Directional movements: These are also called as tropic movements. These movements can be either towards the stimulus or away from it.
• Positive phototropism is seen in shoots which respond by bending towards light. Negative geotropism is seen in shoots by growing away from the ground.
• Roots bend away from light exhibiting negative phototropism. They grow towards the ground exhibiting positive geotropism.
• Hydrotropism is a growth response in which the direction is determined by the stimuli of water.
• Chemotropism is a growth movement of a plant part in response to chemical stimulus. e.g. Growth of pollen tubes towards ovules.
Hormones are the chemical compounds released by stimulated cells. Hormones diffuse all around the cell. They are synthesised at places away from where they act and simply diffuse to the area of action. Different plant hormones help to coordinate growth, development and responses to the environment. Different hormones secreted by the plant are auxins, gibberellins, cytokinins, abscisic acid.
• Auxins are the hormones synthesised at the tip of the stem. These help the plant in growth by cell elongation. Auxin induces shoot apical dominance.
• Gibberellins are hormones that help in the growth of the stem, seed germination, bolting, and flowering.
• Cytokinins are hormones present in the areas of rapid cell division, such as fruits and seeds. They also promote the opening of the stomata.
• Abscisic acid is a hormone that inhibits the growth in various parts. It is also responsible for the closure of stomata. Its effects include wilting of leaves.
