Why Would a Plant Waste Pollen on the Breeze? Because It Is Cheaper Than Flowers

Moving pollen from one plant to another requires either a carrier or luck. Paying an animal means producing nectar, scent, colour and a landing surface, all of which cost energy, and it delivers pollen with reasonable precision to another flower of the same species. Releasing pollen into moving air costs nothing beyond the pollen itself and delivers almost none of it anywhere useful, so the strategy works by producing an enormous quantity and accepting the loss. Which approach is cheaper depends on how far apart the plants are, how dense the vegetation is, how much air moves through it and whether suitable animals are available at the right season.

The features follow directly from the method:

• •Small, dull flowers with no petals worth the name and no scent
• •No nectar at all, since nothing needs attracting
• •Anthers exposed on long filaments, dangling clear of the plant
• •Large feathery stigmas presenting a wide surface to catch what arrives
• •Enormous quantities of very small, light, smooth pollen
• •Flowering before the leaves open in many trees, so the air is unobstructed

The strategy dominates in particular situations rather than being scattered randomly. Grasses use it universally, which suits plants growing in dense single-species stands where a neighbour of the same kind is always close and where wind moves freely. Many temperate trees use it, including oak, birch, hazel, alder and all the conifers, and flowering before leaf emergence is a clear adaptation to it. It is far rarer in tropical forest, where species are mixed and a conspecific may be hundreds of metres away, so animal carriage pays. Several lineages have switched between strategies repeatedly, and island plants show a recurring shift towards wind where pollinating animals are scarce.

Measuring where wind-carried pollen actually ends up produces results that matter for agriculture and for allergy alike. Most grains fall within tens of metres of the plant that released them, since they are heavier than air and settle steadily, and concentration falls sharply with distance. A small proportion is carried into rising air and travels enormously further, with birch pollen routinely detected hundreds of kilometres from any birch and grains recovered over open ocean. That long tail is why allergy sufferers react in city centres with no vegetation nearby, and why isolation distances for seed crops have to be generous. Pollen also survives in sediment for thousands of years, which is what makes reconstructing past vegetation possible.

The connection between the strategy and allergy is direct rather than incidental. Pollen carried by animals is sticky, heavy and produced in modest quantity, so very little of it is in the air. Pollen released to the wind is small, light, smooth and produced in quantities measured in millions per plant, which is precisely what is needed to be inhaled deeply in large numbers. The immune system of a susceptible person responds to proteins on the surface of those grains. That is why grass, birch and ragweed dominate allergy statistics while insect-pollinated flowers, which are far more visible and are blamed constantly, contribute almost nothing. Pollen counts published in season measure exactly these species.

Paying an animal costs nectar, scent and colour and delivers pollen precisely, while releasing it to the air costs nothing beyond the pollen and delivers almost none of it, so the strategy works by enormous overproduction. Dull flowers, no nectar, dangling anthers and feathery stigmas follow from it. Grasses and many temperate trees use it, and the small light abundant pollen it requires is exactly what causes hay fever.