They are plants adapted to life in environments in which water availability is reduced or very low: dry and hot climates or cold countries such as desert, tundra, ice- or snow-covered regions and saline soil in border of oceans. Among groups of xerophytic plants, they are succulent and non – succulent perennials which show a different strategy to survive in dry environment: the first ones will adapt by storing available water in organs and the second by preventing water loss. Both categories develop morphological and anatomical adaptations to decrease loss of water by transpiration and to increase water absorption by the root system. They exhibit a variety of adaptive features or xeromorphic traits.
Ephemeral annuals are another category of xerophytes. They complete their live cycles within a very short period. To escape drought, they survive critical periods as seeds which will germinate immediately in response to rain.
Non-succulent perennials They are drought – enduring resistant plants which are able to support extreme dry conditions in the aerial environment as well as in the soil for long periods. They are considered as true xerophytes and they possess a number of morphological adaptations to reduce loss of water by transpiration: reduction of leaf surface (dwarfism, leaf curl) or replacement of the leaf by the stem. Other adaptations are developed to favor water capture by the roots: extensive root system in surface when water is limited to the surface layer, as in desert plants, or development of a large and deeply penetrating root system, called intensive root system, to have access to water table. Some plants will develop both extensive and intensive roots.
Different anatomical adaptations are observed at the leaf level: thick and waxy cuticle (for reflection of heat and radiations); high density of trichomes to keep humidity, hypodermis to protect mesophyll which is generally compact. The stomata, which are essential for gas exchanges, show one or several adaptations: reduced density, presence only on lower face of the leaf, closing during short periods of the day, hidden in crypts. Finally leaves generally contain more lignified tissues to for higher impermeability and resistance of aerial parts of the plants to wind for plants living in border of oceans.
Succulent They store water in aerial parts of the plants either stems (stem succulence) or leaves (foliar succulence). Some plants such as Crassula, exhibit both foliar and stem succulence.
Succulents show a particular morphology: their aerial organs most often exhibit a high volume for a minimal surface (cylindrical shape), fleshy leaves, small leaves or leaves modified in spines or scales. In cactus leaves are replaced by spines for a maximal protection and to discourage animals from eating them for water.
The root system of succulents is adapted to problems of water supply. They have shallow roots to capture surface water i.e., moisture from rain, condensation, and dew. In addition, they also have extremely long roots which penetrate deep into the soil to reach and capture water table.
At anatomical level succulent leaves exhibit characteristics like those of non- succulent perennials: a thick cuticle, crypts to protect stomata, abundance of lignified tissues, particular cells called bulliform cells to enroll the leaves and aqueous parenchyma in which water is stored in vacuoles.