Cycas is the most prominent genus of the Eastern Hemisphere.
C. circinalis, C. pectinata, C. rumphii, and C. beddomeii are found wild in India. C. revoluta (native of Japan) is the most common cultivated species in Indian gardens. C. siamensis (found in Burma) is sometimes cultivated.
The Sporophyte of Cycas:
The vegetative body of the sporophyte externally resembles a palm tree. The stem is typically un-branched, short, thick, cylindrical and more or less columnar, covered by an armour of persistent leaf- bases, and bearing a crown of leaves. Leaves are pinnate, tough, leathery, large (sometimes up to 3 metres in length), spirally arranged round the free-growing apex of the stem, and show circinate ptyxis characteristic of ferns.
Each leaflet has only one midrib without branches. The primary root elongates as a strong tap root with scanty branches, but numerous secondary roots are present. These secondary roots, near the surface of the ground, branch profusely and dichotomously forming coralloid masses, known as root tubercles.
A cross-section of the stem shows a large pith, a comparatively thin vascular cylinder with conjoint, collateral, open and endarch bundles, and a very thick cortex showing numerous leaf-traces (girdles), limited externally by an epidermis. Numerous canals filled with mucilage also occur in the cortex and in the pith, and these are connected with one another through leaf-gaps forming a network.
The primary cambium is short-lived, and a succession of secondary cambiums is formed in the cortex producing concentric series of vascular cylinders. The first cortical cambium produces a prominent secondary vascular cylinder, the second cambium produces much narrower ones with widely separated vascular bundles, and as this process is continued the constructive power of the successive cambium layers is greatly reduced, which ultimately form isolated patches of smaller vascular bundles here and there at the periphery. These secondary cortical bundles are concentric.
The root is usually tetrarch and the secondary growth begins early but in an irregular fashion. In a cross-section of the petiole, numerous collateral and open vascular bundles are found to be present in a convoluted arc.
The xylem of each bundle lies on the upper face with phloem below, and the former is composed of an endarch protoxylem with both centripetal and centrifugal masses of xylem, which are apparently secondary in origin.
A cross-section of the leaf shows strongly cutinized epidermis on both sides of it and with deeply sunken stomata only on the under-surface. Below the upper epidermis there is a hypodermis, consisting of a few layers of thick-walled cells, and this tissue gives a tough and leathery texture to the leaf.
The mesophyll tissue is differentiated into an upper palisade and a lower spongy parenchyma, full of chloroplasts, and these two tissues are separated by colourless, elongated cells forming the transfusion tissue, which runs parallel to the leaf-surface. The vascular bundle is usually mesarch.
Cycas is strictly dioecious and heterosporous, the micro-and megasporophylls being borne on different plants. In this case, the microsporophylls only are in compact strobili, while the megasporophylls are arranged spirally like the ordinary crown of foliage leaves around the terminal vegetative bud of the shoot-axis. The staminate strobiIi are apparently terminal in position but in reality lateral, being situated close to the growing apex.
Staminate (or Male) Strobilus:
It consists of a central axis on which numerous microsporophylls are arranged in acropetal succession forming a compact, elongated and ovoid structure, often attaining a length of 50 cm. or more. One or more strobili may grow simultaneously about the growing point.
The terminal and basal microsporophylls are sterile, while the remaining ones bear 700 or more microsporangia or pollen sacs on their abaxial (lower) surface in two, more or less distinct patches, separated by a sterile median line.
The microsporangia occur in definite groups of 5, 4, 3, sometimes 2, forming sori. Each microsporophyll, which is not leafy in nature, is narrow below, and broadened above into a more or less sterile expanded portion.
The sporangium is eusporangiate in development, and consists of a wall of several layers of cells surrounding an inner large mass of sporogenous tissue, from the surface of which a comparatively scanty tapetum is developed. From each spore mother cell, by reduction division, four microspores or pollen grains are produced.
Ovulate or Female Strobilus:
The megasporophylls do not form compact strobili, as in the case of microsporophylls, but are spirally and loosely arranged like ordinary crown of foliage leaves around the terminal vegetative bud of the shoot-axis. The megasporophylls, in some cases bear considerable resemblance to foliage leaves (e.g., C. revoluta, C. circinalis, etc.), are devoid of chlorophyll, and covered with short brownish hairs.
Several ovules or megasporangia, sometimes only two (C. siamensis), are borne on the margins of the megasporophyll. In a species of Cycas (C. circinalis) the ovules are the largest, measuring about 6 x 4 cm., and in some cases they are densely hairy (C. revoluta).
The ovule consists of a nucellus surrounded by a single massive integument which develops a testa of three layers; an outer fleshy, a middle stony, and an inner fleshy layer. The outer fleshy layer, in a ripe seed, becomes characteristically coloured, and the inner fleshy one usually becomes thin and membraneous.
The nucellus is fused with the thick integument for most part of its length, except at the apical region, where it forms a beak-like structure, called the nucellar beak, which projects into the micropyle. Within the nucellar beak develops a conspicuous chamber, called the pollen chamber, in which the pollen grains are collected.
The vascular supply of the ovule is divided into two sets, the outer set enters the outer fleshy layer, and the inner set traverses the inner fleshy layer just within the stony coat, and continues beyond the free portion of the nucellus.
Deep within the tissue of the nucellus a spore mother cell soon becomes differentiated which by reduction division, forms a linear tetraid of four megaspores or embryo-sacs, of which the innermost one is functional, while the others disorganize.
The Gametophytes of Cycas::
1. Male Gametophyte:
The microspore, the first cell of the male gametophyte, has two coats:
i. The exine and
ii. The intine.
It germinates while still within the microsporangium; its nucleus divides and two unequal cells are formed, a small persistent prothallial cell or vegetative cell, and a large cell, the antheridial initial.
The larger cell in its turn, divides again into two unequal cells, one, a small cell closely in contact with the prothallial cell, is the generative cell, and the other larger cell is the tube cell. It is at this three-celled condition the microspores or pollen grains are discharged from the microsporangium, and are disseminated by wind. It has been accepted that Cycas is wind-pollinated.
The three-celled microspores are carried by the wind to the ovules of the neighbouring female plants. They are caught by a mucilagenous secretion from the micropyle and as the fluid dries up, they are sucked into the pollen chamber. The tube cell elongates and penetrates the tissue of the nucellus forming the pollen tube, and the tube nucleus passes into it. The pollen tube often branches, and always functions as an absorptive organ.
The generative cell then divides into a stalk cell and a body cell. The former is functionless, but the latter again undergoes division producing two spermatozoid mother cells, in each of which a large spirally coiled, multiflagellate spermatozoid is produced. These spermatozoids are remarkably large, larger than any known in other plants and animals, and are easily visible to the naked eye.
2. Female Gametophyte:
The megaspore is the first cell of the female gametophyte. It germinates within the mega-sporangium (or ovule), and is never shed, but is retained within it.
The development of the gametophyte can be broadly divided into five stages:
a. The megaspore enlarges, its nucleus divides freely forming a variable number of free nuclei distributed in its general mass of cytoplasm;
b. Due to the development of a large central vacuole, all the nuclei are pushed at the periphery of the megaspore;
c. Free nuclear division continues;
d. A peripheral tissue is gradually developed by the formation of cell walls separating the free nuclei;
e. This process is continued, and the tissue grows centripetally until it fills up the cavity of the megaspore.
This gametophytic tissue, which is developed before fertilization, is known as the endosperm, which, at maturity, contains abundant starch grains.
It consists of two regions:
(a) A region of large cells near the base performing nutritive function, and
(b) A region of small cells near the micropylar region.
Within the latter region develops a variable number of archegonia (2-8 in C. revoluta). Any superficial cell of the gametophyte may become an archegonium initial, which divides periclinally into an outer primary neck cell, and an inner central cell.
The primary neck cell divides vertically to form two neck cells, while the central cell enlarges remarkably and gradually becomes surrounded by a special jacket of nutritive cells, known as the archegonial jacket. The nucleus of the central cell finally divides to form a ventral nucleus, and an egg nucleus, and there is no wall in between the two.
The absence of such a wall is regarded as an advanced character. Around the latter an oosphere is gradually differentiated. The ventral nucleus soon disorganizes. The oosphere and its nucleus have been described as the largest among plants, and can be detected by the unaided eye.
During the development of the archegonia at the micropylar surface of the gametophyte, its neighbouring cells continue to grow upwards, so that the archegonia are left in a shallow depression, known as the archegonial chamber.
After penetrating the nucellus the pollen tube bursts, when the spermatozoids are set free in the archegoniaI chamber. The spermatozoids then make their way into the archegonium, and one of them fertilizes the oosphere. The fertilized oosphere surrounds itself by a wall, and forms an oospore.
The New Sporophyte of Cycas:
After fertilization the oospore enlarges, its nucleus by free cell formation produces as many as 256 or probably more (C. revoluta) nuclei, distributed through the cytoplasm of the oospore. A large central vacuole soon appears (C. circinalis), followed by the formation of a peripheral tissue.
This tissue is the pro-embryo. In Zamia the pro-embryo is formed only at the lower end of the developing oospore. The cells below the tip-cells of this pro-embryo elongate remarkably, and ultimately form a long, coiled, flexuous and massive filamentous structure, called the suspensor, which forces the tip-cells out of the archegonium into the nutritive gametophytic tissue (endosperm).
From the tip-cells a dicotyledonous embryo is produced. The embryo and the endosperm remain within the three-layered testa, and the ovule is gradually transformed into a seed. The seed, on germination, gives rise to a new seedling sporophyte, and the mode of germination is hypogeal.
Fossil Cycads:
The Cycadales first appeared in Upper Triassic and are the only group out of all the Cycadopsida that has survived until the present-day. Among the living plant they are extremely ancient and retained primitive features both in their morphology and life cycle.
The fossil cycads are mostly included in the order Nilssoniales by the following Cycas:
Leaves- Nilssonia.
Seed-bearing organs- Beania.
Pollen-bearing organs- Androstrobus.
Nilssonia is a common form that have large frond, and the lamina is often cut into segments of various sizes and shapes. The common Indian species are N. princeps, N. morrisiana, N. medlicottiana, etc.
The leaves are with strong midribs, undivided lamina forked or simple veins pass to the margins at right angles to the midrib or slightly upwardly bent.
Haplocheilic type of stoma typical of the fossil Nilssoniales shows its resemblances with Cycadales. Hence the leaves of Nilssoniales represent the foliage of fossil member of Cycadales.
The pistillate fructification of fossil Cycads is Beania from the Jurassic of Yorkshire. Each of the peltate sporophylls bears two ovules, sporophylls are arranged spirally in a loose cone, and the structure of the ovule shows close resemblance with that of Cycas.
The male cone referred to Androstrobus, belonged to the same plant as Beania, because they are associated together in the rocks, and the pollen grains inside the micropyle of Beania are same as obtained from the male cones of Androstrobus.
Fern-Characters in Cycas:
1. The sporophyte of Cycas has a columnar and an unbranched stem covered over with persistent leaf-bases and bearing a crown of large pinnate compound leaves towards the apex, as in tree ferns.
2. Rachis and leaflets enrolled from the apex in a circinnate manner.
3. Megasporophylls are leaf-like as is indicated by their arrangement on the stem, where they take the place of foliage leaves; by their approach to pinnate character, especially in Cycas revoluta, and by their developing chlorophyll, as in C. circinalis. In ferns also the sporophylls are either leaves or resemble them very much.
4. In the arrangement of microsporangia in sori on the under-surface of the microsporophylls, presence of indusial hairs beneath the sori, structure and eusporangiate development of the microsporangia and their dehiscence and their large output of microspores, Cycas resembles Marattiaceae.
5. Motile and flagellate sperms.
6. Xylem has no vessels and phloem is without companion cells. Sieve plates are present on the longitudinal walls of the sieve tubes and sometimes even concentric bundles are found in the leaf, cone-axis and stalks of sporophylls. The entire cortical bundled of the stem are concentric. All these are fern characters. Further, like them in ferns, pith and cortex are large.
7. Presence of leaf gaps in the cortex.
All these characters lead one to believe that Cycas might have evolved from the ferns.
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