Question

5. Describe and illustrate all of the following life cycles: mosses, ferns, gymnosperms, and angiosperms.

Answer 1

Mosses belong to Bryophytes and in bryophytes, gametophyte is dominant and sporophyte depends on gametophyte. In mosses two types of spores are formed on male and female gametophytes. Antheridia are male reproductive structures developed on male gametophytes. Archegonia are female reproductive structures formed on female gametophytes. The sperm and eggs are formed by mitosis in both male and female gametophytes. Fertilization results in formation of embryo and embryo develops into saporophyte within archegonium. Sporophyte is nourished by gametophte since sporophyte is nonphotosynthetic. Meiosis of spores in sporophytic capsule result in haplod spores that develop into gametophyte. Mossesexhibit haplodiplontic lifecycle. The diploid sporophyte produces haploid gametophyte.

Ferns are pteridophytes and are homosporous producing only a single type of spore. The gametophyte produces both male and female reproductive organs. The sporophyte is photosynthetic and is independent from gametophyte. Meiosis occurs in sporangia formed in sporophytes resulting in haploid spores. These spores undergo mitosis forming gametophytes. The ferns also exhibit haplodiplontic life cycles in which the sporophytes are dominant.

In Gmnosperms and Angiosperms, the life cycles exhibit great similarities. The lifecycles are thought to be diplontic because of reduced gametophytic generation. The sporophyte is dominant. In both gymnosperms and angiosperms heterospory is observed and the spores or pollen reach the eggs through wind, insects or animal pollination methods and requirement of water is rarely observed and is restricted in plants growing in waters (hydrophytes). The sporophyte diploid and male and female plants may be same or seperate. Gymnosperms contain male and female cones and the seeds are naked without coat.

In microsporangium, the cells within the anther undergo meiosis to produce microspores. microspores undergo mitotic divisions resulting in formation of multicellular pollen grain. The megasporangium is protected by two layers of integuments and the ovary wall. Within the megasporangium, meiosis produces four megaspores of which, three are small and one is large. The large megaspore produces the embryo sac. Fertilization occurs by germination of pollen and the pollen tube grows toward the embryo sac. The sporophyte generation is dormant state and is protected by the seed coat. The major difference between angiosperms and gymnosperms is angiosperms contain seeds enclosed in seedcoat while seeds of gymnosperms are naked.