Each pollen grain is a tiny organism in its own right, not a sperm
Pollen is often described as plant sperm, but each grain is really a miniature male plant, a gametophyte that produces sperm cells. Its armour, sporopollenin, is so tough that pollen survives in rock for hundreds of millions of years, letting scientists reconstruct ancient forests and climates from a pinch of sediment.
Seed plants make pollen in the anthers of flowers or the male cones of conifers. Inside each grain sit a vegetative cell, which grows the pollen tube, and a generative cell, which divides into two sperm. Grains form when each diploid pollen mother cell undergoes meiosis to yield four haploid microspores, which then build an inner wall, the intine, and an outer wall, the exine. The exine is the part that fossilises.
Shapes are astonishingly varied and often specific enough to identify a genus, species or even an individual plant. Forget-me-not grains measure just 2.5 to 5 micrometres; corn grains reach about 90 to 100. The surface may bear spines, net-like ridges or furrows and pores, called apertures, that let the grain shrink and swell with moisture, a process named harmomegathy. Eudicots typically have three furrows. Pine, fir and spruce pollen carries spongy air sacs that help it stay aloft.
Wind-pollinated plants such as pines, and flowering plants with inconspicuous blooms, release huge quantities of light pollen. Insect-pollinated plants produce heavier, sticky, protein-rich grains, and some insects and mites, called palynivores, specialise in eating them. On a compatible stigma, a grain sends a tube down the style to an ovule, carrying the sperm in its tip. In pines and gnetophytes the sperm lack flagella, whereas in cycads and ginkgos they swim with many.
Pollen first appears in the fossil record in the late Devonian, when it cannot yet be told apart from spores. Palynology, the study of pollen, now aids archaeology, forensics, dating rock layers and tracing past vegetation. Wind-borne pollen is also behind pollinosis, and grass pollen specifically causes hay fever. Such allergies are common in temperate and polar zones where pollen is seasonal, and rarer in the tropics.
Source: Pollen
