THE METABOLISM OF PLANTS. 265 



essential condition for the formation of chlorophyll, yet it 

 also promotes its decomposition. This is proved to a certain 

 extent by some of the facts with which we have recently 

 become acquainted, namely, that the leaves of etiolated plants 

 become green more rapidly in diffuse daylight than in direct 

 sunlight, and that, when the light is intense, the formation of 

 chlorophyll takes place earlier in blue than in yellow light; 

 but more direct evidence is not wanting. It has been fre- 

 quently observed, especially in the case of Conifers, that those 

 leaves of the plant which are most exposed to sunlight in 

 summer have a yellowish tinge when compared with leaves 

 which are shaded to some extent, and they may become quite 

 yellow. The green colour of these yellow leaves may how- 

 ever be restored, as Batalin has shewn, by covering them with 

 something, a sheet of white paper for example, which di- 

 minishes the intensity of the light which reaches them. The 

 yellow colour which leaves assume in the autumn is doubtless 

 due to the alteration of the chlorophyll under the influence 

 of light, for it has been observed that the autumn colouration 

 makes its appearance first in those leaves which are most 

 fully exposed to light (see, for instance Haberlandt, Askenasy). 

 We have, then, two processes going on in the chlorophyll- 

 corpuscle under the influence of light, the formation of chlo- 

 rophyll, and the decomposition of chlorophyll : when the 

 former is the more active then the corpuscle is green, when 

 the latter, then the corpuscle becomes more and more yellow. 

 The summer yellowness, as we may term it, appears to be 

 due solely to the fact that intense light promotes rather the 

 decomposition than the formation of chlorophyll ; the autum- 

 nal yellowness, on the other hand, appears to be due to other 

 causes, for in the autumn the light is not usually so intense 

 that it would cause the decomposition of the chlorophyll to 

 exceed its formation in the corpuscle. The first of these 

 other causes is, in the case of deciduous leaves at least, that 

 the vitality of the leaf is diminishing as it approaches the 

 term of its existence, that is, that all its various functions, 

 including of course the formation of chlorophyll in its cor- 

 puscles, are being carried on with a rapidly decreasing activity. 



