Page:Popular Science Monthly Volume 43.djvu/816

796 nearest approach being missing links between other neighboring orders and the Cactaceæ, otherwise almost utterly isolated. One is the genus Pereskia, in which, especially in Pereskia grandifolia, there are developed true leaves, succulent and veiny and with spines in their axils. Most species of the genus are shrubs or trees, and, still further remarkable, the flowers are borne in nearly panicled clusters. The thirteen species belong mostly to the West India region, and one produces the so-called "Barbados gooseberry" A decided analogy may be recognized by close comparison between these Pereskias and the Ribeseaceæ, the group of the saxifrage family which includes the currants and spiny-stemmed gooseberries; and this probably points to a distant connection between the progenitors of the Cactaceæ and those of the modern genus Ribes. The other possible "missing link" is the genus Rhipsalis, a curious group, mostly epiphytic, and growing in long, pendent masses from the branches of trees, in some instances resembling mistletoe. In these plants we see a possible approach to the group of so-called "ice-plants," the order Mesembryanthemaceæ. But most peculiar is the fact that one species of Rhipsalis is indigenous to South Africa, Madagascar, and Ceylon—the only instance of an Old World cactus. This probably has its significance.

The other suborder, the Tubulosæ, are undoubtedly the more highly specialized cacti, and further, significant fact, they are for the most part Mexican, while the Opuntiæ are widely scattered northward. Besides several minor genera of Tubulosæ there are three great and distinctive ones, which, as it is interesting to note, mark successive steps in structural specialization—they are Mamillaria, Echinocactus, and Cereus. In mamillaria there is a great departure, in the character of the vegetative body, from the Opuntæ. The plants are more or less globular or subcylindrical, and the original joints of the stem are indicated only by the conical spine-tipped tubercles which make up the surface of the fleshy mass. Echinocactus, the "hedgehog cacti," has the general appearance of mamillaria, but the tubercled surface is modified into a mere series of parallel vertical ribs, bearing clusters of spines along their ridges at points corresponding to the tubercles of mamillaria. Of the two genera, echinocactus is much the more strictly Mexican, while mamillaria has a few representatives spreading northeastward into Kansas and South Dakota. The large genus Cereus is the crowning glory of the cacti. It retains the ribbed structure of echinocactus, but its stems are nearly always columnar and in many instances arborescent. With echinocactus, this genus reaches its greatest development in Mexico, or near the boundary line, where flourishes the monarch of the Cactaceæ, the "giant cactus," Cereus giganteus. In cereus.