At a broad level, cacti have a lot of characteristics in common with other plants. They possess chlorophyll and make sugar by combining carbon dioxide and water through photosynthesis. They have true flowers, reproduce sexually through seeds and have water-absorbing roots. On the other hand, cacti have a number of characteristics that make them quite different, at least superficially, from other flowering plants. Many of these characteristics are adaptations for surviving drought.
In botany, the term "succulent" refers to drought-tolerant plants that have tissues modified for water storage. Jade plants and houseleeks, with their plump water-storing leaves, are two examples. The vast majority of cactus are also succulents, adapted to living in the arid regions of North and South America. In cacti, however, it is the stem that stores water rather than the leaves.
The lack of leaves is another water-preserving characteristic of cacti. Leaves have a lot of surface area and therefore lose a lot of water. Only a handful of cactus species, such as those in the tropical genus Pereskia, have true leaves. These primitive, leafy types of cacti also have non-succulent stems. Normally, cacti have sharp spines instead of leaves. The spines, which are modified leaves, protect the juicy, water-filled stem from thirsty desert animals.
Like most succulent plants, cacti possess a thick, waxy outer covering known as a cuticle. On some cactus species, the cuticle is thick enough that you can scratch wax off the plant with a fingernail, although loss of the wax can damage the plant. The cuticle keeps water stored inside the plant from evaporating into the outside atmosphere. The cuticle also protects the cactus from germs that may try to invade through the skin.
Since a cactus is covered in a waxy cuticle, the main way a cactus can lose water is through its stomata. These are microscopic holes in a plant's skin that let in the carbon dioxide a plant needs to make sugar. When the stomata are open, water vapor inside the plant easily escapes. To minimize this loss, cacti only open their stomata at night, a system known as crassulacean acid metabolism, or CAM. Other plants that use CAM include pineapples and leafy succulents.
All green parts of a plant can photosynthesize, but leaves usually carry out most of the work of photosynthesis. In fact, stems of most leafy plants have few or no stomata to supply nearby photosynthesizing cells with carbon dioxide. In leafless cacti, however, stems are the main photosynthesizers. Therefore, cactus stems have more stomata than do the stems of leafy plants, although less than leaves typically do. According to Arthur Gibson and Park Nobel in "The Cactus Primer," cacti have 15 to 70 stomata per square millimeter, while leaves have more than 100.