is confined to the cystocarp, is the tetrasporiferous plant, as has been suggested, merely a potential gametophyte reproducing by a process analogous to the bud- formation of the Bryophyta? In answer to this question a recent writer, Yamanouchi, states in a preliminary communication that he has found that in Polysiphonia violacea the germinating carpospores exhibit forty chromosomes, and the germinating tetraspores twenty chromosomes. From this it would seem that in this plant reduction takes place in the tetraspore mother- cell, and that the tetrasporiferous plants are sporophytes which alternate with sexual plants. Novel as this result may seem, the tetraspores of Florideae become hereby comparable with the tetraspores of Dictyota, to which reference will be made hereafter. But it is clear that it becomes on this view increasingly difficult to explain the occasional occurrence of tetraspores on male, female and monoecious plants or the role of the carpospores in the life-cycle of Florideae. The results of future research on the cytology of the group will be awaited with interest.
Among Phaeophyceae it is well known that the oospore of Fucaceae germinates directly into the sexual plant, and there is thus only one generation. Moreover, it is known that the reduction in the number of chromosomes which occurs at the initiation of the gametophyte generation in Pteridophyta occurs in the culminating stage of Fucus, where the oogonium is separated from the stalk-cell, so that unless it be contended that the Fucus is really a sporophyte which does not produce spores, and that the gametophyte is represented merely by the oogonium and antheridium, there is no semblance of alternation of generation in this case. The only case among Phaeophyceae which has been considered to point to the existence of such a phenomenon is Cutleria. Here the asexual cells are borne upon the so-called Aglaozonia reptans and the sexual cells upon the plants known as Cutleria. The spores of the Aglaozonia form are known to give rise to sexual plants, and the oospore of Cutleria has been observed to grow into rudimentary Aglaozonia. Latterly, however, as the result of the cytological investigations of Mottler and Lloyd Williams, great advance has been made in our knowledge of the conditions existing in Dictyota. Mottler first observed that a reduction in the number takes place in the mother-cells of the tetraspore. It will be remembered that, as in most Florideae, the male, female and asexual plants are distinct in this genus. Mottler's observation has been confirmed by Lloyd Williams, who has shown, moreover, that the single number occurs in germlings from the tetraspore, and also in the adult stages of all sexual plants, while the double number occurs in germlings from the oospore, and in adult stages of all asexual plants. It is probable, therefore, that we have here a sharp alternation of generations, both generations being, however, precisely similar to the eye up to point of reproduction. Among Chlorophyceae it is often the case that the oospore on germination divides up directly to form a brood of zoospores. In Coleochaete this seems to be preceded by the formation of a minute parenchymatous mass, in each cell of which a zoospore is produced. In Sphaeroplea it is only at this stage that zoospores are formed at all; but in most cases, such as Oedogogonium, Ulothrix, Coleochaete, similar zoospores are produced again and again upon the thallus, and the product of the oospore may be regarded as merely a first brood of a series. It has been held by some, however, that the first brood corresponds to the sporophyte generation of the higher plants, and that the rest of the cycle is the gametophyte generation. Were the case of Sphaeroplea to stand alone, the phenomenon might perhaps be regarded as an alternation of generations, but still only comparable with the case of Bangia, and not the case of the Florideae. But it is difficult to apply such a term at all to those cases in which there intervene between the oospore and the next sexual stage a series of generations, the zoospores of which are all precisely similar.
The difficulty of tracing the relationships of algae is largely due to the inadequacy of our knowledge of the conditions under which they pass through the crucial stages of their life-cycle. Of the thousands of species which have been distinguished, relatively few have been traced from spore to spore, as the flowering plants have been observed from seed to seed. The aquatic habit of most of the species and the minute size of many of them are difficulties which do not exist in the case of most seed-plants. From the analogy of the higher plants observers have justly argued that when they have seen and marked the characters of the reproductive organs they have found the plant at the stage when it exhibits its most noteworthy features, and they have named and classified the species in accordance with these observations. While even in such cases it is obvious that interesting stages in the life of the plant may escape notice altogether, in the cases of those plants the reproduction of which is unknown, and which have been named and placed on the analogy of the vegetative parts alone, there is considerable danger that a plant may be named as a distinct species which is only a stage in the life of another distinct and perhaps already known species. To take an example, Lemanea and Batrachospermum are Florideae which bear densely-whorled branches, but which, on the germination of the carpospore, give rise to a laxly-filamentous, somewhat irregularly-branched plant, from which the ordinary sexual plants arise at a later stage. This filamentous structure has been attributed to the genus Chantransia, which it greatly resembles, especially when, as is said to be the case in Batrachospermum, it bears similar monospores. The true Chantransia, however, bears its own sexual organs as well as monospores. To the specific identity of Haplospora globosa and Scaphospora speciosa, and of Cutleria muitifida and Aglaozonia reptans, reference has already been made. Again, many Green Algae-some unicellular, like Sphaerella and Chlamydomonas; some colonial forms, like Volvox and Hormotila; some even filamentous forms, like Ulothrix and Stigeoclonium- are known to pass into a condition resembling that of a Palmella, and might escape identification on this account.
It is, on the other hand, a danger in the opposite sense to conclude that all Chantransia species are stages in the life-cycle of other plants, and, similarly, that all irregular colonial forms, like Palmella, represent phases in the life of other Green Algae. Long ago Kutzing went so far as to express the belief that the lower algae were all capable of transformations into higher forms, even into moss-protonemata. Later writers have also thought that in all four groups of algae transformations of a most far-reaching character occur. Thus Borzi finds that Protoderma viride passes through a series of changes so varied that at different times it presents the characters of twelve different genera. Chodat does not find so general a polymorphism, but nevertheless holds that Raphidium passes through stages represented by Protococcus, Characium, Dactylococcus and Sciadium. Klebs has, however, recently canvassed the conclusions of both these investigators; and as the result of his own observations declares that algae, so far from being as polymorphic as they have been described, vary only within relatively narrow limits, and present on the whole as great fixity as the higher plants. It certainly supports his view to discover, on subjecting to a careful investigation Botrydium granulatum, a siphonaceous alga whose varied forms had been described by J. Rostafinski and M. Woronin, that these authors had included in the life-cycle stages of a second alga described previously by Kutzing, and now described afresh by Klebs as Protosiphon bolryoides. In Botrydium the chromatophores are small, without pyrenoids, and oil-drops are present; in Protosiphon the chromatophores form a net-work with pyrenoids, and the contents include starch. Klebs insists that the only solution of such problems is the subjection of the algae in question to a rigorous method of pure culture. It is interesting to learn that G. Senn, pursuing the methods described by Klebs, has confirmed Chodat's observation of the passage of Raphidium into a Dactylococcus-stage, although he was unable to observe further metamorphosis. He has also seen Pleurococcus viridis dividing so as to form a filament, but has not succeeded in seeing the formation of zoospores as described by Chodat. While, therefore, there is much evidence of a negative character against the existence of an extensive polymorphism among algae, some amount of metamorphosis is known to occur. But until the conditions under which a particular transformation takes place have been ascertained and described, so that the observation may be repeated by other investigators, scant credence is likely to be given to the more extreme polymorphistic views.
In comparison with the higher plants, algae exhibit so much simplicity of structure, while the conditions under which they grow are so much more readily controlled, that they have frequently been the subject of physiological investigation with a view chiefly to the application of the results to the study of the higher plants. (See PLANTS: Physiology of.) In the literature of vegetable physiology there has thus accumulated a great body of facts relating not only to the phenomena of reproduction, but also to the nutrition of algae. With reference to their chemical physiology, the gelatinization of the cell-wall, which is so marked a feature, is doubtless attributable to the occurrence along with cellulose of pectic compounds. There is, however, considerable variation in the nature of the membrane in different species; thus the cell-wall of Gedogonium, treated with sulphuric acid and iodine, turns a bright blue, while the colour is very faint in the case of Spirogyra, the wall of which is said to consist for the most part of pectose. While starch occurs commonly as a cell-content in the majority of the Green Algae no trace of it occurs in Vaucheria and some of its allies, nor is it known in the whole of the Phaeophyceae and Rhodophyceae. In certain Euphaeophyceae bodies built up of concentric layers, and attached to the chromatophores, were described by Schmitz as phaeophycean-starch; they do not, however, give the ordinary starch reaction. Other granules, easily mistaken for the "starch" granules, are also found in the cells of Phaeophyceae; these possess a power of movement apart from the protoplasm, and are considered to be vesicles and to contain phloroglucin. The colourless granules of Florideae, which are supposed to constitute the carbohydrate reserve material, have been called floridean-starch. A white efflorescence which appears on certain Brown Algae (Saccorhiza bulbosa, Laminaria saccharina), when they are dried in the air, is found to consist of mannite. Mucin is known in the cell-sap of Acetabularia. Some Siphonales (Codium) give rise to proteid crystalloids, and they are of constant occurrence among Florideae. The presence of tannin has been established in the case of a great number of freshwater algae.
By virtue of the possession of chlorophyll all algae are capable of utilizing carbonic acid gas as a source of carbon in the presence of sunlight. The presence of phycocyanin, phycophaein and phycoerythrin considerably modifies the absorption spectra for the plants in which they occur. Thus in the case of phycoerythrin the maximum absorption, apart from the great absorption at the blue end of the spectrum, is not, as in the case where chlorophyll occurs alone, near the Fraunhofer line B, but farther to the right beyond the line D. By an ingenious method devised by Engelmann, it may be shown that the greatest liberation of oxygen, and consequently the greatest assimilation of carbon, occurs in that region of the spectrum represented by the absorption bands. In this connexion Pfeffer points out that the penetrating power of light into a clear sea varies for light of different colours. Thus red light is reduced to such an extent as to be insufficient for growth at a depth of 34 metres, yellow light at a depth of 177 metres and green light at 322 metres. It is thus an obvious advantage to Red Algae, which flourish at considerable depths, to be able to utilize yellow light rather than the red, which is extinguished so much sooner. The experiment of Engelmann referred to deserves to be mentioned here, if only in illustration of the use to which algae have been put in the study of physiological problems. Engelmann observed that certain bacteria were motile only in the presence of oxygen, and that they retained their motility in a microscopic preparation in the neighbourhood of an algal filament when they had come to rest elsewhere on account of the exhaustion of oxygen. After the bacteria had all been brought to rest by being placed in the dark, he threw a spectrum upon the filament, and observed in what region the bacteria first regained their motility, owing to the liberation of oxygen in the process of carbon-assimilation. He found that these places corresponded closely with the region of the absorption band for the algae under experiment.
Although algae generally are able to use carbonic acid gas as a source of carbon, some algae, like certain of the higher plants, are capable of utilizing organic compounds for this purpose. Thus Spirogyra filaments, which have been denuded of starch by being placed in the dark, form starch in one day if they are placed in a 10 to 20% solution of dextrose. According to T. Bokorny, moreover, it appears that such filaments will yield starch from formaldehyde when they are supplied with sodium oxymethyl sulphonate, a salt which readily decomposes into formaldehyde and hydrogen sodium sulphite, an observation which has been taken to mean that formaldehyde is always a stage in the synthesis of starch. With reference to the assimilation of nitrogen, it would seem that algae, like other green plants, can best use it when it is presented to them in the form of a nitrate. Some algae, however, seem to flourish better in the presence of organic compounds. In the case of Scenedesmus acutus it is said that the alga is unable to take up nitrogen in the form of a nitrate or ammoniacal salt, and requires some such substance as an amide or a peptone. On the other hand, it has been held by Bernhard Frank and other observers that atmospheric nitrogen is fixed by the agency of Green Algae in the soil: (For the remarkable symbiotism between algae and fungi see FUNGI and LICHENS.)
Most algae, particularly Phaeophyceae and Rhodophyceae, spend the whole of the life-cycle immersed in water. In the case of the freshwater algae, however, belonging to the Chlorophyceae and Cyanophyceae, although they required to be immersed during the vegetative period, the reproductive cells are often capable of resisting a considerable degree of desiccation, and in this condition are dispersed through great distances by various agencies. Again, as is well known, many species of marine algae growing in the region between the limits of high and low water are so constituted that they are exposed to the air twice a day without injury. The occurrence of characteristic algae at different levels constituting the zones to which reference has already been made, is probably in part an expression of the fact that different species vary in the capacity to resist desiccation from exposure. Thus Laminaria digitata, which characterizes the lowest zone, is only occasionally exposed at all, and then only for short periods of time. On the other hand, Pelvetia canaliculata, which marks the upper belt, is exposed for longer periods, and during neap tides may not be reached by the water for many days. Algae of more delicate texture than either Fucaceae or Laminariaceae also occur in the region exposed by the ebb of the tide, but these secure their exemption from desiccation either by retaining water in their meshes by capillary attraction, as in the case of Pilayella, or by growing among the tangles of the larger Fucaceae, as in the case of Polysiphonia fastigiata, or by growing in dense masses on rocks, as in the case of Laurencia pinnatifida. Such a species as Delesseria sanguinea or Callophyllis laciniata would on the contrary run great risk by exposure for even a short period. A few algae approach the ordinary terrestrial plants in their capacity to live in a sub-aerial habitat subject only to such occasional supphes of water as is afforded by the rainfall. Of this nature are some of the species of Vaucheria. A very few species, like Chroolepus, which grows on rock surfaces, are comparable with the land plants which have been termed xerophilous.
The great majority of the aquatic algae, both freshwater and marine, are attached plants. Some, however, are wanderers, either swimming actively with the aid of cilia, or floating inertly as the result of a specific weight closely approaching that of the medium. To the aggregate of such forms, both animal and vegetable, the term plankton has been applied, and the investigation of the vegetable plankton, both freshwater and marine, has been pursued in recent times with energy and success. The German Plankton Expedition of 1889 added greatly to our knowledge of the floating vegetable life of the North Atlantic Ocean, while many laboratories established on the shores of inland seas and lakes have rendered a similar service in the case of our freshwater phyto-plankton. The quantitative estimate of the amount of this flora has revealed its enormous aggregate amount and therefore its great importance in the economy of oceanic and lacustrine animal life. The organisms constituting this plankton are mostly unicellular, often aggregated together in colonies, and the remarkable structure which they exhibit has added a new chapter to the story of adaptation to environment. The families Diatomaceae, Peridiniaceae and Protococcaceae are best represented in the pelagic plankton, while in addition the Volvocaceae are an important element in freshwater plankton.
The great majority of algae, however, grow like land-plants attached to a substratum, and to these the term benthos is now generally applied. While the root of land-plants serves for the double purpose of attachment and the supply of water, it is attachment only that is usually sought in the case of algae. Immersed as they usually are in a medium containing in solution the inorganic substances which they require for their nutrition, the absorption of these takes place throughout their whole extent. The elaborate provision for the conduct of water from part to part which has played so important a role in the morphological development of land plants is entirely wanting in algae, such conducting tissues as do exist in the larger Phaeophyceae and Rhodophyceae serving rather for the convection of elaborated organic substance, and being thus comparable with the phloem of the higher plants. The attachment organ of algae is thus more properly called a holdfast, and is found to be of very varied structure. It generally takes the form of a single flattened disc as in the Fucaceae, or a group of finger- like processes as in Laminariaceae, or a tuft of filaments as in many instances. When the attachment is in sand or mud, it often simulates the appearance of a true root as in Chara or Caulerpa. It is clear that where the bottom of a lake or sea consists of oozy mud or shifting sand, it is impossible for algae to secure a foothold. Thus a rock emerging from a sandy beach may often be observed to stand covered with vegetation like an oasis in a desert. The rapidity with which walls, piles and pontoons-stone, wood and iron-become covered with marine plants is well known, while the discovery of some effective means of preventing the fouling of the bottoms of ships by the growth of algae would be hailed as a boon by shipowners. While rocks and boulders are the favoured situation for the growth of marine algae, those which readily disintegrate, like the coarser sandstones, are naturally less favoured than the hard and resistant. A large number of algae again live as epiphytes or endophytes. In the case of the freshwater species the host-plants are mostly species of aquatic Graminaceae, Naiadaceae or Nymphaeaceae. In the case of marine algae, the hosts are chiefly the larger Phaeophyceae and Rhodophyceae. A bed of Zostera near the level of low water is, however, on the British coast a favourite collecting ground for the smaller red and brown epiphytes. Of endophytes a distinction must be made between those which occupy the cell-wall only and those which perforate the cells, bringing about their destruction. There can be little doubt that in some cases the epiphytism approaches parasitism. In one case described by Kuckuck the chromaphores of the infesting algae are absent, a circumstance which points to a complete parasitism. Allusion has already been made to the peculiar habit of the shell-boring algae.
In many algae certain branches of limited growth bear a remarkable resemblance to leaves. The Characeae among freshwater algae and the Sargassaceae among marine algae might be cited as examples. Surveying the whole range of algae life, Oltmanns distinguishes bush-forms, whip- forms, net-forms, leaf-forms, sack-forms, dorsi-ventral forms, and cushions, plates and crusts. The similarity of outline in many species to that of trees and shrubs will strike any one who examines algae mounted for the herbarium. Cladophora and Bryopsis among monosiphonous forms, Chara, Polysiphonia, Ceramium and Cystoseira among larger algae, are illustrations of this. The whip-forms are represented by Spirogyra, Chaetomorpha, Scytosiphon, Nemalion, Himanthalia and Chorda. Net-forms are found in Hydrodictyon and Microdictyon. The leaf-forms are very varied and owe their existence to the advantage accruing from the exposure of a large surface to the influence of the light. In some cases such as Delesseria, Neurymenia, Fucus, Alaria, the leaf-like structure is provided with a strengthening mid-rib, and when as in Delesseria it is also richly veined the resemblance to the leaf of a flowering plant is striking. Laminaria, Padina, Cutleria, Punctaria, Iridaea, Ulva, Porphyra, are leaf-like with a rigidity varying from a fleshy lamina to the thin and pliable. Agarum, Claudea and Struvea are leaf-forms which are perforated like Aldrovanda among flowering plants. Enteromorpha, Asperococcus and Adenocystis are sack-forms. Dorsi-ventral algae are rare. Leveillea jungermanneoides bears a remarkable resemblance to a leafy liverwort. In the next group of forms the simplest are crusts attached to the substratum throughout their extent, and growing at the margin. Such are Myrionema, Ralfsia, Melobesia and Hildebrandtia. Others are attached throughout their extent, but also grow vertical filaments so as to form a velvety pile. Such are Coleochaete, Ochlochaete, Elachistea, Ascocyclus and Rhododermis. Peysonellia squamaria, Melobesia lichenoides, Leathesia difformis are forms which are not attached throughout but grow in plates like the foliaceous lichens.
When it is sought to consider algae with a view to the correlation of the external form to the conditions of life, a subject the study of which under the name of ecology has been latterly pursued with great success among land plants, it is difficult as yet to arrive at generalizations which are trustworthy. Among land plants, as is well known, similarity of environment has often called forth similar adaptations among plants of widely separated families. The similarity of certain xerophilous Euphorbiaceae to Cactaceae is a ready illustration of this phenomenon. From what has been already said it is evident that among algae also strikingly similar forms exist in widely different groups. Instances might be multiplied. Compare, for example, the blue-green Gloeocapsa with the green Gloeocystis, the red Batrachospermum with the green Draparnaldia, the red Corallina with the green Cymopolia, the green Enteromorpha with the brown Asperococcus, the green Ulva with the red Porphyra, the red Nemalion with the brown Castagnea, and so on. But on the one hand similar forms seem to grow often under different conditions, while on the other hand different forms flourish under the same conditions. The conceivable variations in the conditions which would count in algal life are variations in the chemical character of the water-whether fresh, brackish or salt; or in the rate of movement of the water, whether relatively quiet, or a stream or a surf; or in the degree of illumination with the depth and transparency of the water. But the laws which determine the associations of various algae under one environment are as yet little understood. The occurrence of a plentiful mucilage in many freshwater forms is, however, doubtless a provision against desiccation on exposure. The fine subdivision of filamentous and net-forms is similarly a provision for easy access of water and light to all parts. The calcareous deposits in Characeae, Corallinaceae and Siphonaceae are at once a protection against attack and a means of support. The whip-forms would seem to be designed to resist injury from surf or current. The vesicles of Fucaceae and Laminariaceae prevent the sinking of the bulkier forms. But why certain Fucaceae favour certain zones in the littoral region, why certain epiphytes are confined to certain hosts, why Red and Brown Algae are not better represented in fresh water Or Green Algae in salt,-these are problems to which it is difficult to find a ready answer.
Algae cannot be regarded as directly important in the industries. On the coasts of Europe marine algae detached by the autumnal gales are commonly carted on to the land as a convenient manure. Porphyra laciniata and Rhodymenia palmata are locally used as food, the latter being known as dulse. Agar-agar is a gelatinous substance derived from an eastern species of Gracilaria. The ash of seaweeds, known in Scotland as kelp, and in Brittany as varec, was formerly used as a source of iodine to a greater extent than is at present the case.
Occurence in the rocks.
Excepting where the thallus is impregnated with silica, as in Diatomaceae, or carbonate of lime, as in Corallinaceae, Characeae and some Siphonales, it is perhaps not surprising that algae should not have been extensively preserved in the fossil form. Considering, however, that it is generally believed that Bryophyta and vascular plants are descended from an algal ancestry, it is natural to suppose that, prior to the luxuriant vegetable growths of the Carboniferous period, there must have existed an age of algae. It was doubtless this expectation that has led to the description of a number of Silurian and Devonian remains as algae upon what is now regarded as inadequate evidence. The geologic record is, as perhaps is to be expected, exceedingly poor, except as regards the calcareous Siphonales, which are well represented at various horizons, from the Silurian to the Tertiary; even the Diatomaceae, which are found in great quantities in the Tertiary deposits, do not occur at all earlier than the chalk. It is believed, however, that the Devonian fossil, Nematophycus, is a Laminarian alga, but it is not until the late Secondary and the Tertiary formations that fossil remains of algae become frequent. (See PALAEOBOTANY.)
The subjoined list includes the larger standard works on algae, together with a number of papers to which reference is made in this article. For a detailed catalogue of Algological literature, see the "Bibliotheca Phycologica" in de Tonii's Syllope Algarum, vo1. i. (1889), with the addendum thereto in vol. iv. (1897) of the same work. GENERAL.-J. G. Agardh, Species, genera et ordines Algarum (vols. i-iii., Algernes Systematik (Lund, 1872-1899); J. E. Areschoug, "Observationes Phycologicae," Nova Acta reg. soc. sci. Upsaliensis (Upsala, 1866-1875); F. F. Blackman, "The Primitive Algae and the Flagellata," Ann. of Botany (vol. xiv., Oxford, 1900); E. Bornet and G. Thuret, Notes agologiques (fasc. i.-ii., Paris, 1876-1880); P. A. Dangeard, "Recherches sur les algues inferieures," Ann. des sci. naturelles, Bot. (vol. vii., Paris, 1888); A. Derbes and A. J. J. Solier, Momoire de la physiologie des algues (Paris, 1856); J. B. de Toni, Sylloge Algarum--vol. i. Chlorophyceae, vol. ii. Bacillariaceae, vol. iii. Fucoideae, vol. iv. Florideae (Padua, 1889-1900); P. Falkenberg, "Die Algen im weitesten Sinne," Schenk's Handbuch der Botanik (vol. ii., 1882); W. G. Farlow, Morine Algae of New England (Washington, 1881); W. H. Harvey, Phycologia Britannica (4 vols., London, 1846-1855); Nereis Boreali-Americana (3 pts., Washington, 1851-1858); Phycologia Australica (5 vols., London, 1858-1863); F. Hauck, "Die Meeresalgen Deutschlands und Osterrichs," Rabenhort's Kryptogamen-Flora (Leipzig, 1885); F. R. Kjellman, The Algae of the Arctic Sea (Stockholm, 1883); F. T. Kutzing, Tabulae Phycologicae (19 vols., Nordhausen, 1845-1869); P. Kuckuck, Beitrage zur Kenntniss der Meercsalgen (Kiel and Leipzig, 1897-1899); G. Murray, Phycological Memoirs (London, 1892-1895) Naegeli, Die neueren Algensysteme (Zurich, 1847); F. Oltmanns, Morphologie und Biologie der Algen (Jena, Band i. 1904, Band ii. 1905); N. Pringsheim, "Beitrage zur Morphologie der Meeresalgen," Abhand. Konigl. Akad. der Wissensch. (Berlin, 1862); J. Reinke, Atlas deutscher Meeresalgen (Berlin, 1889-1892); F. Schutt, Das Pflanzenleben der Hochsee (Leipzig, 1893); J. Stackhouse, Nereis britannica (ed. i., Bath, 1801; ed. ii., Oxford, 1816); G. Thuret and E. Bornet, Etudes phycologiques (Paris, 1878); D. Turner, Historia Fucorum (4 vols., London, 1808-1819); G. Zanardini, Iconographia Phycologia Adriatica (Venice, 1860-1876).
1. CYANOPHYCEAE.-E. Bornet and Ch. Flahault, "Revision des Nostocacees heterocystees," Ann. des sc. naturelles, Bot.(vols. iii.-vii., Paris, 1887-1888); M. Gomont, "Monographic des Oscillariees," Ann. des sc. naturelles, Bot. (vols. xv.-xvi., Paris, 1893); Hegler, "Uber Kerntheilungserscheinungen," Ref. Botan. Centralbl. (vol. lxiv., Cassel, 1895); O. Kirchner, "Schizophyceae", in Engler and Prantl's Pflanzenfamilien (Leipzig, 1900).
2. CHLOROPHYCEAE.-A. Borzi, "Studi anamorfici di alcune alghe verdi," Bull. Soc. Bot. Ital. in N. Giorn. Bot. Ital. (vol. xxii., Pisa, 1890); F. F. Blackman and A. G. Tansley, A Revision of the Classification of the Green Algae, reprinted from the New Phytologist (vol. i., London, 1903); K. Bohlin, "Studier ofver nagra slagten af alggruppen confervales Borzi," Bihang till K. Svenska vel. akad. Handlinger (Bd. xxiii. afd. 3, 1897);-Ufkasttill, De grona algernas och arkegomiaternas bylogeni (Upsala, 1901); R. Chodat, "On the Polymorphism of the Green Algae," Ann. of Botany (vol. xi., Oxford, 1897); M. C. Cooke, British Freshwater Algae (2 vols., London, 1884), British Desmids (London, 1887); G. Klebs, Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen (Jena, 1896); A. Luther, "Uber Chlorosaccus, n.g." Bihang till K. Svenska vel. akad. Handlinger (Bd. xxiv. afd. 3, 1899); H. Grat zu Solms-Laubach, "Monograph of the Acetabulariaceae," Trans. Linn. Soc. (Lond.) Bot. (London, 1895); N. Wille, "Chlorophyceae", in Engler and Prantl's Pflanzenfamilien (Leipzig, 1897).
3. PHAEOPHYCEAE.-E. A. L. Batters, "On Ectocarpus secundus,"
Grevillea (vol. xxi., London, 1893); G. Berthold, "Die
geschlechliche Fortpflanzung der eigentlichen Phaeosooreen,"
Mitth. Zool. Stat. Neapel (vol. ii., Leipzig, 1881); G.
Brebner, "On the Classification of the Tilopteridaceae,"
Proc. Bristol Nat. Soc. (vol. viii., Bristol, 1896-1897);
A. H. Church, "On the Polymorphy of Cutleria multiflda,"
Ann. of Botany (vol. xii., Oxford, 1898); J. B. Farmer
esnd J. Ll. Williams, "Contributions to our Knowledge
of the Life- history and Cytology of Fucaceae," Phil.
Trans. Roy. Soc. (vol. cxc., London, 1898); E. Janczewski,
"Observations sur l'accroissement du thalle des Phaeosporees,"
Mem. soc. nat. de sc. (Cherbourg, 1895); F. R. Kjellmann,
"Phaeophyceae," in Engler and Prantl's Pflanzenfamilian
(Leipzig, 1897); F. Oltmanns, "Beitrage zur Kenntniss der
Fucaceen," Bibliotheca botanica, xiv. (Cassel, 1889); C.
Sauvageau, "Observations relatives a la sexualite des
Phaeosporees," Journal de botanique (vol. x., Paris,
1896); E. Strasburger, "Kerntheilung und Befruchtung bei
Fucus," Cytologische Studien (Berlin, 1897); F. Schutt,
Die Peridinien der Plankton-Expedition (Kiel and Leipzig,
1895); R. Valiante, Le Cystoseirae del Golfo di Napoli
(Leipzig, 1883); J. Ll. Williams, "On the Antherozoids of
Dictyota and Taonia," Ann. of Botany (vol. xi., Oxford, 1897).
4. RHODOPHYCEAE.-G. Berthold, "Die Bangiacen des Golfes von Neapel," Mitth. Zool. Stat. Neapel (Naples, 1882); F. Oltmanns, "Zur Entwickelungsgeschichte der Florideen," Botanische Zeitung (1898); R. W. Philligs, "The Development of the Cystocarp in Rhodymeniales," i. and ii., Annals of Botany (vols. xi. xii., Oxford. 1897-1898); F. Schmitz, "Untersuchungen uber die Befruchtung der Florideen," Sitzungsber. der konigl. Akad.der Wissensch. (Berlin, 1883); "Kleinere Beitrage zur Kenntniss der Florideen," La Nuova Notarisia, 1892-1894; F. Schmitz, P. Falkenberg, P. Hauptfleisch, "Rhodophyceae," in Engler and Prantl's Pflanzenfamilien (1897); W. Schmidle, "Die Befruchtung, Keimung und Haarinsertion von Batrachospermum," Bot. Zeitung.. (1899); Sirodot, Les Batrachospermes (Paris, 1884); N. Wille, "Uber die Befruchtung bei Nemalion multifidum," Ber. d. deutschen bot. Gesellsc. Band xii. (Berlin, 1894); J. J. Wolfe, "Cytological Studies on Nemalion," Annals of Botany (vol. xviii., Oxford, 1904); S. Yamanouchi, "The Life- History of Polysiphonia violacea," Botanical Gazette (vol. xli., Chicago, 1906). (R. W. P.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)