In this article, we propose to discuss about the life cycle of angiosperms with diagram.
With the development of the seed, the life cycle of Angiosperms comes to a close. If we now trace the life-history (Fig. 432) we shall find a regular alternation of generations as in most groups of plants.
The plant is a sporophyte with 2n or diploid cells. The flower is the reproductive shoot or the storobilus with microsporophylls or stamens and megasporophylls or carpels. The microsporophyll contains microsporangia or pollen sacs and the megasporophyll contains integumented megasporangia or ovules.
Within the microsporangium are many microspore mother cells and within the megasporangium there is a single megaspore mother cell. Reduction division or meiosis initiates the gametophytic or haploid generation with n cells resulting in innumerable microspores of pollens and four megaspores.
Each microspore germinates developing a micro- or male gametophyte, i.e., pollen with pollen tube and the functional megaspore forms a mega- or female gametophyte or embryo sac. In the microgametophyte there is no prothallial cell and the generative cell with the vegetative nucleus represent a much reduced antheridium.
In the megagametophyte the antipodal cells are female prothallial cells and the egg apparatus represents a reduced archegonium. The naked sperm cell or gamete now fertilizes the egg cell completing the gametophytic generation and reverting to the sporophytic or diploid generation.
The gametophytic generation, therefore, is extremely brief and completely endosporous (i.e., confined to the walls of the microspore and the megaspore, if we remember that the pollen tube is an extension of the intine). The product of fertilization, the oospore, forms the embryo which, after a period of dormancy in the seed, germinates into the new sporophytic plant.
It should be pointed out again that the gametophytic generation in Angiosperms is reduced to the extreme. As a result, the seed represents three generations. The seed coats (integuments) and the funicle of the seed represent the first sporophytic generation while the embryo within it represents the Second sporophytic, i.e., a third generation.
The intervening second generation is represented by the short-living gametophyte (embryo sac) within which the embryo and the endosperm develop. The perisperm, if present, also represents the first sporophytic generation.
The endosperm of the Angiosperms is something new in the evolutionary history of plants. The endosperm of the Gymnosperms and the heterosporous Pteridophytes is formed of haploid gametophytic tissue. But, because of triple fusion, the Angiospermic endosperm is neither diploid nor haploid but is triploid. So, it belongs neither to the gametophytic nor to the sporophytic generation but forms a separate entity within the seed.
