Question

A seedling germinating under a stone grows at an angle away from the stone and upward. This response to touch is called ________.

a. gravitropism

b. thigmonasty

c. thigmotropism

d. skototropism

Short answer

c. thigmotropism

Step-by-step solution

Step-1 Introduction:

Being rooted in a place, plants must respond to the environmental factors that surround them because they are unable to move from one place to another. Various sophisticated mechanisms are generally possessed by plants to detect physical factors such as light, temperature, gravity, and physical touch. Receptors present in plant cells sense environmental factors, and the effectors receive the information generally through some chemical messengers which result in plant responses.

Step-2 Explanation:

Thigmotropism is the directional change due to touch stimuli. When a seedling germinates under a stone, the directional pressure occurs due to the thigmotropic mechanism. This particular type of change in direction can be towards the stimuli or can be away from the stimuli. Generally in the case of branches, tendrils, and stems these types of responses can be found.

Gravitropism is the response of shoot and roots to gravity that ensures the elongation of shoot part upward and root into the soil.

Although thigmonasty occurs due to touch response, the plant does not necessarily move towards the direction of stimuli. It is mainly seen in petals and leaves.

Skototropism is a kind of negative phototropism. The plant grows away from the source of light.

Step-3 Final Answer:

Therefore, the correct answer is option ‘c’.

Thigmotropism is the type of response that arises due to touch stimuli. The response can be towards the stimuli or away from them.