by adi_101 Member since:
02 January 2008
Total points:
636 (Level 2)
Add to My Contacts
Block User
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 a few exceptions (for example, the genus "Rhodocactus") all cacti are succulent plants, although not all succulent plants are cacti. Like other succulents, these 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 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 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 of a species that has ephemeral leaves is the Opuntia ficus indicia, 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. 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 also 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. This works by preventing water from spreading on the surface and allowing water to trickle down the stem to be absorbed by the roots and used for photosynthesis. Cacti have a thick, hard-walled, succulent stem - when it rains, water is stored in the stem. The stems are photosynthetic, green, and fleshy. The inside of the stem is either spongy or hollow (depending on the cactus). A thick, waxy coating keeps the water inside the cactus from evaporating.
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 the sphere (combining highest possible volume with lowest possible surface area). By reducing its surface area, the body of the plant is also protected against excessive sunlight.
Most cacti have a short growing season and long dormancy. For example, a fully-grown Saguaro cactus (Carnegiea gigantea) can absorb up to 3,000 litres of water in ten days. This is helped by cacti's ability to form new roots quickly. Only two hours after rain following a relatively long drought the formation of new roots begins. Apart from a few exceptions an extensively ramified root system is formed, which spreads out immediately beneath the surface. The salt concentration in the root cells is relatively high, so that when moisture is encountered, water can immediately be absorbed in the greatest possible quantity.
But the plant body itself is also capable of absorbing moisture (through the epidermis and the thorns), which for plants that are exposed to moisture almost entirely, or indeed in some cases solely, in the form of fog, is of the greatest importance for sustaining life.
Most cacti have very shallow roots that can spread out widely close to the surface of the ground to collect water, an adaptation to infrequent rains; in one examination, a young Saguaro only 12 cm. tall had a root system covering an area 2 meters in diameter, but with no roots more than 10 cm. deep.The larger columnar cacti also develop a taproot, primarily for anchoring but also to reach deeper water supplies and mineral nutrients.
One feature distinguishes the cacti from all other plants: cacti possess areoles, as they are known. The areole appears like a cushion with a diameter of up to 15 mm. and is formed by two opposing buds in the angles of a leaf. From the upper bud develops either a blossom or a side shoot, from the lower bud develop thorns. The two buds of the areoles can lie very close together, but they can also sometimes be separated by several centimeters.
Like other succulents in the families of the Crassulaceae, Agavaceae (agaves), Euphorbiaceae (euphorbias), Liliaceae (lilies), Orchidaceae (orchids) and Vitaceae (vines), cacti reduce water loss through transpiration by Crassulacean acid metabolism. Here, transpiration does not take place during the day at the same time as photosynthesis, but at night. The plant stores the carbon dioxide chemically linked to malic acid until the daytime. During the day the stomata are closed and the plant releases the stored CO2 and uses it for photosynthesis. Because transpiration takes place during the cool humid night hours, water loss through transpiration is significantly reduced.