I’ve always found this topic a bit tricky. As a biology tutor, it’s time to give my own explanation.
Alternation of generations refers to the idea that the life cycle of a moss, for instance, comprises two distinct phases: one in which the cells are haploid (n), and the other in which they are diploid (2n). Haploid means they have half the number of chromosomes; diploid means they have the full possible number.
Although all plants have it, moss provides a good example of alternation of generations. People may not notice as they walk over it, but moss looks different depending on the time of its cycle. Most often you are probably seeing the gametophyte (n) generation – likely the green, soft carpet you imagine in forests or bogs. However, if the carpet has brown stalks rising from it, you might well be walking over the sporophyte (2n).
Let’s enter the cycle at the spore stage. A spore is a haploid (n) cell, often borne on the wind, which can land and grow into a new individual – specifically, a new gametophyte. If the spore is male, it grows into a sperm-producing gametophyte; if female, an egg-producing one. In wet conditions, sperm swim from the male stalks to the female ones. (This is why moss needs dampness at least some of the time.) Like you’d expect, the union of sperm with egg produces a zygote. While the sperm and egg were each n, the zygote is 2n, since it receives n chromosomes from the sperm and n from the egg.
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). Cells 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. Whether it receives the chromosome the sperm brought, or the one from the egg, is determined by chance: hence the variability of sexual reproduction. Either way, these new cells are haploid (n): in fact, they will mature into spores. The spores are released when it’s dry and windy, and the life cycle begins anew.
I’ll be mentioning more about this topic in future posts; this was just a toe in the water:)
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.