Leaves of certain plants roll up on a hot sunny day. Explain by giving suitable reasons.
Topic: Rolling of leaves
Answer
On a hot sunny day the rate of transpiration is very high, and the plant is in danger of losing more water than its roots can absorb.
Rolling the leaf REDUCES THE EFFECTIVE SURFACE AREA exposed to the air and encloses the stomata (which are chiefly on the lower surface) within a moist pocket of trapped air. Both effects slow evaporation, so the rate of transpiration falls and the plant conserves water.
Two marks: the PROBLEM (excessive water loss in heat) and the SOLUTION (reduced surface area and trapped moist air).
THE SECOND MECHANISM is the one candidates miss. It is not only that less leaf is exposed — rolling encloses the stomata in a pocket of humid air, and evaporation slows when the air just outside the pore is already moist. That is the same principle behind SUNKEN STOMATA and HAIRY leaves in Question 6(ii) of this paper.
This is a XEROPHYTIC adaptation and is seen in many grasses — which is why a lawn looks visibly narrower-leaved at midday in summer and flattens again by evening.