Page:Popular Science Monthly Volume 66.djvu/342

338 with a meager foliage of compound leaves. The very small leaflets, which are foliar members of the third order, are sensitive, the pairs folding together at night and opening at dawn. Closure again occurs at the period of excessive insolation. All the ultimate branchlets taper into long stiff thorns, an example of direct metamorphosis, since they originate as normal shoots. In the absence of leaves it is evident that the chlorophyll tissues of the stems are chiefly concerned in the food making process, which would appear true also from the fact that the smaller branches and twigs are used as green forage for horses in winter. Although a particular branch removed from the tree and regarded alone is a rather graceless object, the whole tree with its smooth green bark and gradually tapering limbs and twigs is singularly beautiful, its outline most delicate.

Conspicuous as a member of the hillside vegetation is the ocotillo (Fouquieria splendens), a plant with a hypothetical relationship with the willows, but not in the least suggesting them by its habit and more obvious structures. The general disposition of its branches, which are highly suggestive of coach whips, is similar to that in the creosote bush. It is, however, a much taller shrub, with lithe, bespurred stems, bearing in spring each a brilliant mass of scarlet flowers. On the advent of the rains, the stems are quickly and completely clothed with rosettes of light green ovate leaves, each rosette in the axil of a thorn. Most interesting is the manner in which the thorns arise. The new shoots produce first the primary leaves, in the stalks of which a hard tissue is developed. Their leaf-blades rather soon wither away, and split away from the harder part of the stalks, which in this way are left as spines, in the axils of which, as above stated, the secondary