Pollination

From anther to ovule

The mechanics of one successful pollination event:

1. An anther splits open and releases pollen grains. Each grain is a tiny haploid male gametophyte enclosed in a tough, sculpted wall.
2. A vector — wind, animal, water — carries the grain to a receptive stigma of the same species.
3. The stigma's surface chemistry recognizes the pollen and (usually) admits only conspecific grains. Self-incompatibility loci can also block self-pollination.
4. The grain hydrates and germinates a pollen tube that grows down through the style under genetic control of the female tissue.
5. The tube reaches the ovule and discharges two sperm cells. One fertilizes the egg → embryo. The other fertilizes the central cell → triploid endosperm. (This double fertilization is unique to flowering plants.)
6. Each fertilized ovule becomes a seed; the surrounding ovary tissue ripens into a fruit.

The pollen tube can be astonishingly fast — a maize tube grows the length of the silk (often 30 cm!) in under 24 hours. It is the longest single cell in the plant kingdom.

Pollination by every vector

Plants pollinate by every physical means available, and each vector imposes a recognizable design.

	Wind	Insect (bee)	Bird	Bat	Water	Self
Term	Anemophily	Entomophily	Ornithophily	Chiropterophily	Hydrophily	Autogamy
Flower size	Tiny, often clustered	Medium, showy	Medium-large, tubular	Large, open at dusk	Tiny, often submerged	Variable
Colour	Drab, green	Blue, yellow, UV-marked	Red, orange	Pale, dull	Often colourless	—
Scent	None	Sweet, light	None (birds have weak smell)	Musky, fruity, strong	None	—
Reward	None	Nectar + pollen	Copious dilute nectar	Copious nectar + pollen	None	—
Pollen	Small, smooth, dry, abundant	Sticky, sculpted, electrostatic	Sticky	Sticky, large	Filamentous or in slicks	Stays in flower
Stigma	Feathery, large, exposed	Compact, sticky	Compact	Compact	Often filiform	Adjacent to anther
Examples	Grasses, oaks, birches, ragweed	Apple, lavender, sunflower	Hibiscus, fuchsia, ohi'a	Agave, baobab, durian	Sea-grasses, Vallisneria	Peas, peanuts, many weeds

The clusters of co-occurring traits in each column are called pollination syndromes. They are not perfect — many flowers attract more than one pollinator class — but the correlations are strong enough that an experienced botanist can usually predict the pollinator from the flower alone.

A worked example — bee flowers

Bees see further into the ultraviolet than humans, and many flowers exploit it. To us a daisy is uniformly white or yellow; to a bee, the centre is a strikingly different colour from the petals — a UV "bullseye" pointing the bee straight at the nectar reward and, in passing, at the anthers and stigma. Bee-pollinated flowers tend to:

• Be blue, violet, yellow or UV-patterned (red is largely invisible to bees and avoided).
• Provide a landing platform — symmetric, robust petals.
• Be moderately deep, matching bee tongue length.
• Offer both sucrose-rich nectar and protein-rich pollen.
• Time their opening to morning, when bees are most active.

Buzz pollination is a beautiful refinement: blueberry, tomato, eggplant and roughly 8% of all flowering plants store pollen inside tube-shaped poricidal anthers. To get the pollen out, a bumblebee detaches its flight muscles from its wings and vibrates the anther at 200–400 Hz, shaking the pollen out like salt from a shaker. Honeybees cannot do this — which is why commercial tomato glasshouses bring in bumblebees, not honeybees.

When the syndrome breaks

Coevolution can produce extreme specializations. Charles Darwin, on receiving a Madagascar orchid (Angraecum sesquipedale) with a 30-cm-deep nectar spur, predicted there must exist a moth with a 30-cm tongue capable of pollinating it; the moth was found 41 years later (Xanthopan morganii praedicta). Fig trees and fig wasps are obligately bound to each other, and most fig species rely on a single wasp species; lose the wasp, lose the fig. These tight relationships are vulnerable: when the partner is gone, the entire reproductive system fails.

Pollination and food

Pollination is a quietly enormous economic input. Roughly 75% of the world's leading food crops benefit at least partially from animal pollination. Almonds, apples, melons, blueberries, cherries, cucumbers, squash and most stone fruit are heavily pollinator-dependent. Every February, roughly two thirds of all managed honeybee colonies in the United States are trucked to California for the almond bloom — a single crop now consumes most of a continent's bees.

Honeycrisp apple, the dominant supermarket variety in much of North America, is itself a pollination story: a 1960s Minnesota cross whose paternal parent is a Red Delicious-derived seedling and whose maternal parent (long misidentified as a Macoun seedling) is now known to be Keepsake. Without a pollinator carrying Keepsake's pollen to the right flower, no Honeycrisp.

Pollinator decline

Wild pollinators are in measurable decline across most of Europe and North America. Drivers include habitat loss (especially of unsprayed flowering field margins), neonicotinoid pesticides that impair bee navigation and reproduction at sublethal doses, parasitic mites and viruses (notably Varroa destructor in honeybees), and climate-driven mismatches between flowering times and pollinator emergence. Crop-yield drops in pollinator-limited fields are now documented for blueberry, cherry, apple and some squash, with shortfalls of 5–30% depending on region and crop.

Common misconceptions and pitfalls

• "Pollination = fertilization." No. Pollination delivers the male gametophyte to the stigma; fertilization happens days later when the pollen tube reaches the ovule and releases sperm.
• "All bees are honeybees." Honeybees (Apis mellifera) are one species in a global fauna of about 20 000 bee species, most of them solitary. For many crops, native solitary bees are more efficient pollinators than honeybees.
• "Wind pollination is primitive." It is derived. Many lineages — grasses, oaks, willows — secondarily reverted from animal to wind pollination after their ancestors were animal-pollinated.
• "Self-pollination is bad." Self-pollination guarantees seed set when pollinators or mates are absent. Many highly successful crops (peanuts, peas, wheat) self-pollinate.
• "Brightly coloured flowers attract everyone." Bees cannot see most reds; hummingbirds find blue uninteresting; bats are night-foragers and find pale flowers more readable. Different palettes target different pollinators.