The plant body of Cycas is sporophyte (2n). It forms two different types of spores, microspores and megaspores in two separate plants. The spores are haploid which develop into male and female gametophytes.
Male gametophyte
The microspore (=pollen grain) is the first cell of the male gametophyte. Each microspore is rounded and uninucleate covered by two coats; the outer exine and the inner intine. Germination of microspore takes place before being shed from microsporangium.
The microspore divides into two unequal cells i.e. a small prothallial cell or vegetative cell and a large antheridia cell. The antheridia cell divides into two unequal cells; one small generative cell lying close to the prothallial cell and one large tube cell. At this three celled stage, pollination takes place.
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Female gametophyte
The female gametophyte is monosporic. Megaspore is the first cell of the female gametophyte of Cycas. It develops entirely within the mega sporangium or ovule.
The megaspore is surrounded by a tough membrane which later enlarges. Its nucleus divides several times forming numerous free nuclei. The nuclei are distributed in the cytoplasm of the megaspore.
Next a vacuole appears at the centre. Thus, the free nuclei are distributed in the peripheral cytoplasm. Cell wall formation between the free nuclei takes place from the periphery extending towards the centre until the megaspore vacuole is filled up by a cellular tissue. Thus, a solid mass of gametophytic tissue called endosperm is formed, which is haploid.
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The gametophytic tissues show two regions viz, (I) the region of larger cells near the chalaza and (ii) the region of smaller cells towards the micropylar end.
Towards the micropylar end 2-8 archegonia are developed from the cells of gametophytic tissue. At first, the archegonia initial divides transversely to form a neck initial and a central cell.
The neck initial forms two neck cells. The nucleus of the central cell divides to form a central canal nucleus and an egg nucleus.
Each mature archegonium consists of two neck cells, a ventral nucleus and an egg. There is no neck canal cell. In a mature female gametophyte, the archegonia are found below a depression called the archegonia chamber.