1.0 Plant Hormones: The Chemical Messengers
Unlike animals, plants do not have a nervous system. Their growth and responses to the environment are coordinated entirely by chemicals called Phytohormones. These are produced in minute quantities but have profound effects.
Major Growth Regulators
Apical Dominance: The phenomenon where the presence of the terminal bud (auxins) inhibits the growth of lateral buds. Pruning the tips of a hedge encourages bushier growth!
Don't forget Ethylene! It is the only plant hormone that exists in a gaseous state. Its primary function is the ripening of fruits.
The word "Auxin" comes from the Greek word auxein, which means "to grow." It was the first plant hormone to be discovered.
2.0 Tropic Movements (Tropisms)
Tropic movements are directional growth movements of a plant part in response to an external stimulus. The direction of the movement is determined by the direction of the stimulus.
Phototropism: The Auxin Connection
Auxins are synthesized at the shoot tip. When light comes from one side, auxins diffuse to the shaded side. This causes the shaded side to grow faster and longer, making the stem bend toward the light.
3.0 Nastic Movements
Nastic movements are non-directional. The direction of the response is not determined by the direction of the stimulus. These are often much faster than tropic movements.
- 📉 Thigmonasty (Seismonasty): Response to touch. Example: Drooping of leaves in Mimosa pudica (Touch-me-not plant). This is caused by a sudden loss of turgor pressure in the pulvinus (leaf base).
- 🌑 Nyctinasty: "Sleep movements" in response to day/night cycles. Example: Closing of flower petals or leaves at night.
The main difference: Tropism is a growth-dependent, slow movement. Nasty (like in Mimosa) is a turgor-dependent, rapid movement that does not involve growth.
The Clinostat is a mechanical device used in experiments to cancel out the effect of gravity on plants to study geotropism. If the plant rotates, the stimulus of gravity is applied equally to all sides!