Gymnosperms
GYMNOSPERMS, in Botany. The Gymnosperms, with the Angiosperms, constitute the existing groups of seed-bearing plants or Phanerogams: the importance of the seed as a distinguishing feature in the plant kingdom may be emphasized by the use of the designation Spermophyta for these two groups, in contrast to the Pteridophyta and Bryophyta in which true seeds are unknown. Recent discoveries have, however, established the fact that there existed in the Palaeozoic era fernlike plants which produced true seeds of a highly specialized type; this group, for which Oliver and Scott proposed the term Pteridospermae in 1904, must also be included in the Spermophyta. Another instance of the production of seeds in an extinct plant which further reduces the importance of this character as a distinguishing feature is afforded by the Palaeozoic genus Lepidocarpon described by Scott in 1901; this lycopodiaceous type possessed an integumented megaspore, to which the designation seed may be legitimately applied (see PALAEOBOTANY: Palaeozoic).
As the name Gymnosperm (Gr. 7u/w6s, naked, <nrepjua, seed) implies, one characteristic of this group is the absence of an ovary or closed chamber containing the ovules. It was the English botanist Robert Brown who first recognized this important distinguishing feature in conifers and cycads in 1825; he established the gymnospermy of these seed-bearing classes as distinct from the angiospermy of the monocotyledons and dicotyledons. As Sachs says in his history of botany, " no more important discovery was ever made in the domain of comparative morphology and systematic botany." As Coulter and Chamberlain express it, " the habitats of the Gymhosperms to-day indicate that they either are not at home in the more genial conditions affected by Angiosperms, or have not been able to maintain themselves in competition with this group of plants."
These naked-seeded plants are of special interest on account of their great antiquity, which far exceeds that of the Angiosperms, and as comprising different types which carry us back to the Palaeozoic era and to the forests of the coal period. The best known and by far the largest division of the Gymnosperms is that 'of the cone-bearing trees (pines, firs, cedars, larches, etc.), which play a prominent part in the vegetation of the present day, especially in the higher latitudes of the northern hemisphere; certain members of this class are of considerable antiquity, but the conifers as a whole are still vigorous and show but little sign of decadence. The division known as the Cycadophyta is represented by a few living genera of limited geographical range and by a large number of extinct types which in the Mesozoic era (see PALAEOBOTANY: Mesozoic) played a conspicuous part in the vegetation of the world. Among existing Cycadophyta we find surviving types which, in their present isolation, their close resemblance to fossil forms, and in certain morphological features, constitute links with the past that not only connect the present with former periods in the earth's history, but serve as sign-posts pointing the way back along one of the many lines which evolution has followed.
It is needless to discuss at length the origin of the Gymnosperms. The two views which find most .favour in regard to the Coniferales and Cycadophyta are: (i) that both have been derived from remote filicinean ancestors; (2) that the cycads are the descendants of a fern-like stock, while conifers have been evolved from lycopodiaceous ancestors. The line of descent of recent cycads is comparatively clear in so far as they have undoubted affinity with Palaeozoic plants which combined cycadean and filicinean features; but opinion is much more divided as to the nature of the phylum from which the conifers are derived. The Cordaitales (see PALAEOBOTANY: Palaeozoic) are represented by extinct forms only, which occupied a prominent III.
IV.
V.
position in the Palaeozoic period; these plants exhibit certain features in common with the living Araucarias, and others which invite a comparison with the maidenhair tree (Ginkgo biloba), the solitary survivor of another class of Gymnosperms, the Ginkgoales (see PALAEOBOTANY: Mesozoic). The Gnetales are a class apart, including three living genera, of which we know next to nothing as regards their past history or line of descent. Although there are several morphological features in the three genera of Gnetales which might seem to bring them into line with the Angiosperms, it is usual to regard these resemblances as parallel developments along distinct lines rather than to interpret them as evidence of direct relationship.
Gymnospermae. Trees or shrubs; leaves vary considerably in size and form. Flowers unisexual, except in a few cases (Gnetales) without a perianth. Monoecious or dioecious. Ovules naked, rarely without carpellary leaves, usually borne on carpophylls, which assume various forms. The single megaspore enclosed in the nucellus is filled with tissue (prothallus) before fertilization, and contains two or more archegonia, consisting usually of a large egg-cell and a small neck, rarely of an egg-cell only and no neck (Gnetum and Welwitschia). Microspore spherical or oval, with or without a bladder-like extension of the exine, containing a prothallus of two or more cells, one of which produces two non-motile or motile male cells. Cotyledons two or several. Secondary xylem and phloem produced by a single cambium, or by successive cambial zones; no true vessels (except in the Gnetales) in the wood, and no companioncells in the phloem.
I. Pteridospermae (see PALAEOBOTANY, PALAEOZOIC).
II. Cycadophyta.
A. Cycadales (recent and extinct).
B. Bennettitales (see PALAEOBOTANY: Mesozoic). Cordaitales (see PALAEOBOTANY: Palaeozoic). Ginkgoales (recent and extinct).
Coniferales.
A. Taxaceae.
B. Pinaceae.
There is no doubt that the result of recent research and of work now in progress will be to modify considerably the grouping of the conifers. The family Araucarieae, represented by Araucaria and Agathis, should perhaps "be separated as a special class and a rearrangement of other genera more in accord with a natural system of classification will soon be possible; but for the present its twofold subdivision may be retained. VI. Gnetales.
A. Ephedroideae.
B. Gnetqideae.
C. Welwitschioideae (Tumboideae). CYCADOPHYTA. A. Cycadales. Stems tuberous or columnar, not infrequently branched, rarely epiphytic (Peruvian species of Zamia); fronds pinnate, bi-pinnate in the Australian genus Bowenia. Dioecious; flowers in the form of cones, except the female flowers of Cycas, which consist of a rosette of leaMike carpels at the apex of the stem. Seeds albuminous, with one integument ; the single embryo, usually bearing two partially fused cotyledons, is attached to a long tangled suspenspr. Steins and roots increase in diameter by secondary thickening, the secondary wood being produced by one cambium or developed from successive cambium-rings.
The cycads constitute a homogeneous group of a few living members confined to tropical and sub-tropical regions. As a fairly typical and well-known example of the Cycadaceae, a species of the genus Cycas (e.g. C. circinalis, C. reooluta, etc.) is briefly described. The stout columnar stem may reach a height of 20 metres, and a diameter of half a metre; it remains either unbranched or divides near the summit into several short and thick branches, each branch terminating in a crown of long pinnate leaves. The surface of the stem is covered with rhomboidal areas, which represent the persistent bases of foliage- and scale- leaves. In some species of Cycas there is a well-defined alternation of transverse zones on the stem, consisting of larger areas representing foliage-leaf bases, and similar but smaller areas formed by the bases of scale-leaves (F and S, fig. i). The scale-leaves clothing the terminal bud are linear-lanceolate in form, and of a brown or yellow colour; they are pushed aside as the stem-axis elongates and becomes shrivelled, finally falling off, leaving projecting bases which are eventually cut off at a still lower FIG. i. Stem of Cycas. F, foliageleaf bases; S, scaleleaf bases.
level. Similarly, the dead fronds fall off .leaving a ragged petiole, which is afterwards separated from the stem by an absciss-layer a short distance above the base. In some species of Cycas the leaf-bases do not persist as a permanent covering to the stem, but the surface FIG. 2. Cycas siamensis.
is covered with a wrinkled bark, as in Cycas siamensis, which has a stem of unusual form (fig. 2). Small tuberous shoots, comparable on a large scale with the bulbils of Lycopodium Selago, are occasionally produced in the axils of some of the persistent leaf-bases; these are characteristic of sickly plants, and serve as a means of vegetative reproduction. In the genus Cycas the female flower is peculiar among cycads in consisting of a terminal crown of separate leaf-like carpels several inches in length ; the apical portion of each carpellary leaf may be broadly triangular in form, and deeply dissected on the margins into narrow woolly appendages like rudimentary pinnae.
From the lower part of a carpel are produced several laterally placed ovules, which become bright red or orange on ripening; the bright fleshy seeds, which in some species are as large as a goose's egg, and the tawny spreading carpels Eroduce a pleasing comination of colour in the midst of the long dark-green fronds, which curve gracefully upwards and outwards from the summit of the columnar stem. In Cycas the stem apex, after producing a cluster of carpellary leaves, continues to elongate and produces more budscales, which are afterwards pushed aside as a fresh crown of fronds is developed. The young leaves of Cycas consist of a straight rachis bearing numerous linear pinnae, traversed by a single midrib; the pinnae are circinately coiled like the leaf of a fern (fig. 3). The male flower of Cycas conforms to the type of structure characteristic of the cycads, and consists of a long cone of numerous sporophylls bearing many oval pollen-sacs on their lower faces. The type described serves as a convenient representative of its class. There are eight other living genera, which may be classified as follows:
Classification. A. Cycadeae. Characterized by (a) the alternation of scale- and foliage-leaves (fig. i) on the branched or unbranched stem; (6) the growth of the main stem through the female flower; (c) the presence of a prominent single vein in the linear pipnae; (d) the structure of the female flower, which is peculiar in not having the form of a cone, but consists of numerous independent carpels, each of which bears two or more lateral ovules. Represented by a single genus, Cycas. (Tropical Asia, Australia, etc.).
B. Zamieae. The stem does not grow through the female flower; both male and female flowers are in the form of cones, (a) Stangerieae. Characterized by the fern-like venation of the pinnae, which have a prominent midrib, giving off at a wide angle simple or forked and occasionally anastomosing lateral veins. A single genus, Slangeria, confined to South Africa, (b) Euzamieae. The pinnae are traversed by several parallel veins. Bowenia, an Australian cycad, is peculiar in having bi-pinnate fronds (fig. 5). The various genera are distinguished from one another by the shape and manner of attachment of the pinnae, the form of the carpellary scales, and to some extent by anatomical characters. Encephalartos (South and Tropical Africa). Large cones; the carpellary scales terminate in a peltate distal expansion. Macrozamia (Australia). Similar to Encephalartos except in the presence of a spinous projection from the swollen distal end of the carpels. Zamia (South America, Florida, etc.). Stem short and often divided into several columnar branches. Each carpel terminates in a peltate head. Ceratpzamia (Mexico). Similar in habit to Macrozamia, FIG. 3. Cycas. but distinguished by the presence of two horn-like Young Frond, spinous processes on the apex of the carpels. Microcycas (Cuba). Like Zamia, except that the ends of the stamens are flat, while the apices of the carpels are peltate. Dioon (Mexico) (fig. 4). Characterized by the woolly scaleleaves and carpels; the latter terminate in a thick laminar expansion of triangular form, bearing two placental cushions, on which the ovules are situated. Bowenia (Australia). Bi-pinnate fronds; stem short and tuberous (fig. 5).
The stems of cycads are often described as unbranched; it is true that in comparison with conifers, in which the numerous branches, springing from the main stem, give a characteristic form to the tree, the tuberous o.r columnar stem of the Cyca- daceae constitutes a striking distinguishing feature.
Branching, however, occurs not infrequently; m Cycas the tall stem often produces several candelabra-like arms; in Zamia the main axis may break up near the base into several cylindrical branches ; in species of Dioon (fig. 4) lateral branches are occasionally produced. The South African Encephalartos frequently produces several branches. Probably the oldest example of this genus in cultivation is in the Botanic Garden of Amsterdam, its age is considered by Professor de Vries to be about two thousand years: although an accurate determination of age is impossible, there is no doubt that many cycads grow very slowly and are remarkable for longevity. The thick armour of petiole-bases enveloping the stem is a characteristic Cycadean feature; in Cycas the alternation of scale-leaves and fronds is more clearly shown than in other cycads; in Encephalartos, Dioon etc From hotograph of . ^ in the Peradeniya the persistent scale - leaf Gardens, Ceylon, by Professor R. H. Yapp, bases are almost equal in size to those of the foliage- FIG. 4.. Dioon edule.
leaves, and there is no regular alternation of zones sucn as characterizes some species of Cycas. Another type of stem is illustrated by Slangeria and Zamia, also by a few forms of Cycas, (fig. 2), in which the fronds fall off FIG. 5. Bowenia speclabilis: frond.
completely, leaving a comparatively smooth stem. The Cyas type of frond, except as regards the presence of a midrib in each pinna, characterizes the cycads generally, except Bowenia and Stangeria. In the monotypic genus Bowenia the large fronds, borne singly on the short and thick stem, are bi-pinnate (fig. 5) ; the segments, which are broadly ovate or rhomboidal, have several forked spreading veins, and resemble the large pinnules of some species of Adiantum. In Stangeria, also a genus represented by one species (S. paradoxa of South Africa), the long and comparatively broad pinnae, with an entire or irregularly incised margin, are very fern-like, a circumstance which led Kunze to describe the plant in 1835 as a species of the fern Lomaria. In rare cases the pinnae of cycads are lobed or branched: in Dioon spinulosum (Central America) the margin of the segments bears numerous spinous processes; in some species of Encephalartos, e.g. E. horridus, the lamina is deeply lobed ; heteromera. A, part of and in a species of the Australian genus f roll( i ; B, single pinna. Macrozamia, M. heteromera, the narrow pinnae are dichotomously branched almost to the base (fig. 6), and resemble the frond of some species of the fern Schizaea, or the fossil genus Baiera (Ginkgoales). An interesting species of Cycas, C.Micholitzti, has recently been described by Sir William Thiselton-Dyer from Annam, where it was collected by one of Messrs Sanders & Son's collectors, in which the pinnae instead of being of the usual simple type are FIG. 6. Macrozamia dichptomously branched as in Macrozamia heteromera. In Ceratozamia the broad petiole-base is characterized by the presence of two lateral spinous processes, suggesting stipular appendages, comparable, on a reduced scale, with the large stipules of the Marattiaceae among Ferns. The vernation varies in different genera; in Cycas the rachis is straight and the pinnae circinately coiled (fig. 3) ; in Encephalartos, Dioon, etc., both rachis and segments are straight; in Zamia the rachis is bent or slightly coiled, bearing straight pinnae. The young leaves arise on the stem-apex as conical protuberances with winged borders, on which the pinnae appear as rounded humps, usually in basipetal order; the scale-leaves in their young condition resemble fronds, but the lamina remains undeveloped. A feature of interest in connexion with the phytogeny of cycads is the presence of long hairs clothing the scale-leaves, and forming a cap on the summit of the stem-apex or attached to the bases of petioles ; on some fossil cycadean plants these outgrowths have the form of scales, and are identical in structure with the ramenta (paleae)of the majority of ferns. The male flowers of cycads are constructed on a uniform plan, and in all cases consist of an axis bearing crowded, spirally disFlower. posed sporophylls. These are often wedge-shaped and angular; in some cases they consist of a short, thick stalk, terminating in a peltate expansion, or prolonged upwards in the form of a triangular lamina. The sporangia (pollen-sacs), which occur on the under-side of the stamens, are often arranged in more or less definite groups or sori, interspersed with hairs (paraphyses) ; dehiscence takes place along a line marked out by the occurrence of smaller and thinner-wallea cells bounded by larger and thickerwalled elements, which form a fairly prominent cap-like " annulus " near the apex of the sporangium, not unlike the annulus characteristic of the Schizaeaceae among ferns. The sporangia! wall, consisting of several layers of cells, encloses a cavity containing numerous oval spores (pollen-grains). In structure a cycadean sporangium recalls those of certain ferns (Marattiaceae, Osmundaceae and Schizaeaceae), but in the development of the spores there are certain peculiarities not met with among the Vascular Cryptogams. With the exception of Cycas, the female flowers are also in the form of cones, bearing numerous carpellary scales. In Cycas revoluta and C. circinalis each leaf-like carpel may produce several laterally attached ovules, but in C. Normanbyana the carpel is shorter and the ovules are reduced to two; this latter type brings us nearer to the carpels of Dioon, in which the flower has the form of a cone, and the distal end of the carpels is longer and more leaf-like than in the other genera of the Zamieae, which are characterized by shorter carpels with thick peltate heads bearing two ovules on the morphologically lower surface. The cones of cycads attain in some cases (e.g. Encephalartos) a considerable size, reaching a length of more than a foot. Cases have been recorded (by Thiselton-Dyer in Encephalartos and by Wieland in Zamia) in which the short carpellary cone-scales exhibit a foliaceous form. It is interesting that no monstrous cycadean cone has been described in which ovuliferous and staminate appendages are borne on the same axis: in the Bennettitales (see PALAEOBOTANY : Mesozoic) flowers were produced bearing on the same axis both androecium and gynoecium.
The pollen-grains when mature consist of three cells, two small and one large cell; the latter grows into the pollen-tube, as in the Coniferales, and from one of the small cells two large ciliated spermatozpids are eventually produced. A remarkable exception to this rule has recently been recorded by Caldwell, who found that in Microcycas Calocoma the body-cells may be eight or even ten in _ number and the sperm-cells twice as numerous. One of the most important discoveries made during the latter part of the 19th century was that by Ikeno, a Japanese botanist, who first demonstrated the existence of motile male cells in the genus Cycas. Similar spermatozoids were observed in some species of Zamia by H. J. Webber, and more recent work enables us to assume that all cycads produce ciliated male gametes. Before following the growth of the pollen-grain after pollination, we will briefly describe the structure of a cycadean ovule. An ovule consists of a conical nucellus surrounded by a single integument. At an early stage of development a large cell makes its appearance in the central region of the nucellus; this increases in size and eventually forms three cells; the lowest of these grows vigorously and constitutes the megaspore (embryo-sac) .which ultimately absorbs the greater part of the nucellus. The megaspore-nucleus divides repeatedly, and cells are produced from the peripheral region inwards, which eventually fill the sporecavity with a homogeneous tissue (prothallus) ; some of the superficial cells at the micropylar end of the megaspore increase in size and divide by a tangential wall into two, an upper cell which gives rise to the short two-ccUed neck of the archegonium, and a lower cell which develops into a large egg-cell. Each megaspore may contain 2 to 6 archegonia. During the growth of the ovum nourishment is supplied from the contents of the cells immediately surrounding the egg-cell, as in the development of the ovum of Pinus and other conifers. Meanwhile the tissue in the apical region of the nucellus has been undergoing disorganization, which results in the formation of a pollen-chamber (fig. 7, C) immediately above the megaspore. Pollination in cycads has always been described as anemophilous, but according to recent observations by Pearson on South African species it seems probable that, at least in some Microspores ant megaspores.
cases, the pollen is conveyed to the ovules by animal agency. The pollen-grains find their way between the carpophylls, which at the time of pollination are slightly apart owing to the elongation of the internodes of the flower-axis, and pass into the pollen-chamber; the large cell of the pollen-grain grows out into a tube (Pt), which penetrates the nucellar tissue and often branches repeatedly; the pollen-grain itself, with the prothallus-cells, projects freely into the pollen-chamber (fig. 7). The nucleus of the outermost (second small cell (fig. 7, G) divides, and one of the daughter-nuclei passes out of the cell, and may enter the lowest (first) small cell. The outermost cell, by the division of the remaining nucleus, produces two large spermatozpids (fig. 8, a, a). In Microcycas 1 6 sperm-cells are produced. In the course of division two bodies appear in the cytoplasm, and behave as centrosomes during the karyokinesis; they gradually become threadlike and coil round each daughter nucleus. This thread gives rise to a spiral ciliated band lying in a depression on the body of each spermatozoid ; the large spermatozoids eventually escape from the pollen-tube, and are able to perform ciliary movements in the watery FIG. 7. 2 tudinal section. (After Webber.) P, Prothallus. Pt, Pollen-tube. A, Archegonia. Pg, Pollen-grain. N, Nucellus. G, Generative cell jj C, Pollen-chamber.
(second cell of pollen-tube).
liquid which occurs tween the thin papery remnant of nucellar tissue and the archegonial necks. Before fertilization a neck-canal cell is formed by the division of the ovum-nucleus. After the body of a spermatozoid has coalesced with the egg-nucleus the latter divides repeatedly and forms a mass of tissue which grows more vigorously in the lower part of the fertilized ovum, and extends upwards towards the apex of the ovum as a peripheral layer of parenchyma surrounding a central space. By further growth this tissue gives rise to a proembryo, which consists, at the micropylar end, of a sac ; the tissue at the chalazal end grows into a long and tangled suspensor, terminating to a mass of cells, which is eventually differentiated into a radicle, plumule and two cotyledons. In the ripe seed the integument assumes the form of a fleshy envelope, succeeded internally by a hard woody shell, internal to which is a thin papery membrane the apical portion of the nucellus which is easily dissected out as a conical cap covering the apex of the endosperm. A thorough examination of cycadean seeds has recently been made by Miss Stopes, more particularly with a view to a comparison of their vascular supply with that in Palaeozoic gymnospermous seeds (Flora, 1904). The first leaves borne on the seedling axis are often scalelike, and these are followed by two or more larger laminae, which foreshadow the pinnae of the adult frond.
The anatomical structure of the vegetative organs of recent cycads is of special interest as affording important evidence of rela- . . tionship with extinct types, and with other groups of recent plants. Brongniart, who was the first to investigate in detail the anatomy of a cycadean stem, recognized an agreement, as regards the secondary wood, with Dicotyledons and Gymnosperms, rather than with Monocotyledons. He drew attention also to certain structural similarities between Cycas and Ginkgo. The main anatomical features of a cycad stem c may be summarized as follows: the centre is ' occupied by a large parenchymatous pith traversed by numerous secretory canals, and in some genera by cauline vascular bundles (e.g. Encephalartos - and Macrozamia). In addition to these cauline strands (confined to the stem and not connected with the leaves), collateral bundles are often met with in the pith, which form the vascular supply of terminal flowers borne at intervals on the apex of the stem. These latter bundles may be seen in sections of old stems to pursue a more or less horizontal course, passing outwards through the main woody cylinder. This lateral course is due to the more vigorous growth of the axillary branch formed near the base of each flower, which is a terminal structure, and, except in the female flower of Cycas, puts a limit to the apical growth of the stem. The vigorous lateral branch therefore continues the line of the main axis. The pith is encircled by a cylinder of secondary wood, consisting of single or multiple radial rows of tracheids separated by broad medullary rays composed of large parenchymatous cells; the tracheids bear numerous bordered p fi _ p_ ',', " '- ' FIG. 9. Macrozamia. Diagrammatic transverse section of part of Stem. (After Worsdell.) pd, Periderm in leaf-bases. It, Leaf-traces in cortex. ph, Phloem. x, Xylem.
m, Medullary bundles. c, Cortical bundles.
pits on the radial walls. The large medullary rays give to the wood a characteristic parenchymatous or lax appearance, which is in marked contrast to the more compact wood of a conifer. The protoxylem-elements are situated at the extreme inner edge of the secondary wood, and may occur as small groups of narrow, spirallypitted elements scattered among the parenchyma which abuts on the main mass of wood. Short and reticulately-pitted tracheal cells, similar to tracheids, often occur in the circummedullary region of cycadean stems. In an old stem of Cycas, Encephalartos or Macrozamia the secondary wood consists of several rather unevenly concentric zones, while in some other genera it forms a continuous mass as in conifers and normal dicotyledons. These concentric rings of secondary xylem and phloem (fig. 9) afford a characteristic cycadean feature. After the cambium has been active for some time producing secondary xylem and phloem, the latter consisting of sievetubes, phloem-parenchyma and frequently thick-walled fibres, a second cambium is developed in the pericycle ; this produces a second vascular zone, which is in turn followed by a third cambium, and so on, until several hollow cylinders are developed. It has been recently shown that several cambium-zones may remain in a state of activity, so that the formation of a new cambium does not necessarily mark a cessation of growth in the more internal meristematic rings. It occasionally happens that groups of xylem and phloem are developed internally to some of the vascular rings; these are characterized by an inverse orientation of the tissues, the xylem being centrifugal and the phloem centripetal in its development. The broad cortical region, which contains many secretory canals, is traversed by numerous vascular bundles (fig. 9, c) some of which pursue a more or less vertical course, and by frequent anastomoses with one another form a loose reticulum of vascular strands; others are leaftraces on their way from the stele of the stem to the leaves. Most of these cortical bundles are collateral in structure, but in some the xylem and phloem are concentrically arranged; the secondary origm_of these bundles from procambium-strands was described by Mettenius in his classical paper of 1860. During the increase in thickness of a cycadean stem successive layers of cork-tissue are formed by phellogens in the persistent bases of leaves (fig. 9, pd), which increase in size to adapt themselves to the growth of the vascular zones. The leaftraces of cycads are remarkable both on account of their course and their anatomy. In a transverse section of a stem (fig. 9) one sees some vascular bundles following a horizontal or slightly oblique course in the cortex, stretching. /-^ S~~\]r t ^ ing for a longer or shorter I / i'V f V i, distance in a direction conjl-J I* f \ centric with the woody B HI * \ i cylinder. From each leaf- * **&/ Y base two mam Bundles ^f jl \ W /O spread right and left V \ 't '^- --. through the cortex of the r ^ \/ ' ^~ stem (fig. 9, It), and as they \^, y- ~ > curve gradually towards the ^J^ vascular ring they present the appearance of two rather flat ogee curves, usually spoken of as the leaf -trace girdles (fig. 9, It). The distal ends of these girdles give off several branches, which traverse the petiole and rachis as numerous collateral bundles. The complicated girdle-like course is characteristic of the leaf-traces of most recent cycads, but in some cases, e.g. in Zamia floridana, the traces are described by Wieland in his recent monograph on American fossil cycads (Carnegie Institution Publications, 1906) as possessing a more direct course similar to that in Mesozoic genera. A leaf -trace, as it passes through the cortex, has a collateral structure, the protoxylem being situated at_the inner edge of the xylem ; when it reaches the leaf-base the position of the spiral tracheids is gradually altered, and the endarch arrangement (protoxylem internal) gives place to a mesarch structure (protoxylem more or less central and not on the edge of the xylem strand). In a bundle examined in the basal portion of a leaf the bulk of the xylem is found to be centrifugal in position, but internally to the protoxylem there is a group of centripetal tracheids; higher up in the petiole the xylem is mainly centripetal, the centrifugal wood being represented FIG. 10. Ginkgo biloba. Leaves.
FIG. 1 1 . Ginkgo adiantoides. Fossil (Eocene) leaf from the Island of Mull.
by a small arc of tracheids external to the protoxylem and separated from it by a few parenchymatous elements. Finally, in the pinnae of the frond the centrifugal xylem may disappear, the protoxylem being now exarch in position and abutting on the phloem. Similarly in the sporophylls of some cycads the bundles are endarch near the base and mesarch near the distal end of the stamen or carpel. The vascular system of cycadean seedlings presents some features worthy of note; centripetal xylem occurs in the cotyledonary bundles associated with transfusion-tracheids. The bundles from the cotyledons pursue a direct course to the stele of the main axis, and do not assume the girdle-form characteristic of the adult plant. This is of interest from the point of view of the comparison of recent cycads with extinct species (Bennettites), in which the leaf-traces follow a much more direct course than in modern cycads. The mesarch structure of the leaf-bundles is met with in a less pronounced form in the flower peduncles of some cycads. This fact is of importance as showing that the type of vascular structure, which characterized the stems of many Palaeozoic genera, has not entirely disappeared from the stems of modern cycads ; but the mesarch bundle is now confined to the leaves and peduncles. The roots of some cycads resemble the stems in producing several cambium- Roots. rings; they possess 2 to 8 protoxylem-groups, and are characterized by a broad pericyclic zone. A common phenomenon in cycads is the production of roots which grow upwards (apogeotropic), and appear as coralline branched structures above the level of the ground; some of the cortical cells of these roots are hypertrophied, and contain numerous filaments of blue-green Algae (Nostocaceae), which live as endoparasites in the cell-cavities.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
