In this article we will discuss about the process of reproduction in Cycas with the help of diagrams.
Cycas is dioecious; but the male and female plants are indistinguishable until the appearance of male or female cone. The sex of individual plant is described to be determined by X- and Y-chromosomes.
Male cone (Fig. 15.6A) is terminal, singly, on the apex. A young cone is surrounded by brown scales. At maturity, it is large, oval or conical and emits a characteristic smell, perceived from a distance.
The male cones are known for producing heat. The thermogenesis occurs every evening during cone elongation and pollen dispersal. Thermogenesis is also accompanied by the production of odour. High level of salicylic acid serves as inducer of thermogenesis.
On the axis of the cone are many perpendicular and spirally arranged microsporophylls. A microsporophyll is a large, horizontal flattened woody structure having a wedge-shaped portion (Fig. 15.6B) and a sterile extension. Numerous microsporangia in groups of 2, 4 or 5 occur on its basal surface (Fig. 15.6E).
In C. circinalis and C. revoluta some microsporangia are bisporangiate synangia. These are surrounded by epidermal soral hairs (Fig. 15.6C,D), single or bicelled. The sporangia dehisce by longitudinal slits, releasing microspores. The microsporangia are typical eusporangiate in origin, similar to those of a eusporangiate fern, a primitive character.
A microsporangium has 5-6 wall layers which surround the sporogenous cells of a large prominent nucleus and dense cytoplasm. These microspore-mother-cells undergo meiotic division and form tetrad of microspores.
A microspore (Fig. 15.7A) divides to form a small lense-shaped prothallial cell and a large antheridial initial. The latter divides to form another antheridial cell and a tube cell. A microspore or pollen is shed at the 3-celled stage (Fig. 15.7B). Pollen grains of C. revoluta are oblong with 1-sulcate shrunken aperture, when dry.
The antheridial cell divides into a stalk cell (st) and a spermatogenous cell (Fig. 15.7C) the latter divides to form two large top-shaped motile sperms (Fig. 15.7D), with characteristic spiral bands. The spermatozoids are large (Fig. 15.7D) flagellated male gametes and can be seen with the naked eye.
Spermatozoid—A Subcellular Study:
The spermatozoids of cycads are the largest flagellated male gametes visible to the naked eye. They measure about 80-200 µ in C. revolute.
The spermatogenous cell is considerably large and has a prominent nucleus. Two blepharoplasts arise on either side of the nucleus—from these blepharoplasts radiate microtubules. The microtubular apparatus is traceable to centriole and the spiral band, bearing hundreds of cilia.
Some 40,000-50,000 Flagella arise from a spiral band. These are connected through basal bodies to the spiral. The spiral makes six gyres on the body of spermatozoid of C.circinalis (Fig.15.8A). The spiral is flagellated along most of its length, except the first turn which is free of flagella. The anterior tip of the sperm is occupied by an electron dense cap.
Flagella display a typical 9 + 2 arrangement of microtubules at the apex (Fig. 15.8D) other arrangements (E-H) as indicted in figure. The spiral on the body of the spermatozoid consists of four layers: first is electron dense (dl) in which are embedded basal bodies. The dense layer is followed by granular layer (gl) and inner fibrous layer is made into two parts, one towards granular layer, the spline microtubules (smt), and the other towards the cytoplasm – the columns and fibers, the viergruppe (vg).
The viergruppe (Fig. 15.8I) is a four-layered complex of tubules, fins and compartment. The outermost layer (vg1) is composed of parallel bikeeled tubules of 25 µ with cross-connections between the adjacent tubules.
These tubules are continuous with cytoplasmic microtubules of spline (smt). The next layer vg2 consist of parallel fins, about 30 µ tall. At places the fins of vg2 and keels of vg1 are connected by fibres. Below the fins of vg2 and keels of vg 1 is a row of short vertical plates, the vg3.
The microtubules act in two ways: They help in euglenoid movement of spermatozoid or function as cytoskeleton and support the flagellated band along with viergruppe.
Female cone (Fig. 15.6F) is not a compact structure. The female plants bear successive zones of appendages in order, of vegetative leaves, cataphylls, megasporophylls, cataphylls and vegetative leaves.
Each megasporophyll is a large structure (up to 30 cm in length) which bears two rows of ovules (Fig. 15.6G, H) of short stalks. Ovules of Cycas are largest in plant kingdom.
The ovules initiate on the margin of megasporophyll. The cycadean ovule (Fig. 15.6I) is composed of nucellus, which is the female gametophyte (FG), surrounded by an envelope, the integument. In a young ovule a single hypodermal archesporial cell divides to from primary sporogenous cell. The former by replicated divisions gives rise to massive nucellus.
The intregument appears as a ring like outgrowth from the base of the nucellus and extends beyond it to form a micropylar tube. The distinction between the nucellus and integument is not true.
The sporogenous cell functions as a megaspore-mother-cell (Fig. 15.9A) and divides meiotically to form a row of three-cells, the upper dyad cell and two megaspores. Of these two, the lower one is functional. The nucleus of the functional megaspore divides forming many nuclei but without walls. Ultimately, the walls are laid down in free-nuclear female gametophyte.
At the microplylar end of female gametophyte, 2-8 cells become conspicuous by their; large size, dense cytoplasm and prominent nuclei and differentiate as archegonial initials. The nucleus of an archegonial initial divides forming a small neck initial and large central cell (Fig. 15.9D). The former divides to form a neck of four cells. The central cell divides to form an ephemeral venter canal nucleus and a large egg cell. The nucleus of egg enlarges up to 500 microns and can be seen by the naked eye.
Pollination is effected by wind. Pollen grains are caught in pollination drop secreted at the tip of the ovule and are sucked in through micropyle to pollen chamber. Pollen grains germinate forming pollen tube, at the tube cell end. Pollen tube penetrates the nucellar tissue and acts as haustorium to dissolve nucellar cells, instead of the sperm carrier.
In this respect, the male gametophyte of Cycas is considered to be most primitive among the living gymnosperms. On rupture of pollen tube the sperms and pollen tube cytoplasn are released in the archegonial chamber. Pollen tube cytoplasm is osmotically rich causing plasmolysis of neck cells, facilitating the entry of motile sperms.
Cyas is the favorite choice, as a most primitive gymnosperm because of its fern-like features, primitive male and female gametophytes. Hence, it is described as ‘Living fossil’.
Fertilization is possible when a sperm casts off its ciliary band, its nucleus approaches the egg nucleus and gradually sinks into it. The second sperm degenerates.
A free-nuclear proembryo (Fig. 15.10A) results upon repeated divisions of zygote and its progeny. Wall formation occurs when there are 256 or more free nuclei in C. revoluta, whereas it is 512 in C. circinalis. Wall formation begins from the basal end, whereas cells at the upper end elongate to form a suspensor. The suspensor elongates to push the developing embryo into nutrient-rich female gametophyte. A remarkable feature is the length of the suspensor which may attain a length of 8 cm when uncoiled.
The layer of cells covering embryonal mass is the cap. The first region to differentiate in the embryo is coleorrhiza, differentiation of shoot apex occurs at the embryonal mass. As a result of meristermatic activity at a cylindrical shoot apex develop the cotyledons.
A simple polyembryogeny is possible when eggs in more archegonia get fertilized. However, the centrally placed embryo grows virgorously, takes the lead and matures.
The embryo grows slowly, takes about a year, to mature. On maturity of the seed, the integument is an outer fleshy layer brightly coloured, followed by a stony layer and thin inner fleshy layer. At this stage the female gametophyte occupies a large volume and the nucellus is thin and papery.
Seed germinates on emergence of root tip from the micropylar end. The cotyledons remain in seed and perform haustorial function. Germination is complete on the emergence of first primary leaves.