Page:Popular Science Monthly Volume 45.djvu/625

 undershrubs in our Western States, as well as other apparent exceptions among Berberides to the general principle we are now applying.

Along with the firm texture belonging to evergreen leaves there would naturally be retained the marginal spines which protected the mahonia ancestors from browsing animals. But with the establishment of the rosette arrangement the leaves which are borne by a long shoot, in virtue of their position just below the rosettes, come to have a special importance in this protective capacity. For, in the first place, as being already fully developed at a time when the rosette leaves are young and tender, the old leaves can shield the newer ones at the most critical period of their life; and, in the second place, given one stout, spiny leaf in such intimate connection with the mature cluster, and the need for using up material in spinemaking for the latter is much lessened. Accordingly, we find very generally throughout the evergreen Euberberides, along with the differentiation of the branches into long and short, a differentiation of the leaves—those subtending the clusters being decidedly spiny, while those of the cluster are less strongly armed. A particularly good example of such differentiation not far advanced is afforded by a species growing in Chili (Fig. 9). In a number of cases, such as the "box-leaved barberry" (Berberis buxifolia, Fig. 10), the differentiation has been carried so far that the subtending leaf has been completely transformed into a formidable spine, while the rosette leaves have lost all trace of spines except at the tip.

After the plurifoliolate and the unifoliolate types of evergreen barberries had been evolved there was the further possibility of developing from the latter a yet higher type which should be still better adapted to meet the exigencies of a severe and snowy winter, and at the same time safely attain a considerable height. All this would follow from the acquirement of the deciduous habit.

In the series of forms which came to adopt the expedient of defoliation at the approach of winter, several causes may have conspired to bring about in the two sorts of leaves a still further specialization of the two functions of assimilation and defense, which, originally combined in each leaf, began, as we have seen, to be separated more or less in the evergreen Euberberides.

As regards the subtending leaves, not only would their importance as a defense to the young rosette be sufficient to insure their