Question

Which one of the following is not true for the roots of a plant?
(a) Positively geotropic
(b) Positively phototropic
(c) Positively hydrotropic
(d) Negatively phototropic

Answer 1

Hint Tropism is a biological phenomenon indicating growth or movement of a plant made in response to an environmental stimulus, such as light, touch or gravity. If the growth occurs towards the stimulus then it is positive and if it occurs away from the stimulus then it is negative.

Complete answer: To answer this question you must know about the tropism of plants.
Phototropism is the growth of an organism which responds to a light stimulus, growth towards the light source is called positive phototropism, while growth away from the light is called negative phototropism. Stems are positively phototropic and roots are negatively phototropic.
Geotropism is the movement that follows earth's gravitational force. movement of plants downwards to the earth is called positive geotropism and movement of plants upwards of the earth is called negative geotropism. Stem shows negative geotropism and roots show positive geotropism.
Hydrotropism is a growth response in which the direction is determined by the stimuli of water. For example, roots grow towards the source of water.
 The root of the plant shows movement towards the earth and towards the water but they move away from sunlight i.e. positively geotropic, negatively phototropic and positively hydrotropic .

Therefore the correct option of the given question is (B) positively phototropic.

Note: Unlike animals, plants cannot move to more favourable locations. One of the first important studies of plant tropism was phototropism by Charles Darwin. In plants there is a nastic movement also mostly found in specialized plants and organs. There is nyctinastic movement or sleep movement. It helps to protect the leaves and flowers from damage at night, but many plants survive quite well without such movement.