| MadSci Network: Botany |
Charles, The funny thing is that cacti do have leaves but you might not recognise them! The leaves of cacti are in fact the spines. Cacti (and some other plants which live in dry desert environments) have evolved some really cunning adaptations to the searing heat and lack of water. Normal leaves have several functions: they photosynthesise providing food (carbohydrates, like sugars) for the plant, they allow oxygen and carbon dioxide in and out through little holes on the leaves called stomata, they also lose water vapour out of these stomata in a process called transpiration. Plants can lose alot of water in this way (you can measure how much by putting a freshly cut plant in a glass of water and marking the level, come back every couple of hours and see how far the water goes down, put some plastic cling-wrap over the top of the glass so that you know the water lost is just through the plant and not evaporating from the surface. Some of the water is lost through transpiration and some is used by the plant.).
Cacti have very reduced leaves to cut down on the amount of water they lose through transpiration. They can't afford to have a constant stream of water evaporating through the stomata so cacti with smaller and smaller leaves evolved. Being economical, instead of getting rid of leaves altogether they adapted the reduced leaves for another function - defence. Cacti are slow growing and store valuable water in their enlarged stems (the large flattened stems of cacti are called cladodes), it would be very costly for them to lose the stored water to thirsty herbivores so the cacti which developed spines from their reduced leaves survived longer and had more offspring - leading to the evolution of more and more specialised leaves/spines. To read more about plant morphology and the funky adaptations they have evolved to cope with different environments have a look at
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