COORDINATION IN PLANTS
Plants have no nervous system or muscles, but they can respond to stimuli.
Like animals, plants show two types of movement:
- Dependent on growth: For example, when a seed germinates, the root grows downward, and the stem grows upward.
- Independent of growth: For example, when the leaves of a chhui-mui (the ‘sensitive’ or ‘touch-me-not’ plant of the Mimosa family) are touched, they quickly fold up and droop.
IMMEDIATE RESPONSE TO STIMULUS
- In the sensitive plant, movement occurs at a point different from the point of touch, indicating that information about the touch is communicated. Plants use electrical-chemical means to conduct information from cell to cell.
- In animals, some muscle proteins help change the shape of cells. In contrast, plant cells change shape (swelling or shrinking) by altering the amount of water in them.
MOVEMENT DUE TO GROWTH
- Some plants, such as peas, have tendrils to climb up supports or fences. These tendrils are sensitive to touch.
- When tendrils come into contact with a support, the touching part grows more slowly than the part away from the object, causing the tendril to circle around and cling to the support.
- Plants respond to stimuli slowly by growing in a specific direction. This directional growth makes it appear as if the plant is moving.
- Environmental triggers such as light or gravity change the direction of plant growth. These are called tropic (directional) movements, which can be toward the stimulus (positive) or away from it (negative).
TYPES OF TROPIC MOVEMENTS
Phototropism
- The tropic movement in response to light.
- Shoots respond by bending toward light (positive).
- Roots respond by bending away from light (negative).
- This can be demonstrated by the following activity:
- Fill a conical flask with water and cover its neck with a wire mesh containing 2-3 freshly germinated bean seeds.
- Place the flask in a cardboard box such that its open side faces light coming from a window.
- After 2 or 3 days, observe that the shoots bend toward the light and the roots bend away from it.
- Turn the flask so that the shoots are away from the light and the roots are toward it. Leave it for a few days.
- The old parts of the shoot and root show no noticeable change in direction, but the new growth parts show a change: shoots bend toward the light, and roots bend away from it.
Geotropism
- Geotropism is the movement in response to the pull of the Earth or gravity.
- Roots always grow downward (positive).
- Shoots grow upward and away from the Earth (negative).
Hydrotropism
- The movement of a plant toward or away from water.
- For example, roots bend toward moist soil.
Chemotropism
- The movement of a plant toward or away from chemicals.
- For example, the growth of pollen tubes toward ovules.
Controlled movements in plants can be either slow or fast. For example:
- The sensitive plant quickly moves in response to touch.
- Sunflowers slowly move in response to day or night.
- Growth-related movements of plants are slower.
In animal bodies, growth also occurs in controlled directions, such as the growth of arms and fingers.
For fast responses to stimuli, information must be transferred quickly. Electrical impulses are an excellent means for this, but they have limitations:
- They reach only the cells connected to nervous tissue.
- Once an impulse is generated and transmitted, the cell takes time to reset its mechanism to generate a new impulse, so cells cannot continuously create and transmit electrical impulses.
Therefore, most multicellular organisms use chemical (hormone) communication between cells.
In this process, stimulated cells release a hormone that diffuses around the original cell. Other cells detect the hormone using special molecules on their surfaces, recognize the information, and transmit it.
This is a slower process but can reach all cells and can be done steadily and persistently.
Plant hormones coordinate growth, development, and responses to the environment. They are synthesized at specific locations and diffuse to the area of action.
SOME PLANT HORMONES
- Auxin: Regulates growth in plants. When plants detect light, auxin, synthesized at the shoot tip, helps cells grow longer. When light comes from one side, auxin diffuses toward the shady side of the shoot, causing cells on the shady side to grow longer, making the plant bend toward the light.
- Gibberellins: Help in the growth of the stem.
- Cytokinins: Promote cell division. They are present in greater amounts in areas of rapid cell division, such as fruits and seeds.
- Abscisic acid: Inhibits growth and causes wilting of leaves.
The above hormones promote growth.