Cactus and succulents are a very large group of plants, many of which are grown in ornamental gardens for their uncommon appearances. All cactus and succulents have adapted to store water in their leaves and tissues. Cacti are succulents that store water primarily in the stem, and there are about 1,500 species. Cacti may be very large, like the 65-foot (20-m) saguaro, or be so small that some species can be mistaken for rocks or pebbles. Other succulents are even more numerous and usually store water in their fleshy leaves. As they often have similar growing requirements, cactus and succulents of various shapes are often planted together in ornamental gardens.
Although cactus and succulents have similar adaptations, not all succulents are cacti. Cacti are differ from other succulents in their unique growing points called areoles. New growth, spikes, and flowers all appear from these points. All but one species of cactus are native to the Americas, where they can found high in the mountains and even the rainforests of South America. Rainforest cacti are mostly epiphytes that cling to rocks or can be supported in trees, where they collect water and sunlight.
Succulents that store water in leaves are called leaf succulents, but these leaves come in many distinct shapes, sizes, and textures. Root succulents store water underground in their large roots. Root succulents sometimes lose their leaves only to regrow new above-ground foliage when conditions improve. Another class of succulents has a special water-storing structure called a caudex, located where the roots meet the stem. Succulents are widespread in many habitats and climates throughout the world.
One of the more famous cacti is the tall, tree-like saguaro, but this isn't usually cultivated. Tall garden cacti, with heights of 3 to 6 feet (1 to 2 m), include the golden torch cactus, silver torch, and candelabra cactus. Some cacti are known for flowering at night, like the queen of the night and moon cactus. Though many types of cacti are prized for their highly colored flowers, popular flowering varieties include the golden-barrel cactus and the mother of hundreds cactus.
Cactus and succulents differ in appearance because succulents produce a great variety of leaf shapes, many of which are admired by gardeners. Popular genera include aloe, agave, and euphorbia, each of which has many species. Agave come in many sizes but are noted for their thick, long leaves shaped like swords. Aloe plants resemble agave, but they are smaller and often have stripes or streaks of other colors on their leaves. Euphorbia is a large genus of succulents that contains a number of species that are popular as houseplants.