Cactus

A cactus (plural, cacti) is a remarkable example of the way plants have adapted to extreme conditions. Cacti hve the basic structures and processes of plants. But the work done by leaves in most other plants goes on in the stems and branches of cacti. And in the hot, dry region where cacti are among the few green plants, their spin-covered branches and stems and their absence of leaves have allowed them to survive.

The distant ancestors of the cactus had leaves and grew like the more familiar plants of today. But during millions of years the earth’s climate changed. Those parts of the Americas where the cactus ancestors grew became hotter and drier. Gradually these regions turned into desert or near desert. All this time the cactus ancestors were adapting to the changing conditions.
For example, as the climate became drier the roots of cacti gradually spread out, closer to the surface of the ground. That is why cacti quicly absorb water from the earth after a rainfall.

The water taken in trought the roots of a cactus is stored in its spongy or hollow stem. The outer layer of the plant is thick and waxy, preventing the escape of water. The outer skin is also ribbed. Some cacti have ribs that fold and expand like an accordion, depending on how much water is contained within the stem.

Although most cacti are leafless, they carry on the normal food-making activities of plants. The leaves of other plants are thin structures that contain many breathing pores; in the course of the the food-making process, water is given off to the air through these pores. But in cacti the stems and branches have taken over the work of the leaves. The thick skins have few pores, and the water is retained.

There are still some members of the cactus family that have leaves and stems like more familiar plants-the lemon vine of the West Indies is one. In most cacti the leaves have developed into spines, needles, or hairs. These growths now serve to protect the cacti-which are often the only green plants in an area-from animals.

The true cacti are native only to the Western Hemisphere. They grow mainly in the dry land of South America, Central America, and the southwestern United States, Mexico has the greatest number and variety of cacti. A few cacti have extended their ranges as far north as Canada.
In South Africa, Madagascar, and Sri Lanka there is the mistletoe cactus, a small plant that grows on trees. Probably the seeds of these plants were carried from Americas by birds. Cacti have been introduced to many parts of the world by people. ( The African desert have native plants that resemble cacti, but these are actually daisies and milkweeds that have adapted to desert life).