Give biological reasons for the following statements:
In some xerophytes leaves are modified into spines.
Topic: Spines in xerophytes
Answer
To REDUCE TRANSPIRATION. A spine has a very much smaller SURFACE AREA than a leaf, so far less water is lost as vapour — which is essential in a dry habitat. The spines also DEFEND the plant against grazing animals, and in such plants the STEM becomes green and takes over photosynthesis.
The main reason is the reduction of SURFACE AREA, and saying so is what earns the mark. Adding the defensive role shows a second advantage of the same modification.
THE TRADE-OFF worth mentioning: a plant that turns its leaves into spines has given up its photosynthetic organ, so the STEM must take over — which is why a cactus stem is green, thick and fleshy. One adaptation forces another.
The other xerophytic adaptations, all working on the same principle: SUNKEN STOMATA, fewer stomata, a THICK WAXY CUTICLE, hairy surfaces and rolled leaves — either reducing exposed area or trapping moist air against the leaf.