Question

Plants grow downwards and show
A. Positive geotropic
B. Negative geotropic
C. Positive phototropic
D. Negative hydrotropic

Answer 1

Hint: The plant movement refers to the changes in the spatial orientation of its parts. It includes movements toward the light, opening, and closing of flowers, growth of developing roots in search of water and nutrients, etc.

Complete Answer:
 Let us analyze each option to find out the correct answer.
Option A: Geotropism is defined as the response of plant parts to gravity. Positive geotropic is the growth of part of the plant in the direction of gravity. It means the growth of the plants towards the Gravitational force. Hence, the downward growth of roots is an example of positive geotropism. Therefore, this is the correct option.

Option B: Negative geotropic is defined as the response of the plant part of the plant against gravity. For example, primary stems grow against gravity in an upward direction and thus show negative geotropism. Therefore, this is an incorrect option.

Option C: Phototropism refers to the growth of an organism in response to a light stimulus. It may be toward—or away from—a light source. Positive phototropic is growth towards a light source; negative phototropism is growth away from light. For example, shoots, or above-ground parts of plants, generally show positive phototropism as they bend toward the light. Therefore, this is an incorrect option.

Option D: Hydrotropism is defined as the movement of an organ of a plant in response to the stimulus of water. Negative hydrotropic occurs when the organism grows away from water. An example that can be cited for positive hydrotropism is the growth of plant roots towards a higher relative humidity level. Therefore, this is an incorrect option.

Thus, the correct option is A) Positive geotropic.

Note: It is to be noted that during phototropism, phototropin, a protein, is the main photoreceptor responsible for light detection. The phototropin is made up of a protein bound to a light-absorbing organic molecule, called the chromophore.