Leaves of cactus are modified into thorns

To reduce loss of water through transpiration leaves of cactus plants are modified into spines.

Explanation:

leaves are reduced to spines in a cactus , so that the water does not get evaporated with the heat of the SUN. 1. They are present in hot deserts which are scarce in water so in order to reduce the loss of water by Transpiration from the surface of the leaves, its leaves are reduced to spines

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.