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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 the ability to form new roots quickly. Two hours after rain following a relatively long drought, root formation begins in response to the moisture. 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