Then spores are the units of gametophytes. As in cycas there is heterospory so the development of male and female gametophytes takes place from microspores and megaspores respectively.
Male Gametophyte:- The microspore is the first cell or unit of male gametophyte.microspore is a spherical structure which has two walls, an outer thick exine and inner thin intive. In the center is nucleus which is haploid (x) and the cytoplasm.
Development of male gametophyte before pollination:- The microspore present in microsporangium begins to germinate. First of all microspore divides into two to form small prothallial cell and bigger antheridial cell. The antheridial cell by division produces a smaller generative cell in contact with prothallial cell and a large tube cell. Thus the male gametophyte is endospermic and begins its development within microsporangium. Due to loss of water sporangial wall breaks upalong the lower side so what the sporangia become boat shaped and there called microspores come out in the air for pollination.
Pollination:- the microspores or pollen at 3 called stage are light and dry so they may be easily blown away on pollination by wind. At the time of pollination a ground cells at the apex of nucellus break down and form a drop of mucilage which oozes out through the micropyle or ovule. In the absence of any carpel distinguishable into stigma, style and ovary the pollen grains fall directly on the nucilage drop and as the drop driesup the pollen grains (3 celled microspores) are drawn into the pollen chamber below.
Development of Male Gametophyte after pollination:- Further development of male gametophyte from three celled pollen grain of microspore takes place in the pollen chamber of ovule. Due to the enlargement of the tube cell the exine ruptures and the intime protrudes out in the form of a pollen tube which serves as nutritive haustorium. This breaks by nucellar tissue till it reaches the neck of archegonium. The generative cell divides into a stalk cell and a body cell. The prothallial cell penetrates into stalk cell and at the time of fertilization the body cell enlarges and two blepharoplasts develop one at each pole of nucleus of the body cell. Body cell divides into two antherozoids which are multiciliate. Antherozoids or sperms are large and visible by naked eye. The sperms move, pollen tube ruptures and antherozoids pass into archegonial chamber.
Female gametophyte:- During development of ovule the central functional megaspore in the unit of first cell of female gametophyte. The functional megaspore increases in size and its nucleus divide by by a free nuclear division into many nuclei. Now the wall formation is initiated, at first between peripheral nuclei and continues in centripetal direction untill the female gametphyte is entirely cellular.
This is known as female prothallus of endosperm and has uninucleated cells. As the gametophyte develops the nucellus is used up and crushed and at maturity is represented by a papery layer.
The development of archegonoium takes place from the cells of endosperm and female gametophyte and there may be 2 to 6 archegonia in a single ovule. Any superficial cell of gametophyte at the micropyler and functions as archegonial initial cell. This initial cell increases in size and divides by transverse division inot a primary neck cell and central cell. Prmiary neck cell divides by vertical division into two neck cells. There are no neck canal cells. The canal cell becomes covered by special layer cells forming the archegonial jacket layer. Nucleus of central cell divides into a short lived ventral canal nucleus and egg nucleus. Egg nucleus enlarges. Th ventral canal nucleus soon gets disorganised. Archegonium is now ready for fertilization. Mature archegonium is an oval shaped structure with two neck cells, a ventral canal nucleus and an egg nucleus.
Fertilization: At the time of fertlization the end of the pollen tube with tube nucleus and the sperms grow towards the embryo sac the turgid end of pollen tube bursts and discharges its contents in the liquid of the chamber above archegonia towards the necks of which the sperms swim and make their way down to the egg cell. As the sperm passes into the cytoplasm of the egg its own cytoplasmic membrance along with the cilian slip off while the nucleus with the female nucleus forming the oospore or zygote.
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