Coordination in Plants
Plants show two types of movement one dependent on growth and the other independent of growth. Some common observations are:
- The leaves of the touch-me-not plant begin to fold up and droop on being touched. There is no growth involved in this movement.
- When a seed germinates, first the root goes down and the stem comes up into the air. The directional movement of a seedling is caused by growth.
Plants respond to various stimuli like light, gravity, chemicals, water, and touch etc due to the effect of hormones. However, since pLants do not have a nervous system, they do not respond as quickly as compared to animals. It usually takes a considerable time to observe the effect of a stimuli as plants respond to various stimuli very slowly.
If we touch a plant movement happens at a point different from the point of touch. So, information that a touch has occurred must be communicated.
Immediate Response to Stimulus
Nastic movement is the movement of plant organs in response to external stimulus which is independent of the direction of stimulus. For ex, the leaves of touch-me-not plant bend and droop on touching.
- Plants use electrical-chemical means to convey any information from one cell to another. However, there is no specialized tissue in plants for the conduction of information, unlike the nervous tissue in animals.
- Plants cells change their shape in response to an external stimulus by changing the amount of water in them which results in swelling or shrinking, unlike in animals, where specialized proteins
are found in muscle cells due to which muscle cells change their shape and movement occurs.
What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs?
Differences between the manner in which movement takes place in a sensitive plant and the movement in our legs are given below:
|Movement in a Sensitive Plant
|Movement in Our Legs
|1. It is response to stimuli (touch) which is an in-voluntary action
|1. It is a voluntary action
|2. There are no specialized tissues for con-duction of information
|2. There is specialized nervous tissue for conduction of information
|3. Plant cells change their shape and size by changing the amount of water in their cells
|3. Muscles cells have specialized proteins that help in contraction and relaxation of muscles.
Movement due to Growth (Thigmotropism):
- Some plants like the pea plant have tendrils, which are thin, thread-like growths on the stem or leaves of climbing plants.
- These tendrils are sensitive to touch.
- When a tendril touches an object like a bamboo stick, the part of the tendril in contact with the object does not grow as rapidly as the part of the ten¬dril which is away from the object.
- This causes the tendril to bend towards the ob¬ject by growing towards it and circling around the object.
- This type of winding movement of the tendril of a climbing plant is an example of thigmotropism.
Shreya performed an activity to understand tropism in plants. She filled a conical flask, with water and covered the neck of the flask with a wire mesh. She kept two or three freshly germinated bean seeds on the wire mesh.
Case I: She then took a cardboard box which is open from one side and kept the flask in the box in such a manner that the open side of the box faced light coming from a window (Figure). She noted her observations after two or three days.
Case II: She then turned the flask so that the shoots were away from light and the roots towards light. She left the flask undisturbed in this condition for a few days and noted her observations again.
(A) Shreya recorded her observations as given below:
|The direction of Root and Shoot at (I)
|The direction of Old Root and Shoot at (II)
|(a) Both roots and shoot bend towards the light
|(a) Both root and shoot bend towards the light
|(b) Roots bend to-wards light and shoot bends away from light
|(b) Not much change in direction of old roots and shoot
|(c) Shoot bends to-wards light and roots bend away from light
|(c) Not much change in direction of old roots and shoot
|(d) Both root and shoot bend away from light
|(d) Both root and shoot bend away from light
(c) (A) is true, but (R) is false.
Explanation: We observe in Case i, that shoot bends towards light whereas roots bend away from light. However, after 2 to 3 days we observe that there is not much change in the direction of old roots and shoot.
(B) Select the correct statement:
(a) Both roots and shoot show positive phototropism.
(b) Roots show negative phototropism whereas shoot show positive phototropism.
(c) Roots show positive phototropism whereas shoot show negative phototropism.
(d) Both roots and shoot show negative phototropism.
(b) Roots show negative phototropism whereas shoot show positive phototropism. Explanation: Roots bend away from light whereas shoot bends towards light. Roots show negative phototropism whereas shoots show positive phototropism.
(C) Do you observe change in direction of the old parts of the shoot and root when direction of flask is changed?
The change in direction of the old parts of the shoot and root, change in direction is less and not much noticeable , however there is marked and noticeable change in case of new parts of the shoot and root
(D) Are there differences in the direction of the new growth?
Yes, new growth of parts of the shoot and root is visibly more noticeable. Shoots are found bending towards light while roots are found bending away from it
(E) Assertion (A): Direction of old parts of the root and shoot change on changing the direction of flask.
Reason (R): Roots show negative phototropism and shoot shows positive phototropism.
(a) Both (A) and (R) are true and (R) is the cor¬rect explanation of the assertion.
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion.
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
(d) (A) is false, but (R) is true.
Explanation: When the direction of flask is changed in Case II, the old parts of roots and shoot do not change direction much whereas new parts change their direction.
How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light?
The movement of leaves of the sensitive plant is independent of growth whereas the movement of a shoot towards light is dependent on growth.
|Movement of Leaves of Sensitive Plants
|Movement of a Shoot Towards Lights
|1. It is a nastic movement which does not depend on the direction of stimulus applied.
|1. It is a tropic movement which depends upon the direction of applied stimulus.
|2. The stimulus is ‘touch’
|2. The stimulus is light.
|3. It is caused due to change in the amount of water in them resulting in swelling or shrinking.
|3. It is caused due to the unequal growth on the two sides of the shoot.
|4. It is not a growth movement.
|4. It is a growth movement.
The movement of a plant in the direction of an exter-nal stimulus is known as tropism. Environmental trig-gers such as light or gravity will change the directions in which plant parts grow. The growth of a plant part in response to a stimulus can be of two types:
- Positive tropism: If the growth or movement of a plant part is towards the stimulus, it is called positive tropism.
- Negative tropism: If the growth of a plant part is away from the stimulus, it is called negative tropism.
Types of Tropisms
There are five types of tropisms depending on the type of stimuli. These are:
- Phototropism: It is the movement of a plant part in response to light. For e.g., growth of shoot in the direction of light.
- Geotropism: It is the movement of a plant part in response to gravitational forces. For ex, down-ward movement of roots.
- Chemotropism: It is the movement of plant parts in response to a chemical stimulus. For ex, the growth of pollen tube towards a chemical produced by the ovule during fertilization.
- Hydrotropism: The movement of a plant part in response to water is called hydrotropism. For ex, the movement of roots of a plant towards water.
- Thigmotropism: The directional movement of a plant part in response to the touch of an object is called thigmotropism. For ex, coiling of tendrils around support.
Design an experiment to demonstrate hydrotropism.
Take two beakers A and B. Add moist soil in beaker A and sow the seeds. In beaker, B add dry soil in one part and moist soil in another part and sow the seeds. Also, place a small porous pot filled with water just adjacent to it. Keep it for some time so that the plants can grow.
It is observed the roots of the plant in beaker A grow straight whereas the roots in beaker B bend and grow towards moist soil. This activity demonstrates hydrotropism or movement of plant part towards water.
Comparison between Electrical Impulse and Chemicals
Electrical impulses reach only those cells of the body which are connected by nervous tissue, not each and every cell in the animal body. Chemical compounds are secreted by stimulated cells which diffuse all around the original cell and if other cells would have the means to detect this compound, they would be able to recognize this information.
Once an electrical impulse is generated in a cell and transmitted, the cell will take some time to reset its mechanism before it can generate a new impulse.
These are chemical substances known as plant hormones which perform the function of control and coordination in plants. The different types of plant hormones are:
Auxins, Gibberellin, Cytokinin, Abscisic Acid, and Ethylene.
Stimulated plant cells release a chemical compound that diffuses all around the cell.
The cells around have the means to detect this compound using special molecules on their surfaces and are able to recognize information and also transmit it.
This information potentially reaches all cells of the body steadily and persistently.
The plant hormones are synthesized at places away from where they act and simply diffuse to the area of action.
Functions of Phytohormones: The plant hormones regulate many functions in plants which are described below:
|1. Synthesized at the shoot and root tip.
|2. Help the cells to grow longer as they promote cell enlargement and cell differentiation in plants.
|3. Promote fruit growth.
|1. Promote cell enlargement and cell differentiation in plants in the absence of Auxins.
|2. Help in breaking dormancy in seeds and buds.
|3. Promote growth in fruits.
|1. Promote cell division in plants.
|2. Help in breaking dormancy of seeds and buds.
|3. Delay ageing in leaves.
|4. Promote the opening of stomata.
|5. Promote fruit growth.
|1. Functions mainly as a growth inhibitor.
|2. Promotes the dormancy in seeds and buds.
|3. Promotes the closing of sto-mata.
|4. Promotes the wilting and falling of leaves.
Role of auxins in phototropism:
Bending of the stem towards the light:
- When sunlight falls from top, the auxin hormone present in the shoot tip spreads uniformly down the stem and both the sides of the stem grow uniformly.
- When the light falls only on side (say, right side), then the auxin hormone collects in the left side away from light.
- Due to the presence of more auxin hormone on the left side, this side grows faster as compared to the right side.
- The stem therefore bends towards the right side.
Bending of root away from light:
- The effect of auxin hormone on the growth of root is opposite to the effect of auxin on the shoot.
- The side of a root which is away from light will have a greater concentration of auxins.
- Due to this, this side will grow slowly than the side which is facing the light.
- The root bends away from light.
Photoperiodism: The response of plant activities to the duration of light is known as photoperiodism. Flowering and germination of seeds is regulated by duration of light.
What are plant hormones?
The organic substances that are produced naturally in plants and which control growth and other physiological functions in plants are known as plant hormones. Examples are auxins, gibberellins, etc.