Page:Popular Science Monthly Volume 43.djvu/814

794 compact mass of the vegetative body, devoid of leaves or leaf-like appendages, exposes the least possible evaporating surface to the sun; the thick skin, bearing only a few scattered stomata, breathing pores, wraps an almost impermeable covering round the internal moisture; the long, wiry, fibrous roots run hither and thither through the sand and into the finest crevices of rocks; and further, the bristling, spiny armor shields the plant from men and beasts. All this and much more goes to make up the plant of plants that is best fitted to fulfill the mission of vegetation in the Southwestern borderlands. And so the problem solves itself, and we come to realize why the progenitors of the Cactaceæ, spread out and multiplied far and wide, and broke into a myriad varied forms, all retaining amid their diversity the distinctive characteristics of the primal type.

A standard British encyclopædia of scarcely three quarters of a century ago vouchsafes the statement that the order Cactaceæ embraces "twenty-seven" species. Grasp the contrast between this day of science and that! The botanists of the Century Dictionary estimate the species at far above the thousand mark. For this vast stride we thank a host of indefatigable explorers, and of these, a large share of credit goes to the scientists of the Mexican Boundary and the Pacific Railway Surveys, whose discoveries were the main foundations of the labors of Engelmann. Thus duly recognizing the scope of the field before us, we may with interest follow an outline sketch of the order. The general character of the vegetative body may be passed over for the present without further detail, that we may the more particularly notice the floral structure on which the systematic study of the order so largely depends. The inferior ovary, surmounted by the sepals, petals, and stamens, places the order immediately among the highest of the dicotyledonous Choripetalæ. There is a natural division into two suborders. In the first, the Rotatæ, the rotate many-leaved calyx and corolla, with the stamens, directly surmount the ovary. In the second there is a unique and obviously progressive development: calyx and corolla are united toward their bases and prolonged into a tube, with the stamens inserted on its throat—whence the name of the suborder, the Tubulosæ. The typical Rotatæ are the widespread genus Opuntia, numbering about two hundred and fifty species. The greater part of the genus are characterized by broad, thickened, fleshy, jointed stems; but a small subgenus, and this probably the less highly specialized, have cylindrical joints. Most of the species are more or less spreading and prostrate, but a large number are truly arborescent in growth. Several species are thoroughly naturalized in the northeastern quarter of the United States, and these are among the most brilliant acquisitions of our