More than 100 species of cacti grow in Texas, the widest assortment found in any state in the United States. Many are best known by such nonscientific names as blind pear, cow-tongue cactus, night-blooming cereus, Texas rainbow, tree cactus, early bloomer, and devil's head, so-called because its rigid spines are dangerous to the hoofs of horses and cattle. Numerous other varieties are commonly called strawberry cactus, pincushion, and jumping jack. Cacti are used in Texas for foods, for landscaping, and for commercial and private botanical collections. The tunas, or seed pods, of the prickly pear are used in making salads, wines, and jelly; the pads, or nopalitos, with their spines singed off, make a substantial food for cattle and form a minor staple in Tex-Mex food. Other cacti are used to make food colorings, medicines, and candy. The climatic adaptability of cacti and their ease of culture make them useful in gardens and as shrubbery; their unusual forms and multicolored flowers, which vary in shade from green and white to magenta and purple, attract many collectors. The sizes of cacti range from the minute button cactus, smaller than a dime, to the barrel or fishhook cactus, which weighs up to half a ton or more.
The cacti of Texas represent ten genera:
Genus Echinocereus. Echino- ("spiny") refers to the very thorny covering of this genus, and cereus ("wax candle") comes from the stately appearance of its upright species. Echinocerei are oval, conical, or cylindrical cacti, always with ribbed stems. The flowers are usually large and beautiful, though a few have small and inconspicuous greenish flowers. The fruits are always fleshy and thin-skinned and often edible; they are also spiny, but the spines loosen as the fruits mature and may be easily brushed off. The Echinocerei grow mostly in exposed places on dry slopes and hills in the full strength of the southwestern sun.
Genus Wilcoxia. Five species in this genus are usually recognized, four in Mexico and one in South Texas. The cacti have slender stems of about five-eighths inch or less diameter. The spines are very short, a quarter inch or less long. The flower is large and beautiful, bell-shaped or funnel-shaped, reddish to purplish, and diurnal. The ovary surface is scaly, wooly, and covered with bristly or hairlike spines that remain on the fruits.
Genus Peniocereus. The "thread cereus" cacti all have slender stems and an extremely large, fleshy taproot, from which grow stems that are ribbed at first but become round. All have fragrant nocturnal flowers produced from within the spine areole, and all have very short spines on the stems and rigid spines on the fruits.
Genus Acanthocereus. There are about a dozen species in the "acanthus candle" genus. These are more or less shrubby plants that grow upright but cannot support their own weight for long and depend on some support, usually other plants. Supported stems may grow to twenty feet tall. All stems are from one to four inches in diameter, and mature stems have from three to seven conspicuous ribs. The flowers are nocturnal, large, and white, and the ovary is usually spiny. These tropical lowland cacti are never found far from a coast and seem to thrive best on semiarid coastal plains. They tolerate much more moisture than most cacti and when water is adequate grow very rapidly. A light frost will kill the tips of the stems, and 32° F will kill all of the plant above the ground, though the roots may sprout again. In the United States the Acanthocerei have a precarious existence along the coast in South Texas and Florida.
Genus Echinocactus. Most members of this genus, known as the barrel cacti, have strong, rigid, numerous spines. A few have more slender and flexible spines, and the genus includes a some spineless members. The barrels range in size from several hundred pounds to miniature forms only a few inches high. The exterior exhibits from eight to more than twenty vertical or spiraling ribs. Flowers are produced at or near the apex of the plant and have no distinct floral tube; the ovary bears scales and sometimes wool but not spines.
Genus Lophophora. The members of this genus, the "crest-bearers," are small, globose, or depressed globose cacti that grow from comparatively large, carrot-shaped taproots. The stem is about three inches in diameter but stands no more than two inches above the ground. Stems may be single or may branch from the base to form large clusters. Surfaces are blue-green and often glaucous. The plants have no spines after the early seedling stage. The ribs are broad and flat. The areoles are small and round with long white to yellowish wool that often persists, and the flowers are small, bell-shaped, and varied in color. In this genus the ovary and fruit are entirely naked, and the fruit remains always fleshy. The stems are ribbed. Monomorphic areoles produce the flowers from the apexes of young tubercles rather than from the axils. Peyote belongs to this genus.
Genus Ariocarpus. This is a small genus with only one species in Texas and the others in Mexico. The body of the plant consists of one or occasionally a cluster of low flattened stems ranging from two inches in diameter to ten inches across. The smaller species may not project above the surface of the ground, whereas the larger ones reach five inches tall. The stem sits on top of a large carrot-like taproot. The surface of the stem is divided into very distinct, usually imbricated but noncoalescent tubercles. There are no spines after first seedling growth. This genus is unusual in that it flowers in the fall. Flowers open widely, are diurnal, and are white, yellowish, or purplish. The ovary and fruit are both naked. The fruit is fleshy at first, becomes dry at maturity, and disintegrates, leaving the seeds in the wool at the center of the plant.
Genus Epithelantha. Although a number of species of this genus grow in Mexico, only one grows in the United States. The whole stem is covered with very many, very tiny tubercles, apparently the smallest tubercles of any United States cactus. These are hidden almost entirely from view by many tiny spines. The growing tip of the stem is a distinct depression filled with a great deal of hairlike wool and covered over by the converging, later deciduous, tips of the longer spines; this covering makes it difficult to observe the formation of the tubercles, areoles, and flowers. Also unusual about this cactus is the fact that it produces its flowers not in the axil of the tubercle but at the top of it. This cactus does not produce its flower from within a monomorphic spine areole as previously believed. The blossom is produced after a division of the meristem into a determinate spiny portion and a separate, indeterminate, floral or vegetative meristem.
Genus Mammillaria. Members of this genus are small or very small. The stems vary in different species from depressed and almost flat to globular or sometimes columnar and are often referred to as heads. In some species these remain single, but in many others they multiply from the base to become caespitose; one individual sometimes forms a large clump of heads. In a few species branches may occasionally grow from higher up on the stem. Each stem is entirely covered by a system of nipple-like projections called tubercles. These are usually arranged in spiral rows but in a few cases are more loosely organized. The tubercles are usually cylindrical or conical but sometimes have more or less quadrangular bases.
Genus Opuntia. This large genus is generally regarded as more primitive than the others. In more than half of the states of the United States opuntias are the only cacti found; it is this genus that allows the claim that cacti grow over almost the entire United States. Characteristics are jointed stems, cylindrical or conical leaves on young stems, the presence of glochidia (barbed hairs or spines), and the production of spreading, rotate flowers with more or less sensitive stamens and with aeroles that often produce glochidia and spines on the ovaries. The fruits have thick rinds.