There are over 60 families of succulent plants. Three hundred genera have evolved to survive in areas with little moisture, where most other plants are unable to grow. Their name comes from the Latin "succos," meaning juice or sap. Succulents are able to store water to sustain them in harshly arid conditions. The process of absorption of moisture, or assimilation, varies, as does transpiration, depending on the classification of the succulent.
Leaf succulents have several ways to retain water. Moisture loss can be prevented on some succulents, like the jade plant, by a waxlike skin cover on the leaves. The compact leaves of the aloes form rosettes that protect the plant from the sun, retaining the moisture in the plant and the soil directly beneath it.
Small evaporation areas have developed in succulents like the lithops (living stones) to prevent moisture loss. In plants classified as leaf succulents, most of the visual plant is made up of leaves. The stems are so short that they seem almost nonexistent. The leaves are moisture storage tissue. In extremely long, dry periods, the stressed leaf succulent will shed its leaves.
Some succulents, like the Calibanus hookeri, or Mexican boulder plant, store water for survival below ground in the root system. This not only protects the moisture and nutrients from evaporation due to the sun, but also protects from predators. The root systems are shallow and spread out over a large area to collect moisture and nutrients from dew or infrequent rains. They can quickly produce new root hairs to collect extra moisture if the opportunity arises.
Aerial roots, like those of the epiphytes, can collect moisture from the air. Assimilation and transpiration of moisture during the growing season of the root succulent is the job of the leaves and stems. During good conditions, the leaves and stems become additional storage for moisture, but in harsh conditions they are often shed, leaving just the underground tubers and roots of the living plant.
The cacti are in the stem succulent classification. There are few or no leaves, and the stem has cells overlaid with photosynthetic tissue to store water. The absence of leaves means that the stem succulents have less area susceptible to water evaporation. If the stem succulent has leaves or shoots, they can aid the stem in absorption and transpiration. The stems of some stem succulents appear waxlike and are angled or have coatings of hair.
Cyphostemma juttae (wild grape) and ceraria pygmaea are examples of succulents that use more than one organ to store moisture. Called caudiciforms or cryptic succulents, the roots, stems and leaves are all used for moisture storage. Most caudiciforms are deciduous, and when the leaves are shed rely on the roots and stems for survival.
Succulent Survival Techniques
In addition to the varied forms of moisture retention, the succulent has evolved to survive in the environment it finds itself. The lithops are commonly called living stone, and have developed the ability to mimic the stony areas they grow in. This camouflage protects them from predators.
The sap of the euphorbias is poisonous, as is the adeniums (such as desert rose). Thorns protect cacti and other succulents from grazing animals, and the asclepiads (carrion flowers) taste so bad that most predators won't eat them.