The Web of Life (80-9) Shooting Seeds

 Shooting Seeds

Nastic movement was an early adaptation, and it has numerous variations.

Spore-producing plants, including mosses, liverworts, and ferns, propel their spores into the air via mechanical motion triggered by moisture loss. This spreads spores away from the parent plant.

Several flowering plants resort to explosive seed distribution. The conventional mode is a seedpod that is sprung against itself so that, as it dries, it gains torque stress: ready to burst at the slightest touch. The marsh geranium spreads seeds this way, as does the touch-me-not.

The squirting cucumber goes one better. Pressure builds within as its fruits mature, until an entire fruit shoots away, with seeds blasting out from the stalk end at high speed. A squirting cucumber may propel its seeds up to 13 meters.

As the squirting cucumber has self-contained seed dispersal, it most certainly does not want its fruit eaten. To that end the fruit is poisonous: loaded with a bitter, steroid-based poison: cucurbitacin.

Many flowers employ a spring-loaded mechanism to launch seeds or dust a pollinator. The flowers of legumes, such as peas, beans, and alfalfa, hold their stamens between paired petals which form a keel at the base of the flower.

The opening trigger requires just the right heft. Nothing happens if a tiny fly alights. But if a bee lands, the petals burst open. The stamens shoot upwards like an uncoiled spring, dusting the insect with pollen.

This is unpleasant to pollinators. Many bees learn to avoid the experience by alighting on the side of the flower and sipping nectar by cautiously reaching between petals.

Most mistletoe species produce sticky seeds which often adhere to birds, who scrape the seeds off when grooming. The dwarf mistletoe takes a different approach. The fruit of a dwarf mistletoe swells as the seeds within mature, until the fruit bursts, squirting seeds at 100 km/hr; fast enough to go far enough to reach a neighboring tree, thus propagating the parasite.

Even some seeds respond to humidity to get where they need to go. The storksbill plant launches seeds using a spring mechanism as its fruits dry. This is but the start of moisture-based movement. Once a seed is on the ground, its spiral awns (slender bristles) coil and uncoil as humidity changes. This propels a seed across the ground until it encounters a crevice. Then its twisting movement screws the seed into the ground. A storksbill seed manages its own self-burial.