Reproduction Of Plants
REPRODUCTION OF PLANTS The various modes- in which plants reproduce their species may be conveniently classified into two groups, namely, vegetative propagation and true reproduction, the distinction between them being roughly this, that whereas in the former the production of the new individual may be effected by the most various parts of the body, in the latter it is always effected by means of a specialised reproductive cell.
I. Vegetative Propagation.
The simplest case of vegetative multiplication is afforded by unicellular plants. When the cell which constitutes the body of the plant has attained its limit of size it gives rise to two either by division or gemmation; the two cells then grow, and at the same time become separated from each other, so that eventually two new distinct individuals are produced, each of which precisely resembles the original organism. A good example of this is to be found in the germination of the yeast plant. This mode of multiplication is simply the result of the ordinary processes of growth. All plant-cells grow and divide at some time or other of their life; but whereas in > multicellular plant the products of division remain coherent, and add to the number of the cells of which the plant consists, in a unicellular plant they separate and constitute new individuals. In more highly organized plants vegetative propagation may be effected by the separation of the different parts of the body from each other, each such part developing the missing members and thus constituting a new individual. This takes place spontaneously in rhizomatous plants, in which the main stem gradually dies away from behind forwards; the lateral branches thus become isolated and constitute new individuals.
The remarkable regenerative capacity of plant-members is largely made use of for the artificial propagation of plants. A branch removed from a parent-plant will, under appropriate conditions, develop roots, and so constitute a new plant; this is the theory of propagation by " cuttings." A portion of a root will similarly develop one or more shoots, and thus give rise to a new plant. An isolated leaf will, in many cases, produce a shoot and a root, that is, a new plant; it is in this way that new begonias, for instance, are propagated. The production of plants from leaves occurs also in nature, as, for instance, in certain so-called " viviparous " plants, of which Bryophyllum calycinum (Crassulaceae) and many ferns [Nephrodium (Laslraea) Filix-mas, Asplenium (Athyrium) Filix-foemina and other species of Asplenium] are examples. But it is in the mosses, Of all plants, that the capacity for vegetative propagation is most widely diffused. Any part of a moss, whether it be the stem, the leaves, the rhizoids, or the sporogonium, is capable, under appropriate conditions, of giving rise to filamentous protonema, on which new moss-plants are then developed as lateral buds.
In a large number of plants provision is made for vegetative propagation by the development of more or less highly specialized organs. In lichens, for instance, there are the soredia, which are minute buds of the thallus containing both algal and fungal elements; these are set free on the surface in large numbers, and each grows into a thallus. In the Characeae there are the bulbils or " starch-stars " of Char a stelligera, which are underground nodes, and the branches with naked base and the proembryonic branches found by Pringsheim on old nodes of Chara fragilis. In the mosses small tuberous bulbils frequently occur on the rhizoids, and in many instances (Bryum annotinum, Aulacomnion androgynum, Tetraphis pellucida, etc.) stalked fusiform or lenticular multicellular bodies containing chlorophyll, termed gemmae, are produced on the shoots, either in the axils of the leaves or in special receptacles at the summit of the stem. "Gemmae of this kind are produced in vast numbers in Marchantia and Lumdaria among the liverworts. Similar gemmae are also produced by the prothallia of ferns. In some ferns (e.g. Nephrolepis tuberosa and undulata) the buds borne on the leaves or in their axils become swollen and filled with nutritive materials, constituting bulbils which fall off and give rise to new plants. This conversion of buds into bulbils, which subserve vegetative multiplication, occurs also occasionally among Phanerogams, as for instance in Lilium bulbiferum, species of Poa, Polygonum viviparum, etc. But many other adaptations of the same kind occur among Phanerogams. Bulbous plants, for instance, produce each year at least one bulb or corm from which a new plant is produced in the succeeding year. In the potato, tubers are developed from subterranean snoots, each of which in the following year gives rise to a new individual. In the dahlia, Thladiantha dubia, etc., tuberous swellings are found on the roots, from each of which a new individual may spring.
II. True Reproduction.
This is effected by cells formed by the proper reproductive organs. These cells are of two principal kinds. There are, first, those cells each of which is capable of developing by itself into a new organism: these are the asexual reproductive cells, known generally as spores. Secondly, there are the cells which are incapable of independent germination; it is not until these cells have fused together in pairs that a new organism can be developed: these are the sexual reproductive cells or gametes.
In some exceptional cases the normal mode of reproduction, sexual or asexual, does not take place: instead, the new organism is developed vegetatively from the parent. When sexual reproduction is suppressed the case is one of apogamy; when asexual reproduction by spores is suppressed the case is one of apospory. (Apogamy and apospory are discussed below in the section on Abnormalities of Reproduction.)
Asexual Reproduction. Reproduction by means of some kind of spore (using the term in its widest sense, so as to include all asexually produced reproductive cells) is common to nearly all families of plants; it is wanting in certain Algae (Conjugatae, Fucaceae, Characeae), and in certain fungi (e.g. some Peronosporeae). The structure of a spore is essentially this: it consists of a nucleated mass of protoplasm, enclosing, starch or oil as reserve nutritive material, usually invested by a cell-wall. In those cases in which the spore is capable of germinating immediately on its development the cell-wall is a single delicate membrane consisting of cellulose; but in those cases in which the spore may or must pass through a period of quiescence before germination the wall becomes thickened and may consist of two layers, an inner, the endospore, which is delicate and consists of cellulose, and an outer, the exospore, which is thick and rigid, frequently darkly coloured and beset externally with spines or bosses, and which consists of cutin. In some few cases among the fungi, multicellular or septate spores are produced; these approximate somewhat to the gemmae mentioned above as highly specialized organs for vegetative propagation. In some cases, particularly among the algae, and also in some fungi (Peronosporeae, Saprolegnieae, Chytridiaceae, and the Myxomycetes), spores are produced which are usually destitute of any cell-wall, and are further peculiar in that they are motile, and are therefore termed zoospores; they move sometimes in an amoeboid manner by the protrusion of pseudopodia, but more frequently they are provided with one, two, or many delicate vibratile protoplasmic filaments, termed cilia, by the lashing of which the spore is propelled through the water. The zoospore eventually comes to rest, withdraws its cilia, surrounds itself with a cell-wall, and then germinates.
In the simplest case a single spore is developed from the cell of the unicellular plant, the protoplasm of which surrounds itself with the characteristic thick wall. This occurs only in plants of low organization such as the Schizophyta.
In other cases the contents of the cell undergo division, each portion of the protoplasm constituting a spore. Examples of this are afforded, among unicellular plants, by yeast and the Protococcaceae; and in multicellular plants by the Pandorineae, Confervaceae, Ulvaceae, etc., where any cell of the body may produce spores.
In such cases the spore-producing cell may be regarded as a rudimentary reproductive organ of the nature of a sporangium. In more highly organized plants special organs are differentiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a capsule in the interior of which the spores are developed; but in many fungi the spores are formed by abstriction from an organ termed a sporophore. In the Thallophyta the sporangium is commonly a single cell. In the Bryophyta it is a multicellular capsule. In the Pteridophyta the sporangium is mullicellular, but simple in structure, and this is true also of the Phanerogams.
It is important to note that in all the Bryophyta and in some of the Pteridophyta (most of the Filicinae, all existing Equisetinae, and the Lycopodiaceae and Psilotaceae) there is but one kind of sporangium and spore, the plants being homosporous or isosporous, whereas the rest of the Pteridophyta (Hydropterideae, Selaginellaceae) and the Phanerogams are heterosporous, having sporangia of two kinds; some produce one or a few large spores (megaspores), and are hence termed megasporangia, while others give rise to a larger number of small spores (microspores) and are hence termed microsporangia. In the Phanerogams the two kinds of sporangia have received special names: the megasporangium, which produces as a rule only one mature spore (embryo-sac), is termed the ovule; the microsporangium, which produces a large number of microspores (pollen-grains), is termed the pollen-sac.
The development of spores, except in the simpler Thallophyta, is more or less restricted to definite parts of the body. Thus in the Red Algae (Florideae) there are the organs known as stichidia, nemathecia. In the fungi the number and variety of such organs is very great; they may be described generally as simple and compound sporophorcs: but for a description the article FUNGI should be consulted. In the higher plants the organs are less various. In the Bryophyta the production of spores is restricted to the sporogonium. In the vascular plants (Pteridophyta, Phanerogams) the development of sporangia, speaking generally, is confined to the leaves. In most ferns the sporangiferous leaves (sporophylls) do not differ in appearance from the foliage leaves; but in other Pteridophyta (Equisetaceae, Marsiliaceae, some species of Lycopodium and Selaginella) they present considerable adaptation, and notably in the Phanerogams. In the Phanerogams the specialization is so great that the sporophylls have received special names; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the megasporangia (ovules) are termed the carpels. The sporophylls are usually aggregated together on a short stem, forming a shoot that constitutes a flower.
Many terms are employed to indicate the nature of the various kinds of spores, especially among the fungi, but the endless varieties of asexual (and asexually produced) reproductive cells may be grouped under two heads (i) Gonidia, (2) Spores proper.
The distinction between these two kinds of asexual reproductive cells is as fellows.
The gonidium is a reproductive cell that gives rise, on germination, to an organism resembling the parent. For instance, among the algae, the " zoospore " of Vaucheria develops into a Vaucheria-plant. There is thus a close connexion between vegetative multiplication and multiplication by means of gonida. The production of gonida is entirely limited to the Thallophyta, and is especially marked in the fungi, though the nature of all the many kinds of reproductive cells formed in this group has not yet been fully investigated. It is, however, wanting in certain algae (Conjugatae, Fucaceae, Characeae) and fungi (some Peronosporeae and Ascomycetes).
The spore proper is a reproductive cell that as a rule gives rise, on germination, to an organism unlike that which produced it. For instance, the spore of a fern when it germinates gives rise, not to a fern-plant, but to a prothallium. The apparent exceptions to this rule occur only among the Thallophyta, and are explained below in the section on Life-history.
The true spore is developed, usually in a sporangium, after a process of division which presents certain features that call for special notice.
Observation of the process of division of the nucleus ( karyokinesis) in plants generally has shown (for details see CYTOLOGY) that the linin-reticulum of the resting nucleus breaks up into a definite number of segments, the chromosomes, each of which bears a series of minute bodies, the chromalin-disks or chromameres, consisting largely of a substance termed chromatin. In the ordinary homotype divisions of the nuclei the characteristic number of chromosomes is always observable: but when the spore-mother-cells are being formed the number of chromosomes is reduced to one-half. This, if the number of chromosomes of the parent plant be expressed as 2X the number in the spore will be x. To take a concrete case: it has been observed by Guignard and others that in the early divisions taking place in the developing anther and ovule of the lily the number of chromosomes is 24; whereas in the later divisions which give rise to the pollen-mother-cells in the one case and to the mothercell of the embryo-sac in the other, the number of chromosomes is only 12. Thus the development of a spore (as distinguished from a gonidium) is always preceded by a reducing- or heterotypedivision, a process now more generally termed meiosis (Farmer). The reduced number of chromosomes in the nucleus of the spore-mother-cell persists in the spore, and in all the cells of the organism to which the spore may give rise. (Meiosis is discussed below in the section on Sexual Reproduction.)
It should be explained that cells, to which the name " spore " has also been applied, are formed as the result of a sexual act: such are zygospores, oospores, and some carpospores. But these cells differ from spores proper not only in their mode of origin but also in that their nuclei contain the full double number (2*) of chromosomes; hence they may be distinguished as diplospores.
Sexual Reproduction. Sexual reproduction involves the development of sexual organs (gametangia) and sexual cells (gametes). When the organism is unicellular, as in the lower Green Algae (e.g. Protococcaceae, Conjugatae), the cell becomes a sexual organ and its whole protoplasm gives rise to one or more sexual cells: in the higher forms certain parts of the body are specialized as sexual organs. In many of the lower plants the organs present no external distinction of sex (e.g. lower Green Algae: the Chytridiaceae, Mucorinae, and some Ascomycetes among the fungi): it is impossible to distinguish between the male and female organs, although it cannot be doubted that the essential physiological difference exists; consequently the organs are merely described as gametangia. The gap between these plants and those with differentiated sexual organs is, however, bridged over by intermediate forms, as explained in the article ALGAE.
When the sexual organs are more or less obviously differentiated into male and female, they present considerable variety of form in different groups of plants, and accordingly bear different names. Thus the male organ is a pollinodium in most of the fungi, a spermogonium in others (certain Ascomycetes, Uredineae) ; in all other plants it is an antheridium. Similarly the female organ is an oogonium in various Thallophyta (Green and Brown Algae: Oomycetous Fungi); a procarp in the Red Algae; an archicarp in certain Ascomycetous Fungi and in the Uredineae; an archegonium in all the higher plants.
It is generally the case that the protoplasm of the sexual organ is differentiated into one or more sexual cells. Thus the gametangium usually gives rise to cells which, as they are externally similar, are termed isogametes or simply gametes. Certain forms of the male organ, the spermogonium and the antheridium, give rise to male cells which are termed spermatia when they are nonciliate, spermatozoids when they are ciliated and free-swimming. Again, the female organs termed oogonia and archegonia produce one or more female cells called oospheres. But there are important exceptions to this rule. Thus the protoplasm is not differentiated into cells in the gametangium of the Mucorinae; in the male organ (pollinodium), of fungi generally; and in the female organ (procarp) of the Red Algae and (archicarp) of the Ascomycetes and Uredineae.
The immediate product of the fusion of cells, or of undifferentiated protoplasm, derived from sexual organs of opposite sex may be generally termed the zygote; but it is not always of the same kind. Thus when two isogametes, or the undifferentiated contents of two gametangia, fuse together, the process is designated conjugation, and the product is usually a single cell termed zygospore. When an oosphere fuses with a male cell, or with the undifferentiated contents of a male organ, the process is fertilization, and the product is a single cell termed oospore. When, finally, a female organ with undifferentiated contents receives a male cell, the process again is fertilization; here the product is not a single cell, but a fructification termed cystocarp (Red Algae), or ascocarp (Ascomycetes) or aecidium (Uredineae), containing many spores (carpospores).
As a consequence of the diversity in the sexual organs and cells, in the details of the sexual act, and in the product of it, several modes of the sexual process have to be distinguished, which may be conveniently summarized as follows:
I. Isogamy: the sexual process consists in the fusion of either two similar sexual cells (isogametes) , or two similar sexual organs (gametangia) : it is termed conjugation, and the product is a zygospore. Its varieties are:
(a) Gametes ciliated and free-swimming (planogametes), set free into the water where they meet and fuse: lower Green Algae (Protococcaceae, Pandorineae, most Siphonaceae and Confervaceae) ; some Brown Algae ( Phaeosporeae) :
(b) Gametangia fuse in pairs, and a gamete is differentiated in each: the gametes of each pair fuse, but are not set free and are not ciliated (the Conjugate Green Algae) : or, no gametes are differentiated, the undifferentiated contents of the gametangia fusing (Mucorinae among the Fungi).
II. Ooeamy: male and female organs distinct: the protoplasm of the female organ is differentiated into one or (rarely) more oospheres which usually remain enclosed in the female organ: the contents of the male organ are usually differentiated into one or more male cells: the process is fertilization, the product is an oospore.
(A) The sexual organs arc unicellular (or coenocytic as in certain Siphonaceous Green Algae and in the Oomycetous Fungi) ; the female organ is an oogonium.
(a) The male organ is an antheridium giving rise to one or more free-swimming ciliated spermatozoids :
(1) The oogonium contains a single oosphere which is fertilized in situ: higher Green Algae (Volvox, Vaucheria, Oedogonium, Coleochaete, Characeae) ; some Brown Algae (Tilopteris); among the Fungi, Monoblepharis, the only fungus known to have spermatozoids :
(2) The oogonium produces a single oosphere which is extruded and is fertilized in the water : Dictyota and some Fucaceae (Brown Algae) :
(3) The oogonium contains several oospheres which are fertilized in situ : Sphaeroplea (Siphonaceous Green Alga) :
(4) The oogonium produces more than one oosphere (2-8) which are extruded and are fertilized in the water: certain Brown Algae (Pelvetia, Ascophyllum, Fucus):
(0) The male organ is a pollinodium which applies itself closely to the oogonium: the amorphous male cell is not ciliated and is not set free:
(1) The oogonium contains a single oosphere which is fertilized in situ : Peronosporaceae (Oomycetes) :
(2) The oogonium contains several oospheres; Saprolegnia- ceae: but it is debated whether or not fertilization actually takes place.
(B) The male and female organs are (as a rule) multicellular; the male organ is an antheridium, the female an archegonium: the archegonium always contains a single oosphere which is fertilized in situ.
(a) The male cell is a free-swimming ciliated spermatozoid : the antheridium produces more than one (usually very many) spermatozoids, each of which is developed in a single cell: all Bryophyta (mosses, etc.) and Pteridophyta (ferns, etc.): the only Phanerogams in which spermatozoids have been observed are the gymnospermpus species Ginkgo biloba, Cycas revoluta, Zamia integrifolia.
03) The male cell is amorphous and passes directly from the pollen-tube into the oosphere (siphonogamy) : all Phanerogams except the species just mentioned.
It must be explained that in the angiospermous Phanerogams, the male and female organs are so reduced that each is represented by only a single cell: the male, by the generative cell, formed in the pollen-grain, which usually divides into two male cells: the female, by the oosphere. The gradual reduction can be traced through the Gymnosperms.
Attention may here be drawn to the fact (see ANGIOSPERMS) that, in several cases, the second male cell has been seen to enter the embryo-sac from the pollen-tube, and its nucleus to fuse with the definitive nucleus (endosperm-nucleus) or with one of the polar nuclei. The significance of this remarkable observation is discussed in the section on the Physiology of Reproduction.
III. Carpogamy: the sexual organs are (as a rule) differentiated into male and female: the protoplasm of the unicellular or multicellular female organ (archicarp, procarp) is never differentiated into an oosphere: in many cases definite male cells, spermatia, are produced and are set free, but they are not ciliated, and frequently have a cell-wall : the process is fertilization : the product is a fructification derived essentially from the female organ con- taining several (sometimes very many) spores (carpospores): characteristic of the Red Algae and of the Ascomycetous Fungi.
(A) There are definite male cells (spermatia) :
(a) The female organ is a procarp, consisting of an elongated, closed, receptive filament, the trichogyne, and of a basal fertile portion, the carpogonium: on fertilization the latter grows and gives rise directly or indirectly to a cystocarp: the spermatia are each formed in a unicellular antheridium and have no cell-wall at first : they fuse with the tip of the trichogyne : Red Algae (Rhodopnyceac or Florideae) :
03) The female organ (archicarp) resembles the preceding : in fertilization the fertile portion (ascogonium) develops into an ascocarp containing one or more asci (sporangia) each containing usually eight ascospores: the spermatia are formed by abstriction from the filaments (sterigmata) lining special receptacles, the spermogonia, which are the male organs: certain Ascomycetous Fungi (e.g. Laboulbeniaceae, some Lichen-Fungi, Polystigma). For the Uredineae, see Abnormalities of Reproduction, below).
(B) There are no definite male cells: the more or less distinct male and female organs come into contact, and their undifferentiated contents fuse: the product is an ascocarp:
(a) The male and female organs are obviously different : the female organ is an ascogonium, the male a pollinodium: e.g. Pyronema, Sphaerotheca (Ascomycetes) : 03) The male and female organs are quite similar : e.g. Eremas- cus, Dipodascus (Ascomycetes).
It may be explained that carpogamy is the expression of sexual degeneration. In the cases last mentioned, when the sexual organs are quite similar, they have reverted to the condition of gametangia. Still further reduction is observable in other Ascomycetes in which one of the sexual organs, presumably the male, is either much reduced or is altogether wanting. Again in the 'rusts (Uredineae), there are spermatia, but they are functionless (see section on Abnormalities of Reproduction). In the highest Fungi, the Autobasidiomycetes, no sexual organs have been discovered.
Details of the Sexual Act. It has been already stated that the sexual act consists in the fusion of two masses of protoplasm, commonly cells, derived from two organs of opposite sex:" but this is only the first stage in the process. The second stage is the fusion of the nuclei, which usually follows quickly upon the fusion of the cells; but nuclear fusion may be postponed so that the two sexual nuclei may be observed in the zygote, as " conjugate " nuclei, and even in the cells of the organism developed from the zygote (e.g. Uredineae). The result of nuclear fusion is that the nucleus of the zygote contains the double number of chromosomes that is, if the number of chromosomes in each of the fusing sexual nuclei be x, the number in the nucleus of the zygote will be 2X. Moreover, this double number persists in all the cells of the organism developed from the zygote, until it is reduced to one-half by meiosis preceding either the development of the spores, or, less commonly, the development of the sexual cells. But there is yet a third stage, which consists in the temporary fusion of the chromosomes belonging to the two sexual nuclei. This always takes place as a preliminary to meiosis; it may be in the germinating zygote, or after many generations of cells have been formed from it. At the onset of meiosis the (2*) chromosomes are seen to be double, one of each pair having been derived from the male and the female cell respectively: the chromosomes of each pair then fuse so that their chromomeres unite along their length, constituting the pseudo-chromosomes. The paired chromosomes separate and eventually go to form the two daughter-nuclei, one to each, which thus have half (x) the original number of chromosomes. The daughter-nuclei at once divide homotypically, retaining the reduced (x) number of chromosomes to form the four nuclei of a tetrad of spores (more rarely, e.g. Fucus, of sexual cells).
It will have been gathered from the foregoing sections that plants generally are capable of both sexual and asexual reproduction; and, further, that in different stages of their life-history they possess the diploid (2*) number of chromosomes in their nuclei, or the haploid (x) number. It may be at once stated that, in all plants in which sexual reproduction and true meiotic spore-formation exist, these two modes of reproduction are restricted to distinct forms of the plant; the sexual form bears only the sexual organs and is haploid; the asexual form only produces spores and is diploid. Hence all such plants are to this extent polymorphic that is, the plant assumes these two forms in the course of its life-history. When, as in many Thallophyta, one or other of these forms can reproduce itself by means of gonidia, additional forms may be introduced into the life; history, which becomes the more complicated the more pronounced the polymorphism.
The most straightforward life-histories are those presented by the Bryophyta and the Pteridophyta, where there are but the two forms, the sexual and the asexual. In the life-history of a moss, the plant itself bears only sexual organs: it is the sexual form, and is distinguished as the gametophyte. The zygote (oospore) formed in the sexual act develops into an organism, the sporogonium, which is entirely asexual, producing only spores: it is distinguished as the sporophyte. When these spores germinate, they give rise to moss-plants. Thus the two forms, the sexual and the asexual, regularly alternate with each other that is, the life-history presents that simple form of polymorphism which is known as alternation of generations. Similarly, in the life-history of a fern, there is a regular alternation of a sporophyte, which is the fern-plant itself, with a gametophyte, which is the fern-prothallium.
It is pointed out in the preceding section that, as the result of the sexual act, the nucleus of the zygote contains twice as many chromosomes as those of the fusing sexual cells. This 2X number of chromosomes persists throughout all the cellgenerations derived from the zygote, that is, in the cells constituting the sporophyte, up to the time that it begins to produce spores, when meiosis takes place. Again, the cellgenerations derived from the spore, that is, the cells constituting the gametophyte, all have the reduced x number of chromosomes in their nuclei up to the sexual act. Hence the sporophyte may also be designated the diplophyte and the gametophyte the haplophyte (Strasburger) : in other words, the sporophyte is the pre-meiotic, the gametophyte the post-meiotic generation. Twice in its life-history the plant is represented by a single cell: by the spore and by the zygote. The turningpoints in the life-history, the transitions from the one generation to the other, are (i) meiosis, (2) the sexual act.
The course of the life-history in Phanerogams and in those Thallophyta which have been adequately investigated is essentially the same as that of the Bryophyta and of the Pteridophyta as described above, though it is less easy to trace on account of the peculiar relation of the two generations to each other in the Phanerogams and on account of various irregularities that present themselves in the Thallophyta.
In the Phanerogams, as in the Pteridophyta, the preponderating generation is the sporophyte, the plant itself. Inasmuch as they are heterosporous, the gametophyte is represented by a male and a female organism or prothallium, both rudimentary. The male prothallium consists of the few cells formed by the germinating pollen-grain (microspore) ; and though it is quite independent, since the microspores are shed, it grows parasitically in the tissues upon which the microspore has been deposited in pollination. The female prothallium may consist of many cells with well-developed archegonia, as in the Gymnosperms, or of only a few cells with the female organ reduced to the oosphere, as in the Angiosperms. In either case it is the product of the germination of a megaspore (embryo-sac) which is not shed from its sporangium (ovule): hence it never becomes an independent plant, and was long regarded as merely a part of the sporophyte until its true nature was ascertained, chiefly by the researches of Hofmeister, who first explained the alternation of generations in plants. This intimate and persistent connexion between the two generations affords the explanation of the characteristic features of the Phanerogams, the seed and the flower. The ovule containing the embryo-sac, which eventually contains the embryo, persists as the seed a structure that is distinctive of Phanerogams, which have, in fact, on this account been also termed Spermatophyta. With regard to the flower, it has been already mentioned that it is, like the cone of an Equisetum or a Lyco- podium, a shoot adapted to the production of spores. But it is something more than this: for whereas in Equisetum or Lycopodium the function of the cone comes to an end when the spores are shed, the flower of the Phanerogam has still various functions to perform after the maturation of the spores. It is the seat of the process of pollination that is, the bringing of the pollen-grain by one of various agencies into such a position that a part (the pollen-tube) of the male prothallium developed from it may reach and fertilize the oosphere in the embryo-sac. Thus the flower of Phanerogams is a reproductive shoot adapted not only for spore-production, but also for pollination, for fertilization, and for the consequences of fertilization, the production of seed and fruit. However, in spite of these complications, it is possible to determine accurately the limits of the two generations by the observation of the nuclei. The meiosis preceding the formation of the spores marks the beginning of the (haploid) gametophyte, male and female; and the sexual act marks that of the (diploid) sporophyte.
The difficult task of elucidating the life-histories of the Thallophyta has been successfully performed in certain cases by the application of the method of chromosome-counting, with the result that alternation of generations has been found to be of general occurrence. To begin with the Algae. In the Dictyotaceae (Brown Algae) there are two very similar forms in the life-history, the one bearing asexual reproductive organs (tetrasporangia), the other bearing sexual organs (oogonia and antheridia). It has been shown (Lloyd Williams) that the former is undoubtedly the sporophyte and the latter the gametophyte, since the nuclei of the former contain 32 chromosomes, and those of the latter 16. Meiosis takes place in the mother-cell of the tetraspores, which, on germination, give rise to the sexual form. Quite a different life-history has been traced in Fucus, another Brown Alga. Here no spores are produced: there is but one form in the life-history, the Fucusplant, which bears sexual organs and has, on that account, been regarded as a gametophyte. The investigation of the nuclei has, however, shown (Farmer) that the Fwcws-plant is actually diploid, that it is, in fact, a sporophyte; but since there is no spore-formation, meiosis immediately precedes the development of the sexual cells, which alone represent the gametophyte (see below, Apospory).
Similarly, two types of life-history have been discovered in the Red Algae. In Polysiphonia violacea, a species in which the tetraspores and the sexual organs are borne by similar but distinct individuals, it has been ascertained (Yamanouchi) that, as in Dictyota, meiosis takes place in the mother-cell of the tetraspores, so that the nuclei of these spores, as also those of the sexual plants to which they give rise, contain 20 chromosomes: and further, that the nuclei of the carpospores ( diplospores) produced in the cystocarp as the result of fertilization, contain 40 chromosomes, as do also those of the asexual plant to which the carpospores give rise. Hence the sporophyte is represented by the cystocarp and the resulting tetrasporangiate plants: the gametophyte, by the sexual plants. Though it is the rule in the Red Algae that the tetrasporangia and the sexual organs are borne on distinct individuals, yet cases are known in which both kinds of reproductive organs are borne upon the same plant; and to those the above conclusions obviously cannot apply. They have yet to be investigated.
The second type of life-history has been traced in Nemalion. Here there is no tetrasporangiate form, consequently meiosis takes place at a different stage in the life-history. It has been observed (Wolfe) that the nuclei of the sexual plant contain 8 chromosomes; those of the gonimoblast-filaments of the developing cystocarp contain 16, whilst those of the carpospores contain 8: hence meiosis takes place in the carposporangia. Here the plant is the gametophyte; the sporophyte is only represented by the cystocarp. The carpospores here are true spores (haplospores).
Among the Green Algae, Coleochaete is the only form that has been fully investigated (Allen). Here meiosis takes place in the germinating oospore: consequently the plant is the gametophyte, and the sporophyte is represented only by the oospore, so that the life-history resembles that of Nemalion. It is probable that this conclusion is generally true of the whole group; at any rate of those forms (Desmids, Spirogyra, Oedogonium, Chara) which have been more or less investigated.
Turning to the Fungi, somewhat similar results have been obtained in the few forms that have been studied from this point of view. In the sexual Ascomycetes it appears (Harper) that meiosis takes place in the ascocarp just before the development of the spores, so that the life-history essentially resembles that of Nemalion. Again, in certain Uredineae, having an aecidium-stage and a teleutospore-stage, which is apparently a sexual process has been observed (Blackman, Christman) which is described in the section on Abnormalities of Reproduction, and the life-history is as follows. The sexual act having taken place, a row of aecidiospores is developed in the aecidium, each of which contains two conjugate nuclei derived from the sexual nuclei. The mycelium developed from the aecidiospore, as well as the uredospores and the teleutospores that it bears, shows two conjugate nuclei. When, however, the teleutospore is about to germinate, the two nuclei fuse (thus completing the sexual act) and meiosis takes place. As a result the promycelium developed from the teleutospore, and the sporidia that it produces, are uninucleate: so are also the mycelium developed from the sporidium, and the female organs (archicarps) borne upon it. Hence the limits of the sporophyte are the aecidiospore and the teleutospore: those of the gametophyte, the teleutospore and the aecidiospore.
Similar observations have been made upon other Uredineae with a more contracted life-history. Phragmidium Potentillaecanadensis is a rust that has no aecidium-stage: consequently the primary uredospores are borne by the mycelium produced on infection of the host by a sporidium. It has been observed (Christman) that the sporogenous hyphae fuse in pairs, suggesting a sexual act; then the primary uredospores are developed in rows from the fused pairs of hyphae which thus behave as sexual organs (archicarps), and each such uredospore contains two conjugate nuclei. Although the research has not been carried beyond this point, it may be inferred that in this case, as in the preceding, nuclear fusion and meiosis take place in the teleutospore. Here the sporophyte is represented by the uredo-form.
Finally, in some of the fungi in which no sexual organs have yet been discovered, this method of investigation has made it probable that some kind of sexual act takes place nevertheless. Thus in the Uredine Puccinia malvacearum, which has only teleutospore- and sporidium-stages, it has been observed ( Blackman) that the formation of the teleutospores is preceded by a binucleate condition of the hyphae. The same idea is suggested by the binucleate basidia of the Basidiomycetes, which correspond to the teleutospores of the Uredineae.
The life-histories sketched in the preceding paragraphs show that one of the complexities met with in the Thallophyta is that meiosis does not always take place at the same point in the life-history. In the higher plants the incidence of meiosis is generally, though not absolutely, constant: it may be stated as a rule that in the Bryophyta, Pteridophyta and Phanerogams it takes place in the spore-mother-cells. In the Thallophyta this rule does not hold. In some of them, it is true, meiosis immediately precedes, as in the higher plants, the formation of certain spores, the tetraspores (Dictyotaceae, Polysiphonia), the teleutospores (Uredineae): but in others it immediately precedes the development of the sexual organs (Fucaceae), or follows more or less directly upon the sexual act (Green Algae, Nemalion, Ascomycetes).
The life-history of most Thallophyta is further complicated by the capacity of the gametophyte of the sporophyte to reproduce themselves by cells termed gonidia, a capacity that is wholly lacking in the higher plants. The karyology of gonidia has not yet been sufficiently investigated: but when, as in the Green Algae and the Oomycetous Fungi, the gonidia are developed by and reproduce the gametophyte, it may be inferred that they, like the gametophyte, are haploid. One case, at any rate, of the reproduction of the sporophyte by gonidia is fully known, that of the Uredineae just described, in which the uredoform, which is a phase of the sporophyte, is reproduced by the uredo-spores which are binucleate, that is diploid, and may be distinguished as dlplogonidia. In any case the result is that whereas in the higher plants each of the alternating generations occurs but once in the life-history, in these Thallophyta the lifehistory may include a succession of gametophytic or of sporophytic forms This is, in fact, a distinguishing feature of the group. The higher plants present a regular alternation of generations: whereas, in the Thallophyta, though they probably all present some kind of alternation of generations, yet it is irregular hi the various ways and for the various reasons mentioned above.
Sufficient information has been given in the preceding pages to render possible the consideration of the origin of alternation of generations. To begin quite at the beginning, it may be assumed that the primitive form of reproduction was purely vegetative, merely division of the unicellular organism when it had attained the limits of its own growth. Following on this came reproduction by a gonidium: that is, the protoplasm of the cell, at the end of its vegetative life, became quiescent, surrounded itself with a proper wall, or was set free as a motile ciliated cell, having in some unexplained way become capable of originating a new course of life (rejuvenescence) on germination. Then, as can be well traced in the Brown and" Green Algae (see ALGAE), these primitive reproductive cells (gonidia) began to fuse in pairs: in other words, they gradually became sexual. This stage can still be observed in some of these Algae (e.g. Ulothrix, Ectocarpus) where the zoospores (gonidia) may either germinate independently, or fuse in pairs to form a zygote. Gradually the sexuality of these cells became more pronounced : losing the capacity for independent germination, they acquired the external characters of more or less differentiated sexual cells, and the gametangia producing them developed into male and female sexual organs. But this advancing sexual differentiation did not necessarily deprive the plant of the primitive mode of propagation: the sexual organism still retained the faculty of reproduction by gonidia. The loss- of this faculty only came with higher development: it is entirely wanting in some of the higher Thallophyta (e.g. Fucaceae, Characeae), and hi all plants above them in the evolutionary series.
With the introduction of the sexual act, a new kind of reproductive cell made its appearance, the zygote. This cell, as already explained, differs from other kinds of spores and from the sexual cells, in that its necleus is diploid; and with it the sporophyte (diplophyte) was introduced into the life-history. It has been mentioned that in some plants (e.g. Green Algae) the zygote is all that there is to represent the sporophyte, giving rise, or germination and after meiosis, to one or more spores. Passing to the Bryophyta, in the simpler forms (e.g. Riccia), the zygote develops into a multicellular capsule (sporogonium) ; and in the higher forms into a more elaborate sporogonium, producing many spores. In the Pteridophyta and the Phanerogams, the zygote gives rise to the highly developed sporophytic plant.
Thus the evolution of the sporophyte can be traced from the unicellular zygote, gradually increasing in bulk and in independence until it becomes the equal of the gametophyte (e.g. in Dictyota and Polysiphonia), and eventually far surpasses it (Pteridophyta, Phanerogams) . Moreover, the increase in size was attended by the gradual limitation of spore-production to certain parts only, the rest of the tissues being vegetative, assuming the form of stems, leaves, etc. These facts have been formulated in the theory of " progressive sterilization " (Bower), which states that the sporophytic form of the higher plants has been evolved from the simple, entirely fertile, sporophyte of the lower, by the gradually increasing development of the sterile vegetative tissue at the expense of the sporogenous, accompanied by increase in total bulk and in morphological and histological differentiation.
In connexion with the study of the evolution of the sporophyte.
the question arose as to its morphological significance; whether it is to be regarded as a modified form of the gametophyte, or as an altogether new form intercalated in the life-history: in other words, whether the alternation is " homologous "or" antithetic." In certain plants there is a succession of forms which are undoubtedly homologous: for instance, in Coleochaete where a succession of individuals without sexual organs is produced by zoospores (gonidia). The main fact that has been established is that the sporophyte, from the simple zygote of the Thallophyta to the spore-bearing plant of the Phanerogams, is characterized by its diploid nuclei; that it is a diplophyte, in contrast to the haplophytic gametophyte. Were these nuclear characters absolutely universal, there could be no question but that the sporophyte is an altogether new antithetic form, and not an homologous generation. But certain exceptions to the rule have been detected, which are described under Abnormalities of Reproduction: at present it will suffice to say that such things as a diploid gametophyte and a haploid sporophyte have been observed in certain ferns. It can only be inferred that alternation of generations is not absolutely dependent upon the periodic halving in meiosis and the subsequent doubling by a sexual act, of the number of chromosomes in the nuclei, though the two sets of phenomena usually coincide. It must not, however, be overlooked that these exceptional cases occur in plants presenting an abnormal life-history: the fact remains that where there is both normal spore-formation with meiosis, and a subsequent sexual act, the haploid form is the gametophyte, the diploid the sporophyte. But the actual observation of a haploid sporophyte and of a diploid gametophyte makes it clear that however generally useful the nuclear characters may be in the distinction of sporophyte and gametophyte, they do not afford an absolute criterion, and therefore their value in determining homologies is debatable.
IV. Abnormalities of Reproduction.
In what may be regarded as the type of normal life-history, the transition from the one generation to the other is marked by definite processes: there is the meiotic development of spores by the sporophyte, and the sexual production of a zygote, or something analogous to it, by the gametophyte. But it has been mentioned in the preceding pages that the transition may, in certain cases, be effected in other ways, which may be regarded as abnormal, though they are constant enough in the plants in which they occur, in fact as manifestations of reproductive degeneration.
In the first place, the sporophyte may be developed either after an abnormal sexual act, or without any preceding sexual act at all, a condition known as apogamy. In the second, the gametophyte may be developed otherwise than from a postmeiotic spore, a condition known as apospory. APOGAMY. The cases to be considered under this head may be arranged in two groups:
" i. Pseudapogamy: sexual act abnormal. The following abnormalities have been observed :
(a) Fusion of two female organs: observed (Christman) in certain Uredineae (Caeoma nitens, Phragmidium speciosum, Uromyces Caladii) where adjacent archicarps fuse: male cells (spermatia) are present but functionless. (ft) Fusion between nuclei of the same female organ : observed in the ascogonium of certain Ascomycetes, Humaria granulata (Blackman), where there is no male organ; Lachnea stercorea (Fraser), where the male organ (pollinodium) is present but is apparently functionless.
(c) Fusion of a female organ with an adjacent tissue-cell: ob- served (Blackman) in the archicarp of some Uredineae (Phragmidium violaceum, Uromyces Poae, Puccinia Poarum) : male cells (spermatia) present but functionless.
(d) There is no female organ: fusion takes place between two adjacent tissue-cells of the gametophyte; the sporophyte is developed from diploid cells thus produced, but there is no proper zygote as there is in a, 5 and c : observed (Farmer) in the prothallium of certain ferns (Lastraea pseudo-mas, var. polydactyla): male organs (and sometimes female) present but functionless. Another such case is that_ of Humaria rutilans (Ascomycete), in which nuclear fusion has been observed (Fraser) in hyphae of the hypothecium : the asci are developed from these hyphae, and in them meiosis takes place ; there are no sexual organs.
2. Eu-apogamy: no kind of sexual act (a) The gametophyte is haploid :
(o) The sporophyte is developed from the unfertilized oosphere: no such case of true parthenogenesis has yet been observed.
03) The sporophyte is developed vegetatively from the gametophyte and is haploid : observed in the prothallia of certain ferns, Lastraea pseudo-mas, var. cristata-apospora(Farmer and Digby), and Nephrodium molle (Yamanouchi). (6) The gametophyte is diploid (see under Apospory):
(a) The sporophyte is developed from the diploid oosphere : observed in some Pteridophyta, viz. certain ferns (Farmer), Athyrium Filix-foemina, var. clarissima, Scolopendrium vulgare, var. crispum-Drummondae, and Marsilia (Strasburger) ; also in some Phanerogams, viz. Compositae (Taraxacum, Murbeck; Antennaria alpina, Juel; sp. of Hieracium (Rosenberg): Rosaceae ( EuAlchemilla sp., Murbeck, Strasburger): Ranunculaceae (Thalictrum purpurascens, Overton).
09) The sporophyte is developed vegetatively from the gametophyte: observed (Farmer) in the fern Athyrium Filixfoemina, var. clarissima.
In all the cases enumerated under Eu-apogamy, apogamy is associated with some form of apospory except Nephrodium molle, full details of which have not yet been published.
Many other ferns are known to be apogamous, but they are not included here because the details of their nuclear structure have not been investigated. APOSPORY. The known modes of apospory may be arranged as follows:
1. Pseudapospory: a spore is formed but without meiosis, so thai it is diploid- observed only in heterosporous plants, viz. certain species of Marsilia (e.g. Marsilia Drummondii) where the megaspore has a diploid nucleus (32 chromosomes) and the resulting prothallium and female organs are also diploid (Strasburger); and in various Phanerogams, some Compositae (Taraxacum and Antennaria alpina, Juel), some Rosaceae (Eu-Alchemilla, Strasburger), and occasionally in Thalictrum purpurascens (Overton), where the megaspore ( embryosac) is diploid; in some species of Hieracium it has been found (Rosenberg) that adventitious diploid embryo-sacs are developed in the nucellus: these plants are also apogamous.
2. Eu-apospory: no spore is formed of this there are two varieties :
(a) With meiosis: this occurs in some Thallophyta which form no spores; the sporophyte of the Fucaceae bears no spores, consequently meiosis takes place in the developing sexual organs; the Conjugate Green Algae also have no spores, meiosis taking place in the germinating zygospore . which develops directly into the sexual plant.
(6) Without meiosis: the gametophyte is developed upon the sporophyte by budding; that is, spore-reproduction is replaced by a vegetative process: for instance, in mosses it has been found possible to induce the development of protonema, the first stage of the gametophyte, from tissuecells of the sporogonium: similarly, in certain ferns (varieties of Athyrium Filix-foemina, Scolopendrium vulgare, Lastraea pseudo-mas, Polystichum angulare, and in the species Pteris aquilina and Asplenium dimorphum), the gametophyte (prothallium) is developed by budding on the leaf of the sporophyte, and in some of these cases it has been ascertained that the gametophyte so developed has the same number (2x) of chromosomes in its nuclei as the sporophyte that bears it that is, it is diploid.
Apospory has been found to be frequently associated with apogamy; in fact, in the absence of meiosis, this association would appear to be inevitable.
Combined Apospory and Apogamy. Instances have been given of the occurrence of both apospory and apogamy in the same life-history; but in all of them there is a regular succession of sporophyte and gametophyte. The cases now to be considered are those in which one or other of the generations gives rise directly to its like, sporophyte to sporophyte, gametophyte to gametophyte, the normally intervening generation being omitted.
It is possible to conceive of this abbreviation of the life-history taking place in various ways. Thus, a sporophyte might be developed from a haploid spore instead of a gametophyte as is the normal case, but this has not been observed: again, a sporophyte might be developed from a diploid spore (as distinguished from a zygote or a diploid oosphere), a possibility that is to some extent realized in the life-history of some Uredineae in which successive forms of the polymorphic sporophyte are developed from diplogonidia. Similarly a gametophyte might be developed from a fertilized or an unfertilized female cell: the latter possibility is to some extent realized in those Algae (e.g. Ulothrix, Ectocarpus) in which the sexual cells (isogametes), if they fail to conjugate, germinate independently as gonidia, giving rise to gametophytes.
The more familiar mode is that of vegetative budding, as already mentioned. When a " viviparous " fern or Phanerogam reproduces itself by a bud or a bulbil, both spore-formation and the sexual act are passed over: sporophyte springs from sporophyte. Remarkable cases of this have been observed in certain Phanerogams (Coelebogyne ilicifolia, Funkia ovata, Nothoscordum fragrans, Citrus, sp. of Euonymus, Opuntia vulgaris) in the ovule of which adventitious embryos are formed by budding from cells of the nucellus: with the exception of Coelebogyne, it appears that this only takes place after the oosphere has been fertilized. In other plants it is the gametophyte that reproduces itself by means of gemmae or bulbils, as commonly in the Bryophyta, the prothallia of ferns, etc.
The abnormalities described are all traceable to reproductive degeneration; the final result of which is that true reproduction is replaced more or less completely by vegetative propagation. It may be inquired whether degeneration may have proceeded so far in any plant of sufficiently high organization to present spore-formation, or sexual reproduction, or both, as to cause the plant to reproduce itself entirely and exclusively by the vegetative method. The only such case that suggests itself is that of Caulerpa and possibly some other Siphonaceous Green Algae. In this plant no special reproductive organs have yet been discovered, and it certainly reproduces itself by the breaking off of portions of the body which become complete plants: but it is quite possible that reproductive organs may yet be discovered.
V. Physiology of Reproduction.
The reproductive capacity of plants, as of animals, depends upon the fact that the whole or part of the protoplasm of the individual can develop into one or more new organisms in one or other of several possible ways. Thus, in the case of unicellular plants, the whole of the protoplasm of the parent gives rise, whether by simple division or otherwise, to one or more new plants. Reproduction necessarily closes the life of the individual : here, as August Weismann long ago pointed out, there is no natural death, for the whole of the protoplasm of the parent continues to live in the progeny. In multicellular plants, on the contrary,' the reproductive function is mainly discharged by certain parts of the body, the reproductive organs, the remainder of the body being essentially vegetative that is, concerned with the maintenance of the individual. In these plants it is only a part of the protoplasm that continues to live in their progeny; the remainder, the vegetative part, eventually dies. It is therefore possible to distinguish in them, on the one hand, the essentially reproductive protoplasm, which may be designated by Weismann's term germ-plasm, though without necessarily adopting all that his use of it implies, and the essentially vegetative, mortal protoplasm, the somato -plasm, on the other. In the unicellular plant no such distinction can be drawn, for the whole of the protoplasm is concerned in reproduction. But even in the most highly organized multicellular plant this distinction is not absolute: for, as already explained, plants can, in general, be propagated by the isolation of almost any part of the body, that is vegetatively, and this implies the presence of germ-plasm elsewhere than in the special reproductive organs.
If the attempt be made to distinguish between the organs of vegetative propagation and those of true reproduction, the nearest approach would be the statement that the former contain both germ-plasm and somatoplasm, whereas the latter, or at least the reproductive cells, consist entirely of germ-plasm.
The question now arises as to the exact seat of the germ-plasm, and the answer is to be looked for in the results of the numerous researches into the structure and development of the reproductive cells that form so large a part of the biological work of recent years. The various facts already mentioned suffice to prove that the nucleus plays the leading part in the reproductive processes of whatever kind: the general conclusion is justified that no reproductive cell can develop into a new organism if deprived of its nucleus. It may be inferred that the nucleus either actually contains the germ-plasm, or that it controls and directs the activities of the germ-plasm present in the cell. It is not improbable that both these inferences may be true. At any rate there is no sufficient ground for excluding the cooperation of the cytoplasm, especially of that part of it distinguished as kinoplasm, in the reproductive processes.
Pursuing the ascertained facts with regard to the nucleus, it is established that the part of it especially concerned is the linin-network which consists of the chromosomes. The behaviour, as already described, of the chromosomes in the various reproductive processes has led to the conclusion that the hereditary characters of the parent or parents are transmitted in and by them to the progeny: that they constitute, in fact, the material basis of heredity (see HEREDITY). They can hardly, however, be regarded as the ultimate structural units, for the simple reason that their number is far too small in relation to the transmissible characters. It has been suggested (Farmer) that the chromomeres are the units, but the number of these would seem to be hardly sufficient. It seems necessary to fall back upon hypothetical ultimate particles, as suggested by Darwin, de Vries and Weismann, which may be generally termed pangens. The chromomeres may be regarded as aggregates of such particles, the " ids " of Weismann.
The foregoing considerations make it possible to attempt an explanation of the various reproductive processes.
Vegetative Propagation. It is easily intelligible that the two individuals produced by the division of a unicellular plant should resemble the parent and each other; for, the division of the parent-nucleus being homotypic, the chromosomes which go to constitute the nucleus of each daughter-cell are alike both in number and in nature, and exactly repeat the constitution of the parent-nucleus.
In the more complicated cases of propagation by bulbils, cuttings, etc., the development of the new individual, or of the missing parts of the individual (roots, etc.), may be ascribed to the presence in the bulbil or cutting of the necessary pangens.
Reproduction by Gonidia. In this case a single cell gives rise to a complete new organism resembling the parent. The inference is that the gonidium is a portion of the parental germplasm, in which all the necessary pangens have been accumulated.
Reproduction by Spores. In this case, also, an entire organism is developed from a single cell, but with this peculiarity that the resulting organism is unlike that which bore the spore, a peculiarity which has not yet been explained. It has been already stated that the development of true spores involves meiosis, and this process is no doubt related to the behaviour of the spore on germination; but the nature of this relation remains obscure. It might be assumed that, as the result of meiosis, the nucleus of the spore receives only gametophytic pangens. But the assumption is rendered impossible by the fact that the spore gives rise to a sexual organism, the reproductive cells of which, after the sexual act, produce a sporophyte. Clearly sporophytic pangens must be present as well in the spore as in the gametophyte and in its sexual cells. It can only be surmised that they exist there in a latent condition, dominated, as it were, by the gametophytic pangens.
Sexual Reproduction. Here, again, as yet unanswered questions present themselves. The essence of a sexual cell is that it cannot give rise by itself to a new organism, it is only truly reproductive after the sexual act: this peculiarity is just what constitutes its sexuality. Minute investigation has not yet detected any essential structural difference between a sexual cell and a spore; on the contrary, the results so far obtained have established that they essentially agree in being post-meiotic (haploid). Why then do they differ so fundamentally in their reproductive capacities? Again, sexual cells differ in sex; but there are as yet no facts to demonstrate any essential structural difference between male and female cells. What is known about them tends to prove their structural similarity rather than their difference. But it is possible that their difference may be chemical, and so not to be detected by the microscope.
The normal sexual act has been described as consisting in the fusion, first, of two cells, then of their nuclei, and finally, often after a long interval, of their chromosomes and of their chromomeres in meiosis. What causes determined these fusions is a question that is only partly answered. It is known in certain cases (e.g. ferns and mosses) that the male cell is attracted to the female by chemical substances secreted for the purpose by the female organ; that it is a case of chemiotaxis. Probably this is more common than experiment has yet shown it to be. It is quite conceivable that the consequent cell-fusion, as also the subsequent fusions of nuclei and of chromosomes, are likewise cases of chemiotaxis, depending upon chemical differences between the fusing structures.
The sexual process can only take place between cells which are related to each other in a certain degree (see HYBRIDISM); that is, it depends upon sexual affinity. It is the general rule that it takes place between cells derived from different individuals of the same species; that is, cross-fertilization is the rule. This is necessarily the case when the male and female organs are developed upon different individuals, when the plant is said to be dioecious. When both kinds of organs are developed upon the same individual (monoecious), self- fertilization may and often does occur; but it is commonly hindered by various special arrangements, of which dichogamy is the most common; that is, that the male and female organs are not mature at the same time. But though these arrangements favour cross-fertilization, they do not absolutely prevent selffertilization. In some cases, cleistogamic flowers, for instance, self-fertilization alone is possible (see ANGIOSPERMS). The general conclusion is that though cross-fertilization is the more advantageous form of sexual reproduction, still self-fertilization is more advantageous to the species than no fertilization at all.
In considering this subject, it must be borne in mind that the terms used have different meanings when applied to certain heterosporous plants from those which they convey when applied to isosporus plants. In the latter cases their meaning is direct and simple: in the former it is indirect and somewhat complicated. In heterosporous plants generally the actual sexual organs are never borne upon the same individual, there is always necessarily a male and a female gametophyte; so that, strictly speaking, self-fertilization is impossible. But in the Phanerogams, where there is a process preliminary to fertilization, that of pollination, which is unknown in other plants, the terms and the conceptions expressed by them are applied, not to the real sexual organs, but to the spores. Thus a dioecious Phanerogam is one in which the microspores are developed by one individual, the megaspores by another; and again, self-fertilization is said to occur when the microspores (pollen) fall upon the stigma of the same flower (see ANGIOSPERMS); but this is really only self-pollination.
To return to the sexual process itself. Whatever its nature, two sets of results follow upon the sexual act (i) a zygote is formed, which is capable of developing into a new organism, from two cells, neither of which could so develop; (2) the hereditary sporophytic characters of the two parents are possessed by the organism so developed. These two results will now be considered in some detail.
(i) The Relation between the Sexual Act and Reproductive Capacity. In the early days of the discovery of the sexual process, it was thought that the capacity for development imparted to the female cell was to be attributed to the doubling of its nuclear substance by the fusion with the male cell. Reproductive capacity does not, however, depend upon the bulk of the nuclear' substance, for a spore, like an unfertilized female cell, contains but the x number of chromosomes, and yet it can give rise to a new organism. Again, it has been observed (Winkler) that a non-nucleated fragment of an oosphere of Cystoseira (Fucaceae) can be " fertilized " by a spermatozoid and will then grow and divide to form a small embryo, though it necessarily contains only the x number of chromosomes. From this it would appear that some stimulating influence had been exerted by the male cell, and it is probably in this direction that the desired explanation is to be sought. Some important confirmatory facts have been recorded with regard to certain animals (sea-urchins). It has been observed (Loeb) that treatment with magnesium chloride will cause the ova to grow and segment; and similar results have been obtained (Winkler) by treating the ova with a watery extract of the male cells. Hence it may be inferred that the male cell carries with it, either in its cytoplasm (kinoplasm), or in its nucleus, extractable substances, perhaps of the nature of enzymes, that stimulate the female cell to growth.
It may be mentioned that the stimulating effect of fertilization is not necessarily confined to the female cell; very frequently adjacent tissues are stimulated to growth and structural change. In a Phanerogam, for instance, the whole ovule grows and develops into the seed: the development of endosperm in the embryo-sac is initiated by another nuclear fusion, taking place between the second male nucleus and the endosperm-nucleus: the ovary, too, grows to form the fruit, which may be dry and hard or more or less succulent: the stimulating effect may extend to other parts of the flower; to the perianth, as in the mulberry; to the receptacle, as in the strawberry and the apple: or even beyond the flower to the axis of the inflorescence, as in the fig and the pine-apple. Analogous developments in other groups are the calyptra of the Bryophyta, the cystocarps of the Red Algae, the ascocarps of the Ascomycetes, the aecidia of the Uredineae, etc.
(2) The Relation o] the Sexual Act to Heredity. The product of the sexual act is essentially a diploid cell, the zygote, which actually is or gives rise to a sporophyte. The sexual heredity of plants consequently presents the peculiar feature that the organism resulting^ from the sexual act is quite unlike its immediate parents, which are both gametophytes. But it is clear that the sporophytic characters must have persisted, though in a latent condition, through the gametophyte, to manifest themselves in the organism developed from the zygote.
The real question at issue is as to the exact means by which these characters are transmitted and combined in the sexual act. There is a considerable amount of evidence that the hereditary characters are associated with the chromomeres, and that it is rather their linin-constituent than their chromatin which is functional (Strasburger) : that they constitute, in fact, the material basis of heredity. From this point of view it is probable that the last phase of the sexual act, the fusion of the chromomeres in meiosis, represents the combination of the two sets of parental characters. What exactly happens in the pseudo-chromosome stage is not known; at any rate this stage offers an opportunity for a complete redistribution of the substance of the chromomeres in other words, of the parental pangens. It is a striking fact that, in the subsequent nuclear division, the distribution of the chromosomes derived from the male and female parents (when they can be distinguished) seems to be a matter of indifference: they are not equally distributed to the two daughter-nuclei. The explanation would appear to be this, that they are not any longer male and female as they were before meiotic fusion; and that it is because they now contain both male and female nuclear substance that their equal distribution to the daughter-nuclei is unimportant.
The nature of this redistribution of the substance of the chromomeres is still under discussion. Some regard it as essentially a chemical process, resulting in the formation of new compounds: others consider it to be rather a physical process, a new material system being formed in the rearrangement of the pangens; here it must be left for the present.
The various ways in which the parental characters manifest themselves in the progeny are fully dealt with in the articles HEREDITY, HYBRIDISM, MENDELISM. It will suffice to say that the progeny, though maintaining generally the characters of the species, do not necessarily exactly resemble either of the parents, nor do they necessarily present exactly intermediate characters:
they may vary more or less from the type. It is an interesting fact, the full significance of which has not yet been worked out, that, as a rule, plants that vary profusely are those in which the characteristic 2x number of chromosomes is high (60-100).
Brief reference may be made to the cases of abnormal sexual or pseudo-sexual reproduction described above under Apogamy. Taking first the cases of true apogamy, there is clearly no need for any sexual process, for, since no meiotic division has taken place, the gametophyte is diploid; its cells, whether vegetative or contained in female organs, possess the capacity for both development and the transmission of the sporophytic characters. It is not remarkable that such a gametophyte should be able to give rise directly to a sporophyte; but it is remarkable, in the converse case of apospory, that a sporophyte should give rise to a diploid gametophyte rather than to another sporophyte. In the latter case the tendency to the regular development of the alternate form appears to override the influence of the diploid nucleus.
Turning to the various forms of pseudo-apogamy, there are first those in which fusion takes place between two apparently female organs (some Uredineae; Christman), and those in which it takes place between nuclei within the same female organ (Humaria; Blackman). If these are to be regarded physiologically as sexual acts, it must be inferred that the fusing organs or nuclei have come to differ from each other to some extent; for it is unthinkable that equivalent female organs or cells should be able to fertilize, or to be fertilized by, one another. There are finally those cases in which apparently vegetative cells take part in the sexual act, as in Phragmidium (Blackman), where the female organ fuses with an adjacent vegetative cell, and in the fern-prothallium (Farmer), where the nuclei of two vegetative cells fuse. They would seem to indicate that vegetative cells may, in certain circumstances, contain sufficient germ-plasm to act as sexual organs without being differentiated as such.
An interesting question is that of the origin of apogamy. It is no doubt the outcome of sexual degeneration; but this general statement requires some explanation. In certain cases apogamy seems to be the result of the degeneration of the male organ; as in Humaria, where there is no male organ, and in Lachnea, where the male organ is rudimentary. In others, as in the Uredineae, it is apparently the female organ that has degenerated, losing its receptive part, the trichogyne; the male cells (spermatia) are developed normally, and there is no reason to believe that they might not fertilize the female organ were there the means of penetrating it. In yet other cases the degeneration occurs at a different stage in the life-history, in the development of the spores. In the apogamous ferns investigated, meiosis is suppressed and apogamy results. In the heterosporous plants which have been investigated (e.g. Marsilia, Eu- Alchemilla) it has been observed that the microspores are so imperfectly developed as to be incapable of germinating, so that fertilization is impossible; and it is perhaps to this that the occurrence of apogamy is to be attributed. This abnormal development of the spores may be regarded as a variation; and in most cases it occurs in plants that are highly variable and often have a high 2x number of chromosomes.
It will be observed that such physiological explanation as can be given of the phenomena of reproduction is based upon the results of the minute investigation of the changes in nuclear structure associated with them. The explanation is often rather suggested than proved, and some fundamental facts still remain altogether unexplained. But it may be anticipated that a method of research which has already so successfully justified itself will not fail in the future to elucidate what still remains obscure.
BIBLIOGRAPHY. This article should be read in connexion with the following: ALGAE, ANGIOSPERMS, BRYOPHYTA, CYTOLOGY, FUNGI, GYMNOSPERMS.HEREDITY, HYBRIDISM, MENDELISM, PLANTS, PTERIDOPHYTA.
As the bibliographies to these articles include all the publications containing the facts and theories mentioned here, it will suffice to append only a few papers of general importance: Blackman and Fraser. " Further Studies on the Sexuality of the Uredineae," Ann. Dot. (1906) vol. xx. ; Farmer, " On the Structural Constituents of the Nucleus, and their Relation to the Organization of the Individual " (Croonian Lecture), Proc. Roy. Soc. (1907) vol. 79, series B ; Farmer and Digby, " Studies in Apospory and Apogamy in Ferns," Ann. Bot. (1907) vol. xxi. ; Strasburger, Die stpfflichen Grundlagen der Vererbung (1905); " Apogamie bei Manilla," Flora (1907), vol. 97; D. M. Mottier, Fecundation in Plants (1904), Carnegie Institution, Washington. (S. H. V.*)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)