Plants that have adjusted to the arid and semi-arid regions of the world are known as cacti and succulents. Cacti are defined as members of one plant family and are considered separately, although they are actually succulents. The term Succulent encompasses all other fleshy plants, which stem from a variety of plant families. Both succulents and cacti have developed means of survival that assure their growth in these regions. Their leaves have become smaller and their stems larger in order to store more water; in some, the leaves have disappeared leaving only thickened green stems. In plants where the leaves have not disappeared, the leaves may have become thickened or insulated with coverings of hair, wax or scales.