CACTUS. Cacti are succulent perennials that are native to arid and semi-arid regions and are cultivated extensively, except where freezes regularly occur. The land area devoted to cactus cultivation in 2001 was about 1.8 million hectares (4.4 million acres), mostly for fodder, and over half of which was in northern Africa and northeastern Brazil. Cacti are also cultivated in over twenty countries for their fruits, which commercially fall into three categories: cactus pears, which are the fruits of the prickly pear Opuntia ficus-indica and certain other cacti with flat stems (cladodes), and represent over 90% of the cactus fruits sold; pitahayas, which are the fruits of vine cacti in the genera Hylocereus and Selenicereus; and pitayas, which are the fruits of columnar cacti. Young cladodes are consumed as a vegetable (nopalitos), particularly in Mexico. Nearly all cacti employ a photosynthetic pathway...