The plants correctly designated by this name constitute the family Cactaceae. Scarcely any group in the whole vegetable kingdom is more remarkable for its strange and varied forms, the beauty of its flowers, and wonderful adaptation to desert life. It is not, however, confined to desert regions; for in the moist forests of the tropics of the New World it is represented by a number of interesting forms often epiphytal or scrambling in their habit of growth, with beautiful flowers and sometimes with delicious edible fruit.

Tips of Rhipsalis cassytha.

Fig. 712. Tips of Rhipsalis cassytha.

Skeleton of Opuntia stem.

Fig. 713. Skeleton of Opuntia stem.

Pereskia aculeata.

Fig. 714. Pereskia aculeata.

Opuntia joint with leaves.

Fig. 715. Opuntia joint with leaves.

The Cactaceae are confined to America, the only apparent exception being the genus Rhipsalis, composed of plants with the habits of the mistletoe, growing on the trunks and branches of trees, and bearing small pellucid glutinous berries (Fig. 712). This genus, endemic in tropical America, has found its way to Africa, the island of Mauritius and even to Ceylon; and several opuntias, or prickly pears, occur on the shores of the Mediterranean, in South Africa, and Australia, where they have made themselves so thoroughly at home as to be regarded by many writers as indigenous. The Cactaceae are not confined to tropical or even semi-tropical regions. At least two species of Opuntia extend northward into British Columbia, and species of Echinocereus, Echinocactus, and Mamillaria are found in the state of Colorado. The xerophytic forms flourish especially in the southwestern United States, the Mexican plateau, the peninsula of Lower California, where there are great cactus forests, and the vicinity of Tehuacan, in the southern part of the Mexican state of Puebla, a region celebrated for its remarkable and gigantic tree-like forms related to the genus Cereus. For an account of the vegetation of the deserts of the southwestern states and of Mexico, the reader is referred to Frederick V. Coville's "Botany of the Death Valley Expedition," published as Vol. IV of the "Contributions from the United States National Herbarium, 1893;" Coville and MacDougal's "Desert Botanical Laboratory of the Carnegie Institution -1903"; and to D. T. MacDougal's "Botanical Features of North American Deserts," publication No. 99 of the Carnegie Institution of Washington, 1908.

Cactus spines

Fig. 716. Cactus spines

To the southward, the family extends to Chile and Argentina. Giant torch thistles and echinocacti are scattered over the pampas of Uruguay, and melon-shaped echinopses amid the snows of the lofty plateau of Bolivia.

The genus Mamillaria, so well represented in the southwestern United States and Mexico, is almost absent from Central America, the representative genera of that region as well as of the warm Huasteca region of eastern Mexico being Cereus, Pereskia, Pereskiopsis, Nopalea, and Opuntia; while the "turk's-head" or "melon cacti" are chiefly West Indian.

The peculiar structure of columnar, opuntioid, and melon-shaped cacti is undoubtedly the result of excessive dryness of the climates in which they occur, to protect themselves from which they have been obliged to store up water and to reduce their transpiration as low as possible. They have a more or less pronounced woody axis surrounded by pulpy cellular tissue (parenchyma) in which the water-supply is stored. The stomata are usually situated in depressions or grooves in the leathery cuticle; and as an additional means for checking transpiration, the cell-sap is nearly always mucilaginous, while in some forms latex cells are present, filled with milky or gummy fluid which hardens on exposure to the air and effectively heals wounds in the soft fleshy plant. Certain species of Echinocactus (viz-nagas) are like great barrels studded with spines and filled with pulp of the consistency of watermelon rind, which is sometimes made into conserves like citron (dulces de viznaga). Other forms, like species of Pereskia, Pereskiopsis, and arboreous opuntias have hard, woody stems and branches. The reticulated skeletons of certain species of opuntia (Fig. 713) are manufactured into walking-sticks, legs of furniture, napkin rings, and even into veneering for woodwork.

In Lower California and some parts of South America, where other vegetation is lacking, the stems of columnar cerei, or "cardones," are used for constructing habitations, inclosures, and for timbering mines. Columnar cacti are also planted for living fences, or hedges, especially the "organ cactus" (Myrtil-locactus geometrizans) of tropical Mexico. Leaves are present in nearly all cacti, but in some species they are mere vestiges and can scarcely be seen with the naked eye. In other species they are large and perfectly developed, either with distinct petiole and feather veins, as in Pereskia acu-leata (Fig. 714), or sessile and fleshy with only the midrib and several parallel nerves apparent as in the genus Pereskiopsis. They are sometimes caducous, fleshy, cylindrical or awl-shaped, as in the genus Opuntia (Fig. 715). In the axils of the leaves are peculiar cushion-like areoles (corresponding in all probability to aborted branches) clothed with down or felt-like wool, from which spines, and, in some genera, also flowers, issue.

In the genera Opuntia and Pereskiopsis, the areoles also bear minute short barbed bristles called glochidia, which will penetrate the skin and become detached at the slightest contact and are the source of annoying irritation which often persists for many hours.

Opuntia leptocaulis, showing sheathed spines.

Fig. 717. Opuntia leptocaulis, showing sheathed spines.