Cacti have adapted in many ways to prevent water loss. They live in arid climates so they must efficiently use the little water that they have. First, cacti are CAM plants. CAM plants use Crassulacean acid metabolism to conserve water. This means that they only open their stomata to get access to carbon dioxide at night when temperatures and risks of water loss from evaporation/transpiration are low. Additionally, cacti have no visible leaves. This is because water loss is proportional to surface area. A cactus doesn’t lose as much water since it has thick stems instead of thin leaves. A cactus does have spines, however, which are modified leaves. These spines do not contain water and they serve to protect the cactus from herbivores. These spines also trap air near the cactus, creating a moist layer that prevents evaporation/transpiration, and they provide shade to lower the temperature of the cactus. The stems of cacti have adapted to undergo photosynthesis and they are constructed in such a way that 90% of the mass of the cactus is water. The stems take advantage of a low surface area to volume ratio and can absorb large amounts of water. Finally, the stems have a waxy cuticle layer which also prevents water loss.