The control and coordination in plants is done by plant hormones (or phytohormones). The plant hormones coordinate the activities of the plant by controlling one or the other aspect of the growth of the plant. So, the plant hormones are also known as plant growth substances.
The growth of a plant can be divided into three stages: cell division, cell enlargement and cell differentiation (or cell specialisation), and these stages have particular locations in a plant. These three stages of plant growth as well as promotion of dormancy, breaking of dormancy, stomata control, falling of leaves, fruit growth, ripening of fruits and ageing in plants are controlled by the various plant hormones.
There are four major types of plant hormones (or phytohormones) which are involved in the control and coordination in plants. These are:
3. Cytokinins, and
4. Abscisic acid (ABA).
Auxins, gibberellins and cytokinins are the plant hormones which promote growth of plants. On the other hand, abscisic acid is a plant hormone which inhibits (or prevents) the growth. The detailed functions of the various plant hormones are given below.
(i) Auxins are the plant hormones which promote cell enlargement and cell differentiation in plants. Auxins also promote fruit growth. Auxin hormone controls a plant’s response to light and gravity. In other words, auxin hormone is responsible for the phototropic and geotropic responses of plants. Auxin is made by cells at the tip of stems and roots. Auxin moves away from light, and towards gravity. Auxin has opposite effect on the growth of stem and roots. Auxin speeds up growth in stem but it slows down growth in roots. Synthetic auxins are applied in agriculture and horticulture.
(ii) Gibberellins are plant hormones which promote cell enlargement and cell differentiation in the presence of auxins. Gibberellins help in breaking the dormancy in seeds and buds. They also promote growth in fruits. Gibberellins hormone is involved mainly in shoot extensions. Gibberellins stimulate elongation of shoots of various plants.
(iii) Cytokinins are the plant hormones which promote cell division in plants. Cytokinins also help in breaking the dormancy of seeds and buds. They delay the ageing in leaves. Cytokinins promote the opening of stomata. They also promote fruit growth.
(iv) Abscisic acid is a plant hormone which functions mainly as a growth inhibitor. Abscisic acid promotes the dormancy in seeds and buds (this is the opposite of breaking of dormancy). It also promotes the closing of stomata. Abscisic acid promotes the wilting and falling of leaves (which is called abscission). It also causes the detachment of flowers and fruits from the plants.
The plants are fixed at a place with their roots in the ground, so they cannot move from one place to another. That is, plants do not show locomotion (movement of the entire body). However, movements of the individual parts or organs of a plant (like shoot, root, leaves, etc.) are possible when they are subjected: to some external stimuli like light, force of gravity, chemical substances, water, and touch, etc.
These movements of the plant part are usually caused by an unequal growth in its two regions by the action of plant hormones, under the influence of the stimulus. For example, the auxin hormone is made and secreted by the meristematic tissue at the tip of stem (or tip of shoot).
The auxin hormone speeds up the growth in stems. So, if one side of a stem has more auxin than the other side, then the side of stem having more auxin hormone will grow faster than the other side (having less auxin hormone). This will cause the stem to bend. And when the stem bends to one side, we say that the stem is showing movement. This movement (or bending) of the stem has been caused by its growth.
So, we can say that the bending of a stem (or shoot) (when exposed to light from one side) is a growth movement. In fact, the movement in any part of a plant is usually a growth movement. Please note that when a plant part shows movement, it remains attached to the main body of the plant. It does not get detached from it. We will now discuss tropism in which the part of a plant shows movement in response to various stimuli.
The plant movements made in response to external stimuli fall into two main categories: tropisms and nasties. Though all the tropisms are growth movements but nasties may be growth movements or growth- independent movements. In tropisms, the direction of stimulus determines the direction of movement of the plant part but in nasties the direction of movement is not determined by the direction of stimulus.