When you think of the purpose of cactus spines, your first thought may be that they serve as protection from animals, but for many cacti, the spines serve other functions. What else do they do for the cactus?
Answer:
Many cacti use their spines to shade the stem from the blistering desert sun. Normal leaves would be damaged by the heat, but the spines are hardened with pectin and calcium carbonate, and are not harmed by the sun's heat.
Other cacti use their needles for reproduction. The needles stick in the flesh or hair of animals and that part of the plant breaks free. When the animal manages to remove it, the piece of cactus grows into a new plant.
Some cacti even use their needles to move to a new location. When an animal bumps into a fishhook cactus, if the plant is healthy and growing well, the spine breaks off, leaving the plant in place. On the other hand, if the plant is not doing well, its roots give way, and the whole plant is dragged along, to take root in a new spot which will hopefully have better soil or more water.
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