Ch. Ill, 11] COLORATION' OF LEAVES 91 



on by old age or the coming of autumn, a leaf makes no more 

 chlorophyll, while that already present fades rapidly away, 

 permitting other colors which are present to show, and 

 like\^ise some new ones to form under the altered conditions. 

 The rapidity mth which chlorophyll can fade in the light is 

 strikingly sho^^^l by tlie simple experiment of exposing a 

 fresh alcoholic solution to strong Hght in contrast with a 

 control kept in the dark (page 17). In an hour or two the 

 green color is gone, leaving the solution colored yellow by 

 the xanthophyll. This experiment shows why leaves turn 

 yellow in autumn, for the fading of the chlorophyll exposes 

 the xanthophyll, always present -s^ith chlorophyll but far 

 more resistant to destruction by light. Thus all autumn 

 leaves are yellow, though some acquire additional colors. 

 The xanthophyll is easily extracted in a clear solution by 

 simply warming yellow leaves in alcohol ; and it is also ob- 

 tainable by blanching an alcohohc extract from green leaves, 

 as just mentioned. As to the function of this "nidely present 

 xanthophjdl (a mixture of carotin and xanthophyll proper), 

 that is still unkno'nTi, though the constancy of the substances 

 indicates some important functional utihty. Herein lies 

 another of the problems in\iting the future investigator. 



Less abundant but more conspicuous than yellow, as an 

 autumn color, is nd. which is due to the er>-throphyll (an- 

 thocvanin) already described. Being soluble in the cell 

 sap, it is easily removed, in a clear solution, by heating 

 the red autumn leaves in water. It is indeed worth one's 

 while, for testhetic as well as educational reasons, to extract 

 the green, vellow, and red pigments in their beautiful clear 

 solutions, and ^dew them side by side in glass cylinders 

 against the hght ; for these are the three which give almost 

 the entire coloration to all foHage. The er\-throphyll origi- 

 nates in autumn leaves very differently from xanthophyll. 

 for it is not previously present, but is made during the fachng 

 of the chlorophyll. There is much uncertainty about the 

 details, but it seems reasonably certain that it results in- 



