The following points highlight the seven main steps involved in reproduction process of Cycas. The steps are: 1. Male Gametophyte Development 2. Pollination 3. Male Gametophyte Development 4. Mature Sperm 5. Female Gametophyte Development 6. Development of Archegonium 7. Fertilisation.
Reproduction Process of Cycas: Step # 1.
Male Gametophyte Development (Before Pollination):
Microspore is the first cell of the male gametophyte. The microspore (Fig. 8.45 A) starts germination in situ, i.e., while still inside the microsporangium. The microspore nucleus divides into two resulting into the formation of two unequal cells. The smaller one is called prothallial cell while the large one represents the antheridial cell (Fig. 8.45B).
The prothallial cell does not divide any further while the antheridial cell divides to form a generative cell near the prothallial cell and a large tube cell (Fig. 8.45C). The shedding of the microspores takes place at this three-celled stage consisting of prothallial cell, generative cell and tube cell.
Reproduction Process of Cycas: Step # 2.
On dehiscence of microsporangia by a longitudinal slit, the three-celled microspores are blown away by wind for pollination. The pollination in Cycas is, therefore, anemophyllous. On the other hand, certain cells of the nucellar beak are dissolved to form a drop of mucilage. Mucilaginous drop may also be secreted by a few cells lining the nucellus and free part of the integument.
This mucilaginous drop oozes out from the micropyle to represent the pollination drop. Certain microspores, present in the air at their 3-celled stage, are entangled in this pollination drop (Fig. 8.46), which is soon withdrawn into the micropylar chamber through the micropylar canal.
The micropylar chamber at the top of the nucellar beak gets sealed up due to the drying of the mucilage. Thus completes the process of pollination.
Niklas and Norstog (1984) opined that wind alone is not sufficient to effect pollination in some of the cycads. Tang (1987) reported insect pollination in Zamici pumila. According to Wilson (1993) pollination in Bowenia serrulata is amphiphilous with pollens being transferred by the weevil Tranes subopaca.
Reproduction Process of Cycas: Step # 3.
Male Gametophyte Development (After Pollination):
Further development of male gametophyte starts on the nucellar surface of the ovule. The exine breaks up and the intine comes out in the form of a pollen tube. The latter penetrates the nucellar tissue. The pollen tube may be branched or un-branched and acts as an absorbing organ or haustorium.
The generative cell divides soon and forms a stalk cell and a body cell. The stalk cell does not divide any further but due to the accumulation of the starch grains it enlarges in size. The body cell also becomes large- sized.
Now the prothallial cell pushes into the stalk cell and presses it against the cell (Fig. 8.45D). Further development of the male gametophyte takes place after a gap period of about 4 months. During this period the pollen tube penetrates through the nucellus and hangs down in the archegonial chamber.
Now two blepharoplasts appear, one at each pole of the nucleus of the body cells in a transverse position. Blepharoplast gives rise to the cilia.
The body cell divides in the place of the long axis of the tube to form two sperm mother cells (Fig. 8.45E), each having a single nucleus, a small amount of cytoplasm and a blepharoplast. Each sperm mother cell later on develops into a sperm. The sperms are liberated in the pollen tube by breaking of the sperm mother cell (Fig. 8.45F).
Reproduction Process of Cycas: Step # 4.
A mature sperm or spermatozoid contains 5-6 turns of spiral bands with thousands of cilia (Fig. 8.47). Twists of the spiral bands are directed from left to right. In Cycas revoluta the size of the sperm ranges between 180 to 210 μm and they are clearly visible to the naked eye. By means of their cilia the sperms move freely in . the pollen tube.
Ultrastructure of ciliate sperms of some Cycadaceae has been studied by Norstog (1967, 1974, 1977, 1990). The size of the flagellated male gametes of some cycads is given in Table 8.1.
Reproduction Process of Cycas: Step # 5.
Female Gametophyte Development:
The functional megaspore is the first cell of the female gametophyte. It is haploid in nature. The development starts in situ, i.e., within the nucellus (Fig. 8 .48). By absorbing the surrounding cells of the nucellus, the functional megaspore enlarges considerably. The megaspore nucleus divides mitotically to form several free nuclei.
A vacuole now appears in the centre. It pushes free nuclei and cytoplasm of the megaspore towards the periphery. The wall formation starts from the periphery towards the centre and soon a cellular tissue is resulted. This represents female gametophyte or endosperm.
The cells of the endosperm are larger below and smaller towards the micropylar end. The nucellus cells surrounding the endosperm are utilized during the endosperm development, and, therefore, an endosperm jacket or spongy tissue is formed.
Reproduction Process of Cycas: Step # 6.
Development of Archegonium:
An archegonium develops (Fig. 8.49) from a single superficial initial near the micropylar end of the endosperm. This represents archegonial initial. It get enlarged and can be easily differentiated from the surrounding cells of the endosperm.
It divides periclinally to form an outer primary neck cell and an inner central cell (Fig. 8.49B). Primary neck cell divides anticlinally to form two neck cells, which form the neck of the archegonium.
According to De Silva and Tambiah (1952) the neck cells in C. rumphii divide by a second anticlinal division or rarely by a periclinal division to form a neck of 4 cells arranged either in a single layer or in two tiers of two cells each. The inner central cell enlarges in size and its nucleus divides into two nuclei, which represent the ventral canal nucleus and egg nucleus. No neck canal cell is formed in Cycas.
According to Ikeno (1898) no wall is formed between the ventral canal nucleus and egg nucleus in C. revoluta, and, therefore, there is no ventral canal cell. In the later stages the ventral canal nucleus also gets disorganized.
A mature archegonium consists of two neck cells and an egg. The egg in Cycas is largest amongst all living plants. Two to eight archegonia are formed in Cycas revoluta.
The archegonial neck opens in an archegonial chamber formed by the depression in the endosperm Chamberlain (1935) opined that the archegonial chamber is formed because of the rapid growth of cells around the archegonial region to leave a depression near the archegonia.
Reproduction Process of Cycas: Step # 7.
The pollen tubes containing the sperms and the tube nucleus grow downward penetrating the tissue of the pollen chamber. Latter is gradually digested and dis-organised by the growing tubes. The pollen tubes now hang between a cavity formed partly by pollen chamber and partly by archegonial chamber (Fig. 8.50).
In the end the pollen tube bursts and discharges its contents into the archegonial chamber. One of the sperms enters the archegonium through its neck and reaches up to the egg. In this process the cilia and the cytoplasmic membrane of sperm are stripped off and the fusion takes place between the egg nucleus and the naked male nucleus.
Two functions may thus be assigned to the pollen tube:
(i) It acts as a haustorium by absorbing the food for the developing male gametophyte, and
(ii) It functions as a sperm carrier.
In this way the fertilization in Cycas takes place with the help of motile ciliated sperms, a phenomenon known as zoidogamy. This phenomenon was first reported by Ikeno (1896). Zoidogamy in Cycas is accompanied by the pollen tube formation, a phenomenon called siphonogamy. The pollen tube acts as a sperm carrier.