Coordination in Plants | Biology for Grade 10 PDF Download

Plant movements & plant hormones 

Plant movement : The movements in plants are not as apparent as in case of animals. Plants generally show movements at a very slow rate. The higher plants are fixed to the substratum by means of roots. They cannot move from one place to another. Therefore, they show movement of their organs only.

Classification of plant movements :

Plant movements are broadly classified into two types :

1. Tropic movements :- Directional movements or orientations of specific part of a plant in response to external stimuli such as light, force of gravity, chemicals, water are called tropisms or tropic movements.

Tropic movements are very slow. The movement of the plant part can be either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).

(i) Phototropism :- Definite direction movement in relation to light.

 (ii) Geotropism :- Definite direction movement in relation to gravity.

 (iii) Chemotropism :- Definite direction movement in relation to chemicals.

e.g. - Movement of pollen tubes and fungal hyphae.

 (iv) Hydrotropism :- Definite direction movement in relation to water.

e.g. - Roots of seedlings.

(v) Thigmotropism (Haptotropism) :- Definite direction movement in relation to contact or support. The pea plant develops tendrils which help it to climb up other plants or fences or some other support. These tendrils are sensitive to touch.

e.g. - Tendrils, haustoria of Cuscuta 

(2) Nastic movements :- Induced by external stimuli such as light, temperature, touch. However, these are not directional movements. Here, the part of the plant does not respond towards or away from the stimulus. Nastic movements include :-

 (i) Seismonasty :- The turgor changes occur in thin-walled cells of pulvinus, causing folding and drooping of the compound leaves. Such movements occur in response to touch (shock). These movements are very quick and are best seen in 'touch-me-not' plant (Mimosa pudica), also called 'Chhui-mui' or 'Lajwanti' or 'sensitive plant'. If we touch the leaves of the chhui-mui plant with our finger, we find that all its leaves immediately fold up and droop. After sometime, the leaves regain their original status. Here, no growth is involved. Instead, plant cell changes shape by changing the amount of water in them (turgor changes), resulting in folding up and drooping of leaves.

(ii) Nyctinasty :- Sleep movements - Due to day and night.
e.g. - Daily movement of flowers, leaves, stomata. 

   Chemical Coordination in Plants 

• A phytohormone can be defined as a chemical substance which is produced naturally in plants and is capable of translocation and regulating one or more physiological processes.
• Different plant hormones help to coordinate growth development and responses to the environment. They are synthesized at places away from where they act and simply diffuse to the area of action.
• First plant hormone, discovered by Went was auxin.
• Main phytohormones are -

 (i) Auxins
 (ii) Gibberellins
 (iii) Cytokinins|
 (iv) Abscisic acid
 (v) Ethylene

(i) Auxins :-

• When growing plants detect light, a hormone called auxin, synthesized at 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.
• 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.

Functions of Auxins 

• Promote cell division and elongation
• Cause apical dominance (terminal or apical bud inhibits the development of lateral buds)
• Used in parthenocarpy (production of seedless fruits without pollination and fertilization).
• Help in root initiation in cutting or in callus differentiation. 

(ii) Gibberellins (GA) :-

• Yabuta and Sumiki (1938) were the first to extract a substance from the Gibberella fungus, which they named as Gibberellin.

 Functions of Gibberellins:

• Stimulate stem elongation.
• Help in seed germination.
• Rossette plants show bolting effect when treated with gibberellins.

  (iii) Cytokinins (CK) :- 

• Cytokinins promote cell division, and it is natural that they are present in greater concentration in areas of rapid cell division such as in fruits and seeds. 

Functions of Cytokinins: 

Promote cell division and elongation 

• Suppress apical dominance (promotes lateral branches in the presence of apical bud).
• Help in secondary growth (growth in thickness).
• Promote production of female flowers.
• It helps in braking the dormancy of seeds and buds. 

(iv) Abscisic Acid (ABA) :- It is also known as stress hormone or dormin. 

 Functions of Abscisic Acid (ABA) 

• Inhibit growth hence called anti-auxins or anti-gibberellins
• Reduce transpiration by closing stomata under water stress conditions, hence called stress hormone
• Stimulate the formation of abscission zone (zone of separation), causes wilting of leaves.

(v) Ethylene (Gaseous hormone) :-  

Functions of Ethylene (Gaseous hormone) 

• Promote fruit growth and ripening
• Cause ageing (senescence)