Answer:
The correct answer is Option D) "Stems grow toward a gap of light in a forest."
Step-by-step explanation:
- The phenomenon due to which plant shoot shows a tendency to move towards light is known as "Phototropism".
- This is caused by a plant hormone called "Auxin".
- Auxin is produced at the apical ends (both shoot tip and root tip) of the plants and it promotes cell elongation.
- However, auxin is sensitive to light and gets degraded when exposed to light, so in the shoot tip it is produced on that side of the stem which is not exposed to light.
- As a result of this, the side of the stem away from light elongates more (due to exposure to auxin) as compared to that which is exposed to light.
- This causes the stem to bend towards the direction of light.
- This is the reason for both the sunflower's diurnal movement and movement of the stem towards the light source in forest.
- Roots grow down the soil due to the phenomenon called Geotropism. The hormone auxin is responsible for the elongation of root-tip towards gravity.
- The phenomenon due to which vines wrap around the pole they touch is called Thigmotropism (directional movement induced by touch). The plant hormone auxin has an essential function here.
- Water drawn from soil by roots is used to maintain a turgor pressure (water pressure) in the leaves that keeps them erect. Absence of water results in a reduction of the turgor pressure in the leaves. This causes the leaves to wilt. Wilting of leaves reduce the surface area of the leaves that is exposed to the sunlight. This again lowers the rate of transpiration through the stomata in absence of water thereby protecting the plant from dehydration.