The Prevalence of Green Color in Plants 43 



turn red at all, the others being restricted to yellow. The Maples 

 and the Oaks are trees well-known for their richness in sugar or 

 tannin, which helps to explain why those particular trees are 

 more brilliantly red than most others. It happens, furthermore, 

 that erythrophyll formation, contrary to the usual rule with 

 chemical processes, is promoted by lower temperature; and this 

 explains why it is that a cool season promotes the brilliance of 

 color, which indeed reaches its highest perfection in seasons or 

 places where the skies are very bright and the frosts come 

 early. 



Thus much for the facts as to the yellow and red autumn color- 

 ation. We have now to take notice that two conflicting views 

 exist as to its significance to plants. Many botanists believe that 

 since erythrophyll seems to have definite functions in spring 

 vegetation (as we have seen a few pages earlier), it has also an 

 identical function in the leaves of the autumn, acting usefully 

 as a selective light screen. The argument runs thus: chloro- 

 phyll fades away in the leaf before the protoplasm has wholly 

 ceased its activity: full exposure to bright sunlight, especially 

 the untempered blue-violet rays, would injure this protoplasm, 

 and act unfavorably on the translocation of the valuable materials 

 from the leaf into the stem : an erythrophyll screen must temper 

 the blue-violet rays while permitting the passage of the red rays 

 which are not simply harmless but, being warm rays, would actually 

 aid the final vital processes of the leaf during the cooling days of 

 autumn. And those who hold this view assume that xanthophyll 

 must have something of the same action, though inferior in degree 

 to erythrophyll. On this view autumn colors are believed to be 

 useful if not indispensable to the plants which possess them, and 

 inferentially, have been developed adaptively to such use. 



Sharply contrasting, however, with this utility explanation of 

 autumn coloration is the view that it is merely incidental. While 

 the utility theory has certainly some facts in its favor, the most 

 of the evidence seems to me heavily against it. Thus the utility 



