Control And Coordination In Plants CBSE Class 10 Notes

 CONTROL AND COORDINATION IN PLANTS

Control And Coordination In Plants are sensitive to environmental stimuli like light, temperature, touch and gravity etc. The response of the plants towards these stimuli occurs mainly in the form of: 

1. Movements 
2. Flowering

These responses are also controlled by various internal factors such as growth regulative hormones (or phytohormones).

(A) Response in the Plants (In the Form of Movements)

Plants can not move from one place to another. But still they show various types of movements. Since, plant does not move from its place, the movements are of bending type. Such movements are called movements of curvature. 

Movements may be sudden (as in 'touch me not' plant or they may be slow. Slow movements are due to growth nature of the plant.

Movement Due to Growth

These are permanent movements, which are caused by the activity of meristem (living tissues) and result in permanent change in the position of organs. Growth movements are caused by various internal/external factors.

(a) Movement by Internal Factors

The most important factors are the phytohormones :

Role of Phytohormones in Coordination

Five important types of growth regulators are found in plants-Auxin, gibberelin, cytokinin, abscisic acid (ABA) and ethylene. Out of these, the auxin, gibberelin and cytokinin are growth promoting hormones. They coordinate the growth activities in the plant. The remaining two hormones work in opposite direction. They check excessive growth of the plant parts.

The synthesis and action of phytohormones is greatly influenced by external stimuli such as light, temperature etc. Phytohormones control and coordinate various functions in plants such as flowering growth of roots, stem and leaves, seed germination, ripening of fruit and growth movements.

 

Plant Hormones : At a Glance

Growth inducing Hormones:

1. Auxin : It is the most important plant hormone, which is developed in shoot and root tips. It promotes cell enlargement and cell division. It also promotes the growth of root and shoot, mainly in the apical parts and tendrils. (Tendrils are spring like supportive structures in some plants. These are sensitive to touch. Their growth is promoted by the auxins). It was discovered by Went in 1928.

2. Gibberelin : It is another growth promoting hormone. Its main effect is on stem growth. It greatly increases the height of the plant. It also promotes the growth of seed, flower and fruits. It was discovered by Kurosova in 1926.

3. Cytokinin : It is also a growth promoting hormone. Along with auxin, it promotes division of cells. It helps in seed germination and also promotes stomatal opening. Miller et.al. (1926) isolated it for the first time from the DNA of yeast and it was called kinetin. Skoog (1965) obtained a similar compound from the coconut water. He called it as kinin, which was later on called cytokinin. Letham obtained the naturally occuring cytokin in form the maize seed. He called it as zeatin.

Growth inhibiting Hormones:

1. Abscisic acid : It is a growth inhibiting hormone. It promotes all those activities, which are against, the normal growth of the plant. It closes stomata. It promotes abscision (fall) of leaves and fruits. It also causes wilting of leaves. Corns and Addicott (1961-1965) isolated this chemical from the cotton plant. It prevents cell division, therefore, it is called growth reducing hormone. It is transported mainly through phloem by active transportation

2. Ethylene : It is a gaseous hormone which is also growth inhibiting in nature. It helps in the ripening of fruits, so it is also called 'fruit ripening hormone'. It was discovered by Berg in 1962.

(b) Movement due to External Stimuli

These movements are caused in response to some external stimulus such as light, touch, gravity. chemicals and wind etc. These are mainly of two types : 

1. Tropic movements 
2. Nastic movements

(i) Tropic Movements

When the stimulus has a particular direction and movement of plant occurs in the direction of this stimulus, the movement is called tropic movement. In this case only a particular part shows movement.

For example:

1. If a plant is given unilateral light, its shoot will bend in the same direction (phototropism).
2. Similarly, roots are positively geotropic and move towards gravity (gravitropism or geotropism).
3. Pollen tube in flowering plants grows towards ovule. This is due to some chemical stimulus. This is called chemotropism.

(ii) Nastic Movements

When the stimuli have no fixed direction, and have a diffused impact, then the movement of plant occurs irrespective of the direction of the stimuli. In this case, whole of the plant is affected and shows movement.

For example:

1.  Opening and closing of petals of a flower in response to diffused light is an example of photonasty.
2. Change in temperature also influences opening and closing of flower in Oxalis. This is an example of thermonasty.
3. Touch, in some plants causes sudden change in turgor pressure, resulting in a peculiar movement. Such movements are called seismonastic movement, This is found in "Touch me not” (Mimosapudica) plant.
4. The movement in "Touch me not' plant is an example of immediate response to stimulus.

(B) Response in the Plants, (In the Form of Flowering)

The response of the plants to various external stimuli is also exhibited in the form of flowering. Plants are highly sensitive to the duration and quality of light they receive and also to the temperature. The duration of light, which is available to the plant, is called photoperiod and the response of the plants for duration of light is called photoperiodism. 

Some plants undergo flowering even if the duration of light is less than 6 hours. Such plants are called short day plants. But some plants require longer duration of light (about 14 to 16 hours) for flowering. Such plants are called long day plants.

To receive the stimulus of light duration, plants have a specialised pigment, called phytochrome. In many plants low temperature promotes flowering and seed germination. This low temperature requirement of the plant for flowering is called vernalisation.