cacti are flowering plants that have become adapted to life in hot desert. since the major drawback to life in the desert is lack of water,most adaptations of cacti are for efficient water absorption and conversation. thus,most cacti have huge spreading root systems which cover enormous areas of soil close to the surface. such systems pick up a great deal of the infrequent rainfall before it evaporates.
cacti,like most plants,consist of 90 percent water. at the same time,they are extraordinarily efficient in keeping what water they have. they are compact in shape - barrel shaped,spherical,globular, or tubular and this keeps theirs surface area relatively small compared with their volume. a reduced surface area cuts down the amount of water lost to the atmosphere. other factors which cut down water loss are a thick outer covering,and reduced leaves.
cactus leaves are reduced and modified into protective prickles called spines. the food-making job that is the responbility of the leaves in all normal plants is taken over in cacti by the green stems. the stems prossess only a very few stomata (pores) in their surface, so again ,water loss is kept to minimum. the most important modification of all is water storage tissue in the stems. the cells absord and hold water strongly, and the fat,fleshy stems can expend enormously when water is freely available.
cacti are perennial plants. most of them live in the deserts of south-western north america.