Leaves and the tree trunks, and finally gain support upon the branches of the trees themselves. Unlike ordinary vines, however, which only injure the plants that support them, the barberry may be 
Fig. 5.—Berberis aquifolium. Leaves and branches. of some service, as its armament of spines is well calculated to repel intruders.
The great aim of all this spread and strengthening of branch work is of course to secure the most advantageous exposure of foliage to light; to the attainment of this object the form and arrangement of the leaves themselves also contribute not a little. Wherever we find such rosettes of wedge-shaped leaves as the barberry produces, the likelihood of one leaf overshadowing its neighbor is much reduced, and when as in shady localities this matter is of special importance, it is a noteworthy fact that the leaves commonly adapt themselves to each other so perfectly that a cluster becomes almost equivalent to a single large shield-shaped blade. Moreover, on the more horizontal shoots the margins of contiguous rosettes dovetail into each other so neatly that the result may be justly compared to a mosaic of leaves.
Another peculiarity connected with that abbreviation of the branchlets which results in the rosette arrangement is the method of defoliation. When the time arrives, the leaves, instead of separating entirely, drop only the blade, while the flattened overlapping leafstalks remain attached to the stem and perform the function of bark for several years.
It will thus be seen that we have in the barberry one of those rare cases (paralleled by certain species of orange, grapevine, and creeper) in which an apparently simple leaf has the blade articulated with the stalk after the manner so characteristic of the leaflets of compound leaves (Fig. 8). Of the hundred or more known species of Berberis, about twenty (forming the subgenus Mahonia) have compound leaves of from three to many leaflets all plainly articulated at the base (Figs. 6 and 7), just as is also the case with certain species of the genera Citrus and Vitis, to which the orange and the grape respectively belong. Moreover, throughout the Berberidaceæ we find almost all the species to pos-
