Control and Coordination in Plants

Plant shows two types of movements- one dependent on growth and the other independent of growth.

Movements Due to growth or Tropic Movements

A growth movement of a plant part in response to an external stimulus in which the direction of a stimulus determines the direction of response s called tropism. Thus tropism is a directional movement of the part of a plant caused due to its growth. The tropic movements can be either towards the stimulus or away from it e.g. the shoot of a growing plant bends towards the light, while roots of a plant move away from light.

Types of tropism:

1. The movement of a plant part in response to light is called phototropism. The stem of a growing plant bends towards the light (positive phototropism), while roots of a plant move away from light (negative phototropism).
2. The movement of a plant part in response to gravity is called geotropism. The roots of a plant move downwards in the direction of gravity. On the other hand the stem of a plant grow upwards and away from the earth.
3. The movement of a plant part in response to chemicals is called chemotropism. The growth of pollen tubes towards ovules during the process of fertilization.

4. The movement of a plant part in response to water is called hydrotropism. The roots of a plant always grow towards water.
5. The directional growth movement of a plant part in response to the touch of an object is called Thigmotropism.

There are some plants called climbing plants which have weak stems, cannot stand erect on their own. They have climbing organs called tendrils. Tendrils are thin, thread like structures on the stem or leaves of climbing plants. Tendrils are sensitive to touch. When they come in contact with an object, wind around the object and cling to it. This is due to the growth of a tendril towards the object.

Nastic Movements:
In nastic movements the movement of plant part is neither towards the stimulus nor away from the stimulus. This movement is not a directional movement of the plant part with respect to the stimulus. In nastic movements growth may or may not take place.
The folding up of the leaves of a sensitive plant or touch me not plant (Mimosa pudica) on touching is an example of nastic movement. Here stimulus is touch. This information is traveled from cell to cell by electrical chemical means.
This plant has pad like swellings called pulvini at the base of each leaf. The folding up of the leaves of a sensitive plant on touching is due to the sudden loss of water from the pulvini.
Plant cells change shape by changing the amount of water in them, resulting in swelling or shrinking, and in changing shapes.

Hormones in Plants

Different plant hormones help to coordinate growth, development and responses to the environment. They are produced at places away from where they act simply diffuses to the area of action.
Types of Hormones:
1. Auxin: It is produced at the shoot tip. It helps plant to grow. It promotes cell enlargement and cell differentiation in plants
2. Gibberellins: It also helps in the growth of the stem along with Auxin.
3. Cytokinins: It promotes cell division. They are present in large amount in areas of rapid cell division, such as fruits and seeds. It promotes the opening of stomata.
4. Abscisic acid: It inhibits growth. It effects wilting of leaves. It also promotes the closing of stomata.