Ch. III. 11] COLORATION 01' LEAVES 93 



chlorophyll, or by air-spaces, hairs, or other structural fea- 

 tures. In a few cases no bro'mi color appears, and by the 

 slow fading of the xanthophyll the tissues are left nearly 

 white, as happens to some extent in our Birches. 



All autumnal coloration of fohage rests upon these five 

 colors, either singly or in combinations, modified somewhat 

 by other substances, or by the leaf structure. The student 

 will notice how different they are in their significance to the 

 plant, for while chlorophyll has a well-known and va.stly 

 important function, and xanthophyll an unknown but prob- 

 ably important function, erjnhrophyll and the browns are 

 mere chemical resultants of the physical and chemical con- 

 ditions prevaiUng in djlng leaves, and white is the natural 

 color of the unaltered leaf structure. In autimin leaves, 

 obviously, none of the colors seem to have any fimctional 

 utility to the plants, and autumnal coloration as a whole 

 appears to represent simply a gigantic chemical incident, 

 comparable with the blue of the sky and the red of a sunset. 

 Though thus but an incident, it is a happy one for mankind, 

 in whose elevated enjojinent of nature it forms a great 

 factor. 



Everybody knows that autumnal coloration is far more 

 brilhant in some chmates and some seasons than others, 

 thus showing a marked sensitiveness to external conditions. 

 Something depends on the kinds of plants which constitute 

 the flora, for plants differ in their susceptibility to the 

 color changes. Again, the coloration is notable only in those 

 regions where the transition from su mm er to autumn is 

 rather abrupt, and the ^itahty of the leaves is suddenlj' 

 checked while they are stiU full of sap : and it is relatively 

 poor in places of gradual transition from summer to autumn 

 where the leaves lose their sap before d}ing. It is through 

 the abrupt check to the ^itahty of the leaves that early 

 frosts help the coloring, though they do not cause it, as 

 popularly beheved. In fact, any cause which hastens the 

 waning of leaf ^itaUty brings on the coloration more ciuickly. 



