INTRODUCTION: 

Control and coordination is essential in maintaining a stage of stability and a steady state between the internal conditions of an organism and the external environment In animals control and coordination takes place by neural control as well as chemical control while in plants it takes place by chemical control only.

2. PLANT HORMONES

2.1 Auxions

 Auxions take part in a number of plant processes. Some are as follow:-

• • • • – Auxin Promote apical daninance.
      • -Auxin participates in molecular reaction
      • -Affects osmotic pressure by increasing solutes
      • -Affects enzyme action Affects nucleic acid activities
      • -Stimulates respiration
      • -Promotes root formation
      • -Helps in inhibition of leaf and fruit abscission.

2.2 Gibberellins

• • • •  -These hormones were first identified in studies of a disease of rice in Japan, the bakanal (foolish seedling) disease caused by Gibberella fujikuroi.

• • • • -These are second important growth hormones found in plants. Normally Gibberellins causes increased growth, especially in height of So Gibberelline are defined as the stem. growth hormones which causes ceil elongations.

2.3 Cytokinins: 

• • • • -Cytokinins are defined as compounds having a highly specific hydrophilic group or adenine and one nonspecific lipophilic group.

• • • • -Name of some cytokinins are-

• • • • -Kinetin, dihydrozeation, methylthiozeation and Riboxylzeatin.

2.4  Abscisic Acid (ABA):

• • • • -Acts as growth inhibitor and induces dormancy of buds towards the approach of winter.

• • • • -Inhibition of cambium activity- Towards the approach of winter, the activity of combium is inhibited due to the formation of abcisic acid.

• • • •  -Abscission :-   Abscisic acid promotes discission of flowers and fruits.

• • • • – Senescene It stimulates senescence of leaves.

• • • • -Closure of stomata :-  The normal causes closure of stomata (by inhibuiting K+ uptake.

• • • • -Inhibition of germination :-  Abscisic acid inhibits sprouting of cereal grains.

• • • • -Resistance :-  Abscisic acid increases resistance of plant to cold.

• • • • – Flowering :-  ABA delays flowering in long day plants

• • • • -Tubarization in potato:-   ABA helps in tuber formation in potato.

• • • • -Rooting :-  ABA promotes not imitation in stem cutting of some plants e.g. lvy,  Poinsettia.

2.5 Ethylene (CH2=CH2) 

• • • • -Ethylene is a gaseous hormone which stimulates transverse growth but retards the longitudinal one.

• • • • -Transverse growth :-   transverse growth sothat stem looks swollen.

• • • •  -Inhibition of geotropism :-   Ethylene nullifies geotropism.

• • • • -Fruit ripening :-   Ethylene is a ripening agent, such fruits as apple, banana, mango, citrus etc,Ethylene is used for artificial ripening of these fruits.

• • • • -Apical dominance :-  Ethylene inhibits the growth lateral buds and thus cause apical dominance.

• • • • -Root initation :-  In low concentration, ethylene stimulates root initiations.