Plants do not show the type of movements that animals exhibit. Plants are rooted or anchored to a substratum and perform their activities while remaining stationary. However, plants also show certain movements of their body or body parts in response to changes in the environment. Most of these movements are hard to see; however with special techniques one can see that plants are in continuous motion. The movements that plants exhibit come under two broad categories, namely growth movements and turgor movements.
Growth movements are brought about by the unequal growth in different regions of the same organ. If shoot growth occurs at a faster rate alternately on the right and left sides of the axis, resulting in a zig-zag axis, the movement is said to be nutation. Examples: the inflorescence axis of wheat and barley. The weak stems of climbers and twiners as they coil around a support exhibit a movement called circumnutation. This spiral movement of stems is due to unequal growth on the two sides of the stem. The differential growth of young leaves, buds and floral parts resulting in the curving upwards of these organs, are movements coming under the category of hyponasty. This is because of a higher level of growth of the lower surface of the organ as a result of which the bud gets enclosed. The circinate coiling of fern leaves is an example of such movements. When the leaves mature and grow in size and the flower buds open, more rapid growth occurs on the upper surface. This results in unfolding of coiled leaves and opening of the flowers. This type of movement is called autonomic growth movement.
These responses of plants parts are initiated by external stimuli (paratonic). The movements result from changes in the turgor (that is, water) of given cells or cell groups. Unlike growth movements which are slow and produce more or less permanent results, turgor movements are rapid, and they are transient and repeatable. In some cases at least the phytohormone, auxin is the chief controlling agent. The examples of turgor movements are the opening and closing of the guard cells of stomata in plants. The contact movement of leaves in sensitive and carnivorous plants and the sleep movement of leaves in certain plants, are also examples of turgor movements.
When a plant organ exhibits movements regardless of the direction of the stimulus, it is called nastic movement. The direction of the movement of the organ is determined by the structural characteristic of the responding organ. The movement in plant organs in response to day and night, as in sleep movement, is called nictynasty.
These movements always bear a definite relation to the direction from which the stimulus acts. The stimuli involved in tropic movements are gravity (geotropism), light (phototropism), contact (thigmotropism), water (hydrotropism) and chemicals (chemotropism).
These are movements of plant organs under the influence of gravity. The primary root always grows downward in the direction of gravity (positive geotropism) and the stem grows upward away from gravity (negative geotropism).
It is the movement of plant organs in response to the direction of light. The stems generally show a curvature towards the source of light (positive phototropism) and the roots grow away from the source of light (negative phototropism). Since the leaves are attached to the stem, at right angles to the incident sunlight, they are said to be diaphototropic.
The phototropic curvature of root and stem is determined by unequal distribution of auxin under the influence of light. Light pushes auxins to the shaded side resulting in higher auxin concentration on the shaded side of the stem, thus promoting growth on that side which in turn causes a curvature towards the lighter side. On the contrary, the higher auxin concentration on the shaded side of the root inhibits growth on that side so that the root exhibits a curvature away from the source of light.
These are movements in response to the stimulus of touch.
This is the movement of plant parts in response of moisture or water. The roots of higher plants and rhizoids of lower plants (bryophytes) grow towards water. The stems normally grow away from water.
Curvature of fixed plant organs in response to chemicals is called chemotropism. In germinating pollen grains, the pollen tubes can be demonstrated to grow towards a nutrient medium rich in boron and calcium. This is an example for chemotropism.