http://homeguides.sfgate.com/plants-adapted-prevent-water-loss-104586.html
by Carolyn Csanyi, Demand Media
Century plant’s waxy coating reflects sunlight and prevents water loss.
Where water is plentiful and temperatures are moderate, plants have wide, thin leaves with lots of surface area for maximum photosynthesis. Plants adapted to drought conditions, however, exhibit a number of structural features that prevent water loss, helping them survive hot, dry conditions. They also have physiological mechanisms that aren’t directly observable and that help them conserve water. Many of these plants are suitable drought-tolerant landscaping subjects.
Reduced Leaves
A typical leaf has three main layers. The upper and lower layers consist of epidermis, which is usually one cell thick. The mesophyll is in the leaf’s middle; it is moist and is where photosynthesis occurs. The epidermis contains breathing holes, called stomata, where gas exchange occurs. On leaves with thin epidermis and numerous stomata, water escapes through the epidermis and stomata. Some arid-climate plants are able to conserve water because of their reduced leaf size. Less leaf surface area results in reduced water loss through the epidermis. Small leaves have fewer stomata than larger leaves, and that adaptation also reduces water loss. Some dry-land plants have stomata only on the bottom epidermis, which further reducing water loss, and some have several layers of epidermal cells. An example of a plant with small leaves is “Rosy Dawn” manzanita (Arctostaphylos edmundsii “Rosy Dawn”), which is perennial in U.S. Department of Agriculture plant hardiness zones 8b through 10; it has gray-green leaves tinged with pink and pink flowers. Prickly pears (Opuntia spp., USDA zones 3b through 11) have very reduced, cylindrical, fleshy leaves that occur on only new growth.
Water Storage
Succulent plants have developed multiple structural mechanisms that prevent water loss. When water is available, they absorb it through their roots and bind it in place in interior water storage cells. The water is held there without the danger of being lost until the plants need it. Succulent plants include golden barrel cactus (Echinocactus grusonii, USDA zones 9 through 11), which stores water in its stem, and coral aloe (Aloe striata, USDA zones 9 through 11), which stores water in its leaves.
Coated Leaves
Coatings of wax or hairs also help prevent water loss in plants. Waxy layers can give either a sheen or dull, grayish or bluish cast to a leaf surface. Century plant (Agave americana, USDA zones 8 through 11) has a dull, waxy coating that adds a gray color to its long leaves. A shiny wax and a coating of hairs are on the leaves of carob tree (Ceratonia siliqua, USDA zones 9 through 11), a native of the Eastern Mediterranean. The hairs help slow air movement over the tree’s leaves, reducing transpiration and water loss. Brittlebush (Encelia farinosa, USDA zones 8 through 11) has silvery white leaves covered with reflective, white hairs that can reduce the plant’s temperature by several degrees.
Physiological Mechanisms
Many succulents and arid-climate plants have a specialized form of photosynthesis called Crassulacean acid metabolism. Their stomata open only at night and store the carbon dioxide they absorb; the plants use the carbon dioxide for photosynthesis during daylight hours. During extended droughts, these plants can decrease their metabolism rate, keeping their stomata closed day and night, and maintaining in moist internal tissues a low level of activity sufficient to sustain life.
