6o6 THE POPULAR SCIENCE MONTHLY. 



persistence fhrougli the winter, but the same reason wliicb. 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, assimila- 

 tion 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 de- 

 fensive organ, so much the more efficient 

 because having no other function to per- 

 form. Our common barberry exhibits es- 

 pecially well (Figs. 11 and 4) not only the 

 more highly developed spines, but the 

 , - intermediate stages connecting these with 



Fig. 10. Berberis buxifolia. ^ ^ ^ . 



Leaf rosette aud spiue. the primitive spiuy 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 character- 

 istics 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 warm- 

 er 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 sujDerseded in func- 

 tion, 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 re- 

 mained 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 



