Why do some plants have thorns?
In short, thorns are leaves; yet adapted to arid environments. In nature, living things tend to adapt to their environments, and plants are not an exception.
Allow me to elaborate. Leaves are a very important part of the plant, as it is through them that plants can breathe and photosynthesise their food. Yet leaves are also the part through which a plant sweats. Can you imagine how dangerous could this be if a plant lives in the desert? Plants know, so some of them reduced the size and thickness of some or all of their leaves to its minimum, or close to its minimum —the small spikes we call thorns. This way, they prevent losing water uselessly by sweating.
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