The Prevalence of Green Color in Plants 45 



white-transparent when alive, but turn brown on their death and 

 decay. In fact the conditions prevailing in the ripening and dying 

 leaf are most complex, for not only are different chemical sub- 

 stances and physical forces interacting in large number, but their 

 interrelations are constantly changing as the death of the proto- 

 plasm weakens its regulatory control upon them. This combina- 

 tion of complexity and changeability produces a state of unstable 

 equilibrium, which permits even very minor external influences 

 to exert relatively great effects, — and thus is explained the differ- 

 ences in the coloration of the same plants in different seasons or 

 different places. In general, however, the effects of the weather 

 upon the intensity of coloration are clear. Thus a bright autumn 

 (and, equally, a sunny climate) intensifies the coloration, at least 

 for the red, while dull weather is accompanied by dull coloration. 

 Early frost helps somewhat to intensify color, partly by hastening 

 the death of the leaf, and partly by aiding the chemical formation 

 of the erythrophyll ; though frost is not, as many suppose, a cause 

 of the coloration itself. Furthermore, the coloration can be 

 brought on much earlier in the season than usual by any injury, 

 — a break in the bark, a split in the trunk, some damage to the 

 roots, — which weakens the vitality of the tree and hence pro- 

 motes the waning of life in the leaves; and this is the explana- 

 tion of the occasional reddening of a single branch, or even 

 whole tree, which one finds turning sometime ahead of its 

 neighbors. 



The reader will feel, I am sure, that this is an unsatisfying answer 

 to his natural wish for a definite knowledge of the causes of 

 autmim coloration, but it is all that the present state of our 

 knowledge permits. The subject has been studied heretofore 

 by botanists from their side, and by chemists from theirs; but 

 its problems will not be solved until some competent investiga- 

 tor takes autumn coloration as his unit, and attacks it by any and 

 all methods,— chemical, physical, physiological, observational, 

 experimental, or any others essential for attaining his ends. 



