Cactus
4
Some environments, such as deserts, semi-deserts, and dry steppes, receive little water in the form of precipitation. Plants that inhabit these dry areas are known as xerophytes, and many of them are succulents, with thick or reduced, "succulent", leaves. Apart from a few exceptions (for example, the genus Pereskia) all cacti are succulent plants. Like other succulents, cacti have a range of specific adaptations that enable them to survive in these environments. Cacti have never lost their leaves completely; they have only reduced the size so that they reduce the surface area through which water can be lost by
Adaptations to dry environment
transpiration. In some species the leaves are still remarkably large and ordinary while in other species they have become microscopic but they still contain the stomata, xylem and phloem. Certain cactus species have also developed ephemeral or deciduous leaves, which are leaves that last for a short period of time when the stem is still in its early stages of development. A good example is Opuntia ficus-indica, better known as the prickly pear. Cacti have also developed spines which allow less water to evaporate through transpiration by shading the plant, and defend the cactus against water-seeking animals. The spines grow from specialized structures called areoles, homologous to the nodes on other plants. Very few members of the family have leaves, and when present these are usually rudimentary and soon fall off; they are typically awl-shaped and only 1–3 mm. long. Two genera, Pereskia and Pereskiopsis, do however retain large, non-succulent leaves 5–25 cm. long, and non-succulent stems. Pereskia has now been determined to be the ancestral genus from which all other cacti evolved. Enlarged stems carry out photosynthesis and store water. Unlike many other succulents, the stem is the only part of a true cactus where this takes place. Much like many other plants that have waxy coatings on their leaves, cacti often have a waxy coating on their stems to prevent water loss and potentially repel water from their stems. The bodies of many cacti have become thickened during the course of evolution, and form water-retentive tissue and in many cases assume the optimal shape of a sphere or cylinder (combining highest