Class - 10 (biology) Science

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Nastic movements

Movements that occur in response to a stimulus but whose direction of movement is independent of the direction of stimulus are known as nastic movements.
These movements are independent of Growth.

1. Seismonasty 
      It refers to movements which occur in response to touch. 
When the leaves of touch-me-not plant are touched, the leaves fold and droop down. 

2. Nyctinasty
     It refers to movements in response to day and night. These are also called sleep movements.
The movements of leaves and flowers in response to day and night belongs to this category.

Phytohormones or plant Hormones
Phytohormones are organic substances, other than nutrients, which in low concentrations, regulate plant growth and differentiation.

Plant Hormones can be divided into two main types : 
Growth promoters which have a positive effect on a process and thus promote it and growth inhibitors which have negative effect on a process And thus inhibit it. 
Auxins, gibberlins and cytokinins are Growth promoters While abscisic acid 
is a growth inhibitor.

1. Auxins 
   Auxins were first to be discovered and studied in detail. Auxin is indole -3-acetic acid. 

Effects
• Auxins promote cell division and elongation.
• Apical dominance of terminal bud is due to secretion of auxin. 
• Auxins are also used for production of seedless fruits. 

2. Gibberellins
  The major sites of gibberellin production in plants are roots, embryo and young leaves near the shoot tip.

Effects : 
• Gibberellins stimulate stem elongation and leaf expansion. Cell division and cell elongation induced by gibberellic acid causes stem elongation.
• They cause elongation of genetically dwarf varieties of plants such as corn.
• They induce cell elongation in rosette plants. This is called bolting.

3. Cytokinins
   Cytokinins are non-motile. The discovery of cytokinins is an offshoot of the efforts of Miller and skoog.
Cytokinins are produced in actively growing tissues, such as embryos, developing fruits and roots.

Effects : 
• Cytokinins promote cell division.
• They stimulate the growth of lateral buds and thus suppress apical dominance.
• delay ageing of plant organs by controlling protein synthesis and mobilisation of resources.

4. Abscisic Acid 
     Abscisic acid is a growth inhibitor and causes retardation of growth. 

Effects :
• It induces bud dormancy in a number of plants.
• It also induces dormancy in many seeds.
• It promotes Seperation of leaves, flowers and fruits in most of the plants.

5. Ethylene 
   Ethylene is the only gaseous phytohormone which stimulates transverse growth but retards the longitudinal one.

Effects :
• It stimulates the growth of fruits in some plants.
• It inhibits the elongation of stem and roots in longitudinal direction.
• It accelerates the abscission of leaves, flowers and fruits.

Movement in sensitive plant
• It is caused due to changes in water content of cells. 
• It occurs in response to external stimulus.
• Nerves are not involved.

Movement in Legs 
• It is caused due to contraction and relaxation of muscles 
• It occurs in response to our will. 
• Nerves are involved in the movement of legs.

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