Every plant lives two lives, and pollen is one of them
Plants cycle between two distinct bodies: one carrying a single set of chromosomes, the other carrying two. In mosses the single-set generation is the green plant you see. In flowering plants it has shrunk to a handful of cells, and a grain of pollen, often just three cells, is an entire generation in miniature.
The cycle works like a relay. A mature sporophyte, the diploid generation, uses meiosis to make haploid spores with half the chromosomes. Each spore grows into a multicellular gametophyte, which later produces eggs or sperm by ordinary cell division. When two gametes fuse, the diploid zygote grows into a new sporophyte, and the loop starts again. All land plants and most algae reproduce sexually this way. Animals do not: they make gametes directly, so they have no multicellular haploid stage, apart from oddities such as insects whose males hatch from unfertilised eggs.
The balance between the two phases varies enormously. In the sea lettuce Ulva lactuca both generations live independently and look virtually identical. In liverworts, mosses and hornworts the gametophyte dominates, and the sporophyte depends on it for water, minerals and extra food even when it can photosynthesise. Vascular plants flip this: a fern's gametophyte is a small flat prothallus that briefly feeds the young sporophyte, and in seed plants the female gametophyte develops wholly inside the sporophyte. Fossils show that the Devonian ancestors of vascular plants had two generations of roughly equal complexity.
The idea began with animals. Adelbert von Chamisso, watching colonial salps between 1815 and 1818, and Japetus Steenstrup, studying flukes in 1842, described sexual and asexual generations taking turns. The term was later reserved for plants. Wilhelm Hofmeister demonstrated the alternation in plant form in 1851, and in 1874 Eduard Strasburger showed the underlying swap between diploid and haploid nuclei.
The two kinds of alternation do not always match. In many red algae one nuclear phase takes two different bodily forms, and some ferns that abandoned sex keep alternating generations with no change in chromosome number. Where one generation lives inside the other, researchers note they behave like a single organism, and 'alternation of phases' may be the better name.
Source: Alternation of generations
