geotropic

Geotropic is an adjective describing any movement or growth oriented with respect to gravity. The term derives from the Greek ge, meaning earth, and tropos, meaning turn. In biology, geotropism (gravitropism) refers to growth responses directed by gravity.

In plants, geotropism is a key mechanism by which roots and shoots orient themselves. Roots exhibit positive geotropism, growing downward toward gravity, while shoots show negative geotropism, growing upward away from gravity. The response is coordinated by gravity-sensing cells called statocytes, which contain dense starch-filled organelles known as amyloplasts that sediment under gravity. This physical cue influences the distribution of the plant hormone auxin: in shoots, higher auxin concentration on the lower side promotes elongation, causing the stem to bend upward; in roots, higher auxin on the lower side inhibits elongation, causing the root to bend downward. The result is a coordinated growth pattern that helps the plant stabilize and access resources.

Historically, geotropism was studied by early plant physiologists such as Charles Darwin and his son Francis, who observed tilted shoots bending toward gravity, and by later work that identified auxin as a driving factor in gravitropic responses.

In medicine, the term geotropic also appears in descriptions of eye movements, notably geotropic nystagmus in certain forms of benign paroxysmal positional vertigo (BPPV). In this context, geotropic nystagmus refers to fast eye movements directed toward the ground in the horizontal canal, helpful in distinguishing canalithiasis from cupulolithiasis.

See also: gravitropism, phototropism, auxin, statolith.