Cacti are succulent plants that have a variety of adaptations that enable them to survive in hot and dry environments. In most species of cacti the leaves have evolved into spines which not only defend the cactus against herbivores but also provide shade that lowers the plant's water loss through transpiration. The spines grow from specialized structures called areoles, homologous to the nodes on other plants.
Areoles are unique features to cacti. The areole appears like a cushion with a diameter of up to 15 mm, and is formed by two opposing buds in the angles of a leaf. The upper bud develops into either a blossom or a side shoot, while the lower bud develops into spines. The two buds of the areoles can lie very close together or be separated by several cm.
Cacti often have a waxy coating on their stems to prevent water loss and potentially repel water from their stems. Because of the plants' high water-retention ability, detached parts of the plant can survive for long periods and then grow new roots from anywhere on the plant body when rain comes.
The bodies of many cacti have become thickened during the course of evolution, and form water-retentive tissue that is in the optimal shape of a sphere or cylinder (combining highest possible volume with lowest possible surface area). By reducing its surface area, the body of the plant is also protected against excessive sunlight. The plant body itself is also capable of absorbing moisture, through the epidermis and the spines, which is especially important for plants that receive most of their moisture in the form of fog.
Cacti often have very shallow roots that spread out widely close to the surface to collect water, an adaptation to infrequent rains.