Grass Seedling

Grass seedlings were the subject of Charles Darwin's final experiment in 1880. Darwin covered the tips of grass seedlings with tiny opaque caps and exposed them to directional light. Uncapped seedlings bent toward the light within hours, while capped seedlings grew straight up, ignoring the light source entirely.

Darwin concluded that 'some influence is transmitted from the upper to the lower part, causing the latter to bend.' This influence was later identified as auxin by Frits Went in 1926. Grass seedlings respond to light direction within 30 minutes, representing some of the fastest tropism responses in the plant kingdom.