1. In case of plants, growth and development are in a coordinated manner.
2. The movement of plants in the direction of stimulus is known as ‘tropism‘. There are mainly three types of tropism. They are:
(i) Phototropism – Bending towards light
(ii) Geotropism – Downward movement in response to gravity
(iii) Chemotropism – Movement in response to chemical activity.
3. Apart from these, nastic movement may also be observed in some plants. This is the movement of plant parts caused by an external stimulus but unaffected in direction by it. Example: The leaves of the ‘touch -me – not’ plant droop on touching.
4. Flowering and seed germination are regulated by the duration of light. This is known as photoperiodism.
5. Plants show two different types of movement – one dependent on growth and the other independent of growth.
6. Some plants like the pea plant climb up other plants or fences by means of tendrils. These tendrils are sensitive to touch. When they come in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part of the tendril away from the object. This causes the tendril to circle around the object and thus cling to it. More commonly, plants respond to stimuli slowly by growing in a particular direction. Because this growth is directional, it appears as if the plant is moving.
7. Growth substances are also called the phytohormones. The phytohormones have been put into five different categories based on their actions. They are:
(i) Auxins – Auxins are phytohormones that are mainly concerned with cell enlargement. They affect the plasticity of the cell walls and induce them to grow. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of auxin stimulates the cells to grow longer on the side of the shoot which is away from light. Thus, the plant appears to bend towards light.
(ii) Gibberellins – Gibberellins are plant hormones that are mainly responsible for cell elongation. They cause the cells to grow in length. Help in the growth of the stem.
(iii) Cytokinins – They are phytohormones that induce cell divisions even in mature tissues. They are present in greater concentration in areas of rapid cell division, such as in fruits and seeds.
(iv) Ethylene – Ethylene is a gaseous growth regulator that speeds up the ripening process. It is a gas produced by most of the plant organs.
(v) Abscissic acid – It is a growth inhibitor that results in dormancy and abscission. Its effects include wilting of leaves.
8. Plants show tropism in response to other stimuli. This upward and downward growth of shoots and roots, respectively, in response to the pull of earth or gravity, is geotropism. Hydrotropism means movement towards the water. Chemotropism is the growth of pollen tubes towards ovules.
9. Electrical impulses are an excellent means for communication in cells but they have two limitations: –
(i) Firstly, they will reach only those cells that are connected by nervous tissue, not each and every cell in the animal body.
(ii) Secondly, once an electrical impulse is generated in a cell and transmitted, the cell will take some time to reset its mechanisms before it can generate and transmit a new impulse. Therefore, cells cannot continually create and transmit electrical impulses.