Many mistakenly assume pollen to be the sperm of flower plants, produced by male flowers to fertilize female ones. This is not the case. A pollen grain is in fact an independent, but highly reduced, individual.
In the diplontic life cycle familiar to animals, the gametes (sperm and egg cells) fuse to form a diploid zygote which is diploid, i.e. it carries two copies of each gene, one from each parent. The zygote then grows to form a body in which all cells are diploid, until new gametes are produced by meiosis, which cuts their DNA content in half.
The haplontic life cycle, found in fungi, is in some ways the opposite. The zygote immediately undergoes meiosis to form haploid spores, which then grow into haploid adult bodies (the mycelia). The filaments of the mycelia then fuse to form new diploid zygotes, with no new meiosis needed.
Plants went for something stranger: the haplodiplontic life cycle.
The newly formed zygote grows into a diploid body, as in animals. This body then generates spores by meiosis, and these spores in turn grow into a haploid body, as in fungi, which produces sperm and egg cells. The spore-making diploid body is known as sporophyte, and the gamete-making haploid body as the gametophyte. Two distinct multicellular bodies, for one life cycle!
The relation between sporophyte and gametophyte varies in plant evolution. In ferns, we see a true separation: the tiny, leafy prothallus which is the gametophyte, and the more familiar plant with fronds which is the sporophyte. In mosses, most of what you see is the gametophyte: the sporophyte is only generated to produce spores, and never detaches from the mother gametophyte. It is visible in moss as the yellow stalks growing over the green carpet:
(tall yellow sporophyte growing out of the low, green gametophyte)
But in flower plants, it's quite the opposite. When you look at a tree, you see a massive growing sporophyte. Its spores are kept close, to develop in the shelter of a flower, and they grow into a tiny gametophyte, genetically distinct from the mother sporophyte but unable to survive on its own. Female gametophytes never leave the flower: they develop into the embryo sac, comprised of a single egg cell and some nurse cells. But the male gametophyte does leave, and that is what a pollen grain is: a tiny haploid plant that is carried away from the mother by wind or rain or bees, laying dormant until it falls into a receptive flower, and then it starts producing and releasing actual sperm.
So basically what flower plants do is as if men, to reproduce, had to give birth to tiny goblins who run around to have sex with women and impregnate them. Hope this helps
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