Why do desert plants like cacti have sharp spines instead of broad leaves?
Broad leaves lose a lot of water through transpiration, which a desert plant can't afford.
Broad leaves lose a lot of water through transpiration, which a desert plant can't afford. So the leaves are reduced to spines, which have almost no surface area to lose water and also defend the plant from thirsty animals. The job of photosynthesis is taken over by the flattened green stem, which stores water too. It's a water-saving redesign of leaf and stem.
Key point
Leaves become spines to cut water loss and deter grazers; the green stem does photosynthesis.
Common mistake
thinking the spines are for photosynthesis.
Memory tip
spines save water + protect; the fleshy stem photosynthesises and stores water.
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