40 The Living Plant 



ever bright light, a relatively low temperature, much sugar, and 

 some tannin happen to come together in a living cell, then the 

 substance erythrophyll, of which the composition-color happens 

 to be red, is formed incidentally as a purely passive chemical 

 result. On this view the red color may be purely accidental, 

 and may have no utility whatever to the plants which possess 

 it, though the possibility is not thereby excluded that the plant 

 may bring those conditions together, adaptively, in a cell where 

 it has need for the red color to appear. The substance of the whole 

 matter in reality is this, that we do not yet know surely the 

 significance of erythrophyll hi the plant; and herein lies another of 

 the problems which make science so ever alluring. 



Connected with chlorophyll in a different way is one of the 

 most striking and beautiful of all the phenomena of nature, the 

 transition in the foliage each season from the uniform green of 

 summer to the brilliant colors of autumn. Strangely enough, for 

 a subject so important, our knowledge thereof is still very im- 

 perfect, and there is even a difference of opinion as to the very 

 significance of the colors to the plant. A basal fact, however, 

 upon which there is agreement, is this, that the autumn color- 

 ation results from changes connected with the death and fall of 

 the leaf. We know that in late summer our trees are preparing 

 for the annual leaf fall, in anticipation of which they are gradually 

 bringing the activities of the leaves to a close, ceasing to make new 

 chlorophyll, withdrawing certain precious materials into the stem, 

 and building right across the bases of the leaves those corky layers 

 which both cut them away from the stem, and also heal in ad- 

 vance the wound that is thus to be made. Now chlorophyll, as 

 the reader's own experiments will have shown, is soon destroyed 

 by bright light ; this destruction, indeed, is continually in progress 

 throughout the summer in the living green leaves, where the color 

 is maintained only through virtue of its constant renewal. It 

 was formerly believed (and I mention the matter because the 

 statement persists even yet in some writings), that this chloro- 



