Succulents are plants from more than 60 families and 300 genera. They have evolved special water-storage tissues in thickened or swollen leaves, stems or roots as an adaptation to arid environments. By making the most of scarce available moisture, succulents can survive in habitats that are far too dry for most other plants.

Leaf Succulents: Leaves are almost entirely composed of water storage cells covered by a thin layer of photosynthetic tissue.
Examples: Aloe, Haworthia, Lithops, Sempervivum.

Stem Succulents: Fleshy stems contain water storage cells overlaid by photosynthetic tissue. Leaves are almost or entirely absent, reducing surface area to prevent evaporative loss of water.
Examples: most cacti, Euphorbia obesa, Stapelia.

Root Succulents: Swollen fleshy roots store water underground away from the heat of the sun and hungry animals. Stems and leaves are often deciduous and shed during prolonged dry seasons.
Examples: Calibanus hookeri, Fockea edulis, Pterocactus kunzei, Peniocereus striatus.