Lisa Rinehart, age 11, of Prescott, Ariz., for her question:

ARE THERE MANY KIDS OF CACTUS?

Cactus is the common name for a peculiar group of spiny, fleshy plants that are native to the New World. The order consists of a single family, the Cactaceae, with about 90 genera and 2,000 species, most of which are adapted to arid climates.

Because cacti require little care and exhibit bizarre forms, they are very popular for home cultivation and are coming under increasing pressure as a result. More than 17 kinds of cacti now face extinction because of plundering by avid collectors and professional poachers, especially in the southwestern part of the United States.

Cactus plants usually consist of spiny stems and roots. Leaves are greatly reduced or entirely absent. The stems are usually swollen and fleshy, adapted to water storage, and many are shaped in ways that cause rain to flow directly to the root system for absorption.

The flowers of the cacti are often large and showy and occur singly rather than in clusters of several flowers.

The most distinctive vegetative feature of cacti is the areoles, specialized areas on the stems on which stiff, sharp spines usually grow. Some cacti lack spines but have hairs or sharp, barbed structures called glocids on the areoles.