Large Root Systems
Even the tiniest of cacti can have several feet of roots surrounding them. These roots lay close to the desert surface so they can catch water almost as soon as it hits the ground. When there's a heavy rain, a cactus's roots can sprout smaller "feeder roots" quickly to spread out even more. After the storm ends, the plant cuts these new roots off and they lay dormant or die. There's no need to have more parts to feed or to risk water seeping out of them and into the ground.
Stems
Most cacti have thick stems and few or no leaves. These stems serve several purposes. First of all, they are flexible, so they can grow during heavy rains to accommodate more water and store it. Second, they are covered in a waxy substance, or cuticle, that impedes transpiration, the evaporation of water off the plant. You may notice that most stems grow vertically instead of spreading out like tree branches. This helps water run down the cactus and reach the roots instead of settling on the plant itself. Another feature of cactus stems are stomata, pores that help all advanced plants get the carbon dioxide they need for photosynthesis. Because water can get out when the cactus opens these pores, many species can close them more quickly than other plants. They can also open them at night when the sun isn't out to speed up absorption and complete the photosynthetic process the next day. This is known as crassulacean acid metabolism photosynthesis.
Needles
The needles, or spines, on a cactus are not there just to prick your finger. They're specifically designed to help the cactus survive. They keep animals at bay so they don't harm the plant by eating the stems or siphoning out vital water. Some also provide shade to the main part of the plant, which helps the cactus keep its internal temperature down. Needles can help to catch water and draw it inward and downward.
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