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 leaf-like 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 suspensor. Stems and roots increase in diameter by secondary thickening, the secondary wood being produced by one cambium or developed from successive cambium-rings.
| Fig. 1.—Stem of Cycas. F, foliage-leaf bases; S, scale-leaf bases. |
| Fig. 2.—Cycas siamensis. |
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. revoluta, &c.) 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. 1). 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 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 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 produce a pleasing combination 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 bud-scales, 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:—
| Fig. 3.—Cycas. Young Frond. |
Classification.—A. Cycadeae.—Characterized by (a) the alternation of scale- and foliage-leaves (fig. 1) on the branched or unbranched stem; (b) the growth of the main stem through the female flower; (c) the presence of a prominent single vein in the linear pinnae; (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, &c.).
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, Stangeria, 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, &c.).—Stem short and often divided into several columnar branches. Each carpel terminates in a peltate head. Ceratozamia (Mexico).—Similar in habit to Macrozamia, but distinguished by the presence of two horn-like 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 scale-leaves 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).
| From a photograph of a plant in Peradeniya Gardens, Ceylon, by Professor R. H. Yapp. | Fig. 5.—Bowenia spectabilis: frond. |
| Fig. 4.—Dioon edule. |
| Fig. 6.—Macrozamia heteromera. A, part of frond; B, single pinna. |
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 or columnar stem of the Cycadaceae Stem and leaf. constitutes a striking distinguishing feature. Branching, however, occurs not infrequently: in 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, &c., the persistent scale-leaf bases are almost equal in size to those of the foliage-leaves, and there is no regular alternation of zones such as characterizes some species of Cycas. Another type of stem is illustrated by Stangeria and Zamia, also by a few forms of Cycas (fig. 2), in which the fronds fall off 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; and in a species of the Australian genus 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. Micholitzii, 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 dichotomously 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, &c., 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 phylogeny 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 disposed sporophylls. These are often wedge-shaped and angular; in some cases they consist of a short, thick Flower. 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-walled cells bounded by larger and thicker-walled 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 sporangial 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.