Native to the tropics and subtropics, this rich fruit is known for its lush, buttery texture and mild, faintly nutlike flavor. The fruit's name comes from ahuacatl, the Nahuatl word for "testicle," which is assumed to be a reference to the avocado's shape. Florida was the site of the first U. S. avocado trees in the 1830s but almost 80 percent of today's crop comes from California. Known early on as alligator pear, the many varieties of today's avocado can range from round to pear-shaped. The skin can be thick to thin, green to purplish black and smooth to corrugated. The flesh is generally a pale yellow-green and softly succulent. The two most widely marketed avocado varieties are the pebbly textured, almost black Haas and the green Fuerte, which has a thin, smooth skin. Depending on the variety, an avocado can weigh as little as 3 ounces and as much as 4 pounds. There are even tiny Fuerte cocktail avocados (also called avocaditos) that are the size of a small gherkin and weigh about 1 ounce. Like many fruits, avocados ripen best off the tree. Ripe avocados yield to gentle palm pressure, but firm, unripe avocados are what are usually found in the market. Select those that are unblemished and heavy for their size. To speed the ripening process, place several avocados in a paper bag and set aside at room temperature for 2 to 4 days. Ripe avocados can be stored in the refrigerator several days. Once avocado flesh is cut and exposed to the air it tends to discolor rapidly. To minimize this effect it is always advisable to add cubed or sliced avocado to a dish at the last moment. When a dish containing mashed avocado, such as guacamole, is being prepared, the addition of lemon or lime juice helps to prevent discoloration. (It is not true that burying the avocado pit in the guacamole helps maintain good color. ) Though avocados are high in unsaturated fat, the California Avocado Advisory Board states that half of an 8-ounce avocado contains only 138 calories. In addition, avocados contain a fair amount of vitamin C, thiamine and riboflavin.