Alternation of generations refers to the idea that the life cycle of a moss, for instance, comprises two distinct phases: \u00a0one in which the cells are haploid (n), and the other in which they are diploid (2n). \u00a0Haploid means they have half the number of chromosomes; diploid means they have the full possible number.<\/p>\n
Although all plants have it, moss provides a good example of alternation of generations. \u00a0People may not notice as they walk over it, but moss looks different depending on the time of its cycle. \u00a0Most often you are probably seeing the gametophyte (n) generation – likely the green, soft carpet you imagine in forests or bogs. \u00a0However, if the carpet has brown stalks rising from it, you might well be walking over the sporophyte (2n).<\/p>\n
Let’s enter the cycle at the spore stage. \u00a0A spore is a haploid (n) cell, often borne on the wind, which can land and grow into a new individual – specifically, a new gametophyte. \u00a0If the spore is male, it grows into a sperm-producing gametophyte; if female, an egg-producing one. \u00a0In wet conditions, sperm swim from the male stalks to the female ones. \u00a0(This is why moss needs dampness at least some of the time.) \u00a0Like you’d expect, the union of sperm with egg produces a zygote. \u00a0While the sperm and egg were each n, the zygote is 2n, since it receives n chromosomes from the sperm and n from the egg.<\/p>\n
The zygote starts to divide, eventually becoming a sporophyte found on top of what was the female gametophyte (which is where the sperm joined the egg). \u00a0Cells in the mature sporophyte undergo meiosis, a special kind of division in which each cell produced receives only one chromosome from each set of two available. \u00a0Whether it receives the chromosome the sperm brought, or the one from the egg, is determined by chance: \u00a0hence the variability of sexual reproduction. \u00a0Either way, these new cells are haploid (n): \u00a0in fact, they will mature into spores. \u00a0The spores are released when it’s dry and windy, and the life cycle begins anew.<\/p>\n
I’ll be mentioning more about this topic in future posts; \u00a0this was just a toe in the water:)<\/p>\n