Question

Define the following :

1) Tropism 2) Hydrotropism 3) Stress hormone 4) Thigmotropism

Answer 1

1) Tropism

• When any plant part or organ grows in response to the external stimuli like water, light, touch, gravity or chemicals to the direction of the particular stimulus ,this phenomenon is known as tropism or tropic movements.
• The direction of movement of the plant parts is dependent on the direction of the particular stimulus.
• The respond of the plant includes bending and growth of the plant parts.
• If the growth of the plant part is towards the direction of the stimulus then it is known as positive tropism and if the growth of the plant part is opposite to the direction of the stimulus then it is termed as negative tropism.

2) Hydrotropism

• The phenomenon of directional growth and movement of the plant parts in response to the stimulus of water or moisture is known as hydrotropism.
• For example, the growth of root towards the water.
• Hydrotropism involves turning or curling of plant part.

3) Stress hormone

• Abscisic acid(ABA) is the plant hormone or phytohormone which regulates abscission and dormancy in plants.
• This hormone is called as stress hormone because it increases the tolerance and resistance of plants against cold and other adverse conditions that plants face.

4) Thigmotropism

• The phenomenon of directional growth and movement of the plant parts in response to the stimulus of contact or touch with a solid surface is known as thigmotropism.
• The soft stemmed plants such as bitter gourd, pea have tendrils which search a surface for support. After finding a support it coil itself around the support and grow properly.
• Another excellent example of thigmotropism is exhibited by touch-me-not (Mimosa pudica) plants, where the plants curl their leaves whenever it is touched.