What is alternation of generations mean?

Answer 1

In most plants meiosis and fertilization divide the life of the organism into two distinct phases or "generations". * The gametophyte generation begins with a spore produced by meiosis. The spore is haploid, and all the cells derived from it (by mitosis) are also haploid. In due course, this multicellular structure produces gametes - by mitosis - and sexual reproduction then produces the diploid sporophyte generation. * The sporophyte generation thus starts with a zygote. Its cells contain the diploid number of chromosomes. Eventually, though, certain cells will undergo meiosis, forming spores and starting a new gametophyte generation. Two points revealed by plant life cycles: * Mitosis can occur in haploid cells as well as diploid ones. * A haploid set of chromosomes, and hence a single set of genes (one genome), is sufficient to control cell function in these organisms (but not in most animals). In fact, the gametophyte generation is the major stage in the life of mosses and an independent plant in ferns.