Pollen

A seed plant alternates between two generations: the plant you can see, which produces spores, and a much reduced generation grown from those spores, which produces gametes. In a flowering plant the second generation is almost invisible — an ovule containing a handful of cells inside the flower, and a pollen grain of two or three cells on the outside. Both are complete organisms of that generation.

So a pollen grain is not a sperm cell; it makes sperm cells. And it is not a seed: a seed contains an embryo of the next full plant, formed after fertilisation, with a food store and a coat. Pollen arrives before any of that has happened.

Grains are produced in the anthers, in enormous numbers where the wind does the carrying and in more modest ones where an animal does. A single ragweed plant can release a billion grains in a season; an orchid packages its entire output into two waxy masses and glues them to one insect.

The outer wall, the exine, is built from sporopollenin — a polymer whose exact structure is still argued about and whose resistance is not. It survives strong acids, strong alkalis and the digestive enzymes of anything that eats it, which is why the standard laboratory method for extracting fossil pollen involves dissolving away everything else.

The exine is also sculptured, and the sculpturing is diagnostic. Spines, ridges, reticulate networks and the number and position of the apertures the tube emerges through differ between families and often between genera. A palynologist counting grains in a sediment core is reading a dated record of which plants grew nearby, one centimetre of mud at a time.

Colour varies far more than the yellow of the popular imagination: pollen ranges from white through yellow and orange to deep red, purple, green, grey and almost black. Beekeepers use the colour of the loads coming into a hive to work out what the colony is foraging on.

Wind transport is cheap per grain and catastrophically inefficient per delivery, so wind-pollinated plants compensate with volume — and with flowers stripped of everything that costs money. No petals, no scent, no nectar, anthers dangling in the airflow and feathery stigmas held out to catch what passes. Grass, most northern trees, and every cereal crop work this way.

Animal transport is expensive per visit and vastly more accurate, because an animal that has learned one flower species tends to visit more of the same. That behaviour — floral constancy — is what makes an insect worth paying: a visitor that sampled randomly would deliver mostly the wrong pollen.

• Wind: enormous production, short effective distance for most grains, occasional very long transport.
• Animals: fewer grains, better aimed, dependent on the visitor’s behaviour and grooming.
• Water: rare, and mostly in seagrasses, where pollen is released in filaments that drift to stigmas.
• Self-pollination: no transport at all, in plants whose flowers pollinate themselves before opening.

A grain arriving on a stigma is still not finished. Compatible pollen is hydrated by the stigma and allowed to germinate; incompatible pollen — including, in many species, the plant’s own — is recognised and stopped. Self-incompatibility is a genetic system, and it is what keeps a plant that carries both sexes in the same flower from simply fertilising itself.

Germination is the grain growing a tube out through one of its apertures and down through the style towards the ovule. The tube is a single cell that extends only at its tip, and it can travel a long way: in maize, down a silk that may be thirty centimetres long, which is on the order of ten thousand times the diameter of the grain that started it.

The tube carries two sperm cells. In flowering plants both are used — one fertilises the egg to make the embryo, the other fuses with a second cell to make the endosperm, the tissue that feeds the seedling. Double fertilisation is unique to flowering plants and is the reason a grain of wheat is mostly food.

This is also where pollination and fertilisation separate, and why NatureHQ keeps insisting on the distinction. Pollination is arrival. Fertilisation is what may happen afterwards, sometimes hours later, sometimes — in some orchids and oaks — many months later. A flower can be thoroughly pollinated and set no seed at all.

Pollen is the only substantial protein source most bees ever encounter. Adult bees run on nectar; larvae are built out of pollen, and a colony’s ability to raise young is limited by how much of it comes in. That is why bees have structures for carrying it — the corbicula on the hind leg of honey bees and bumblebees, dense brushes of hair on the abdomen in many solitary bees — and why a foraging bee grooms pollen deliberately into them.

This creates the tension at the centre of pollination. Every grain a bee successfully packs away is a grain that will never reach a stigma. The plant is trying to get its pollen carried; the bee is trying to collect it as food. What the plant recovers is the fraction the animal fails to groom off — which is why flowers place anthers where a visitor cannot easily reach, and why the least tidy visitors are sometimes the most useful.

• What is the molecular structure of sporopollenin?

  Why it matters: The material is central to plant reproduction and to the entire fossil pollen record, and its exact chemistry has resisted determination for over a century precisely because it resists everything else.

• How much pollen actually moves long distances?

  Why it matters: Most grains fall close to the parent, but the rare long-distance events determine gene flow between populations — and rare events are the hardest to measure.

• How far can pollen production bias be corrected in fossil reconstructions?

  Why it matters: A pine produces orders of magnitude more pollen than an insect-pollinated tree, so a sediment core over-represents the wind-pollinated flora unless the correction is right.