Question: why does cacti have to be spiky? :)

Oli Wilson answered on 6 Nov 2017:

The main reason cacti are spiky is so they can hold on to their water. In the deserts water is one of the most precious things living things need, so plants and animals do everything they can to get as much of it as possible, and to make sure they don’t lose it. Plants mainly lose water through holes in their leaves called stomata (which also let gases in for photosynthesis), so cacti have evolved to have as few stomata as they can – and they’ve rolled up their leaves into spines! (An added advantage is that they put off animals that would try and eat them – again, protect the water at all costs!) That’s also why lots of cacti look green – they’ve moved photosynthesis from their leaves (now spines and not good at catching sunlight) to their stems 🙂