7. CONTROL AND COORDINATION
Plants have no nervous system or muscles. But they can respond to stimuli.
Like animals, plants show 2 types of movement:
o Dependent on growth: E.g. When a seed germinates, root goes down and stem comes up.
o Independent of growth: E.g. When we touch the leaves of a chhui-mui (the ‘sensitive’ or ‘touch-me-not’ of Mimosa family), they quickly fold up and droop.
In sensitive plant, movement happens at a point different from the point of touch. It means information about touch is communicated. Plants use electrical-chemical means to conduct information from cell to cell.
In animals, some muscle proteins help to change the shape of cells. But plant cells change the shape (swelling or shrinking) by changing amount of water in them.
Some plants (e.g. pea) have tendrils to climb up or fence. They are sensitive to touch.
When tendrils contact with a support, the touching part does not grow as rapidly as the part away from object. So, tendril circles around and clings to the object.
Plants respond to stimuli slowly by growing in a direction. Because of the directional growth, it appears as if the plant is moving.
Environmental triggers such as light, gravity etc. change the directions of plant growth. They are called tropic (directional) movements. These can be towards the stimulus (positive), or away from it (negative).
Types of tropic movements
It is the tropic movement in response to light.
Shoots respond by bending towards light (positive).
Roots respond by bending away from light (negative).
It can be proved by the following activity.
- Fill a conical flask with water and cover its neck with a wire mesh having 2-3 freshly germinated bean seeds.
- Keep the flask in a cardboard such that its open side faces light coming from a window.
- After 2 or 3 days, it is noticed that the shoots bend towards light and roots away from light.
- Now turn the flask so that shoots are away from light and roots towards light. Leave it for a few days.
- The old parts of the shoot and root have no noticeable change in direction. But new growth parts show change in direction. i.e., shoot bends towards light and roots bend away from it.
Movement in response to the pull of earth or gravity.
Roots always grow downwards (positive).
Shoots grow upwards and away from earth (negative).
The movement of a plant toward or away from water.
E.g. Roots bend towards the moist soil.
The movement of plant towards or away from chemicals.
E.g. growth of pollen tubes towards ovules.
Controlled movements can be either slow or fast. E.g.
o Sensitive plant quickly moves in response to touch.
o Sunflowers slowly move in response to day or night.
o Growth-related movements of plants are slower.
In animal bodies also have controlled directions to growth. E.g. growth of arms & fingers.
For fast responses to stimuli, information must be transferred very quickly. Electrical impulses are an excellent means for this. But it has some limitations:
o They reach only the cells having connection with nervous tissue.
o Once an impulse is generated and transmitted, the cell will take some time to reset its mechanism to generate a new impulse. So, cells cannot continually create and transmit electrical impulses.
So most multicellular organisms use chemical (hormone) communication between cells.
In this, stimulated cells release a hormone and it diffuses around the original cell. Other cells detect hormone using special molecules on their surfaces. Then they recognise information and transmit it.
This is slower process but can reach all cells and can be done steadily and persistently.
Plant hormones can coordinate growth, development and responses to the environment. They are synthesised at some places and diffuse to the area of action.
Some Plant Hormones
o Auxin: Regulates growth in plants. When plants detect light, auxin, synthesised at the shoot tip, helps the cells to grow longer. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. As a result, the cells on the shady side grow longer. Thus, the plant bends towards light.
o Gibberellins: Help in the growth of the stem.
o Cytokinins: Promote cell division. They are present in greater amount in areas of rapid cell division, such as fruits and seeds.
The above hormones help in promoting growth.
o Abscisic acid: Inhibits growth. Causes wilting of leaves.