Question

The stimulus involved in the process of thigmotropism is
A. Gravity
B. Touch
C. Light
D. Water

Answer 1

Hint: Tropism is a natural phenomenon observed when an organism shows directional growth in response to external stimuli like touch, gravity, chemicals and light. If an organism shows growth towards the stimulus, then it is called positive tropism, and if it is away from the stimulus, then it is called negative tropism.

Complete answer:
When an organism changes its orientation of growth in response to touch, it is referred to as thigmotropism. Here touch acts as a contact stimulus. It is an important characteristic of certain plants and is very useful for their existence. Thigmotropism is notably demonstrated by climbers or creepers. They develop tendrils for assisting the plant to climb and grow.
Thigmotropism brings about a change in the arrangement and orientation of the plant during its growth. It is responsible for inducing a pattern in differential growth of a plant. When a plant like sweet pea comes in contact with a solid surface like a wall or fence (which act as a stimulus), the plant produces a growth hormone auxin. Auxin stimulates the tendrils and the plant stem to grow faster around the solid surface and help the plant to climb up. Special epidermal cells of tendrils exhibit a rapid differential growth rate which is marked in their growth pattern also. The curve in a tendril is formed when the tendril side opposite to the side of contact grows faster and gets elongated to turn around the solid surface. This allows the coils to form around the surface. The growth and shape of cells is controlled by another hormone ethylene which also assists during the process.

So, the correct answer is option (B), Touch.

Note:
Thigmotropism is also called haptotropism. Some tendrils demonstrate rapid contact coiling also. When there is a change in cell turgor, the shape of the tendril changes and a curve is formed. This is a very rapid process but short-lived, whereas, differential growth is a slow process but long-lived or permanent.