Control and coordination are essential for living organisms to respond appropriately to changes in their environment. In animals, these functions are performed by the nervous and endocrine systems, while in plants, they are achieved through chemical messengers called hormones.
Plants lack a nervous system and muscles. They show two types of movement:
Independent of Growth: Quick responses to stimuli, like the sensitive plant (Mimosa pudica) folding its leaves when touched. This occurs due to changes in water amount (swelling/shrinking) in cells.
Dependent on Growth (Tropism): Directional movements toward or away from stimuli.
Phototropism: Growth toward light (shoots).
Geotropism: Response to gravity (roots grow down, shoots grow up).
Hydrotropism: Response to water.
Chemotropism: Response to chemicals (e.g., pollen tube growth toward ovules).
Plant Hormones (Phytohormones):
Auxin: Promotes growth at shoot tips and helps in phototropism.
Gibberellins: Assist in stem growth.
Cytokinins: Promote rapid cell division (found in fruits and seeds).
Abscisic Acid: Inhibits growth and causes wilting of leaves.
Growth Hormone (Pituitary Gland): Regulates body growth and development.
Insulin (Pancreas): Regulates blood sugar levels.
Diabetes
If insulin is not secreted in proper amounts, the sugar level in the blood rises, causing many harmful effects. This condition is known as diabetes and is often treated with insulin injections.
Testosterone & Oestrogen: Regulate changes associated with puberty in males and females respectively.
Feedback Mechanism:
The timing and amount of hormone release are regulated by feedback mechanisms. For example, the pancreas increases insulin production when blood sugar levels rise and reduces it when they fall.