Page:Popular Science Monthly Volume 45.djvu/626

 persistence through the winter, but the same reason which. made the defoliation of the rosette advantageous—namely, decrease of the surface on which snow might lodge—would favor a reduction of lateral spread in the persisting leaf blades. Moreover, assimilation could not, of course, be carried on during the winter, and so the green parts of the leaf could well be spared to afford the material necessary for making the spines firmer and longer. Thus would finally result a purely defensive organ, so much the more efficient because having no other function to perform. Our common barberry exhibits especially well (Figs. 11 and 4) not only the more highly developed spines, but the intermediate stages connecting these with the primitive spiny leaf. Toward the tip of the uppermost shoots we find slender, one-pronged spines; the next below these are three-pronged, while those toward the base of the same shoot may have the prongs five or more in number. Passing now to one of those shoots, known as "suckers," which spring from older (mostly subterranean) parts of the plant, we find in addition to the forms of spines already noticed, others (Fig. 11, A-D) in which foliar characteristics become more and more evident as we approach the base of the shoot, where occur spiny leaves (A) essentially like what we have assumed to be the ancestral form. In regard to the position which these different forms occupy in relation to the ground or to their proximate basis of support, it is worthy of note how nicely all this accords with the theory of their having been developed under the influence of snowy winters.

To the rosette leaves the limiting of their duration to the warmer part of the year would permit a much thinner texture than was formerly necessary, and in consequence a more extended spread. This would of course involve a corresponding weakening of the marginal spines, but these being now so fully superseded in function, might safely be reduced to such slender cilia as we now find on the leaves of our common barberry (Fig. 8), or indeed be done away with altogether, as not infrequently happens in the same plant. They are clearly rudimentary organs tending to disappear.

A further consequence of the increasing severity of climate was the need of some special means to protect the tender organs of the bud against harmful changes of temperature. So long as these changes were comparatively slight and one set of leaves remained in place while the others were developing, the sheathing bases of the former served as a loose protective covering which answered every purpose. This supplementary function obviously