Cacti are succulent xerophytes, able to store large quantities of water in their leaves, stems, or roots, which give them a fleshy (succulent) character. Many succulent plants grow in drought-prone, nearly arid climates or physiologically dry soil (e.g. frequently frozen or partially or periodically salty soils). This involves adaptations that enable them to make do with a minimal amount of available water, and to survive long periods without rainfall. In botanical ecology, plants with these qualities are called xerophytes. The stems of cacti are thick and succulent. The soft, succulent tissues of the cortex and pith provide a large volume for storing food materials and water. A thick waxy cuticle covers the epidermis and restricts loss of water. The stems may be simple columns with no side shoots or may be branched either near ground level or far above it. The stem surface may be smooth, but is most commonly covered with tubercles, or rows of them, more or less conne cted, called ribs. which represent the sublimation of all branches. Each tubercle bears an areole, a minute hump of tissue in which spines are propagated. The buds of areoles can eventually differentiate into flowers and then fruit depending on the presence or absence of appropriate environmental stimuli and particular precursors. Areoles are found only in cacti. Spines develop from the axillary buds and can be considered to be modified leaves. The spines are always located on the areole. In most cases two forms of spines can be distinguished: central spines that are located close to t he apex of the areole, and radial spines which grow around the circumference of the areole apex. Commonly, the root system of cacti is very shallow and widely spread, which enables them to exploit water deposited in surface horizons by short periods of rain. Cacti reproduce both sexually and asexually. In asexual reproduction all new plants are genetically identical to their parent plants. Asexual reproduction can occur naturally from detached joints, offsets, cuttings and cell propagati on, and including scions in grafting which produce roots after they have separated from the parent plant. Joints which do not develop roots eventually die.