The movement of plants, while almost undetectable to humans, with certain exceptions, is as incessant, various and necessary to life as it is with animals. Growth movements, which arise from cell enlargement or numerical increase can be categorised into three types: Tropic movements, nastic1 movements and nutations2.
Tropic movements are responses to directional stimuli such as sunlight (called phototropism), gravity (geotropism), chemotropism (chemical substances), water (hydrotopism), electrical (galvanotropism), mechanical (thigmotropism) and wounding (traumatropism). Most tropic movements are directed towards the source of stimuli.
Nastic movements such as the opening or closing of flower petals at night are caused by response to factors such as diffuse light levels (photonasty) or temperature (thermonasty). Nastic movement primarily occurs in leaves, the structure of which can restrict movement in certain directions. The opening of flowers for example is caused by the more rapid growth of the upper side of the petals - this is termed epinasty. Conversely the rapid growth of the lower part of a plant organ is termed hyponasty.
Nutation is best illustrated by bean or pea plants and describes the characteristic twining of shoots and tendrils. This is the result of the inequality of growth rates in tissues at the periphery of the growing tip. More rapid growth usually takes place on the outermost and lower sides of the stem. This results in an upward swinging motion of the growing tip.
Turgor movements are among the most rapid of plant movements. The Mimosa pudica begins to react 0.075 seconds after the plant is stimulated. Stimuli which generate turgor movements include electrical shock, physical contact, insufficient water supply, injury or light changes. Many turgor movements are initiated by pressure changes in special plant cells or organs. Turgor movements are also responsible for the opening and closing of stomates which allow gaseous interchange between the plant and its surrounding atmosphere.
By far the most rapid movement in plants are to be found in the insectivores such as the Venus flytrap. These reactions must be quick enough to trap an alert insect, something even a speedy newspaper- wielding mammal may find difficult. In the Venus flytrap, the snapping together of its leafy cage is caused by a release of tension built up from differences in growth rate between the upper and lower surfaces of the leaf. The mechanism is triggered by stimulation to hairs on the surface of the leaf.