A cactus (plural : Cacti) plant is able to exist even in extreme conditions. For this, the plant has certain basic structures and processes. The work that is done by leaves in other plants is done by stems and branches of the Cacti.
Since leaves are absent and Cacti have spine-covered branches and stems, they are able to survive in hot dry regions. The thick skins of the stems and branches with a few pores, prevent the loss of water. The root of cacti are spread out close to the surface of the ground thus it quickly absorb water.
The water is stored in the spongy hollow stems of the cactus. The outer layer of the plant is thick and waxy, therefore prevents the water to escape.
Below Cacti info from Wikipedia (Read More):-
A cactus (plural: cacti') is any member of the plant family Cactaceae, native to the Americas. They are often used as ornamental plants, but some are also crop plants. Cacti are grown for protection of property from wild animals, as well as many other uses. Cacti are part of the plant order Caryophyllales, which also includes members like beets, gypsophila, spinach, amaranth, tumbleweeds, carnations, rhubarb, buckwheat, plumbago, bougainvillea, chickweed and knotgrass.
http://en.wikipedia.org/wiki/Cacti
