CHLOROPHYLL AND ENERGY 169 



and this first led to the belief that it was present in the molecule 

 of chlorophyll. It would seem to take part in a synthetic process 

 involving albumins, for the etiolation caused by its absence 

 is accompanied by an accumulation of them. 



Although we do not know precisely what part they take 

 in it, manganese, copper, zinc, potassium and sulphur are 

 also necessary for the synthesis of chlorophyll. 



Chlorophyll is not a stable substance which, once formed, 

 persists throughout the life of the leaf; its quantity depends 

 on the equiUbrium estabHshed between its rate of formation 

 and its rate of disappearance. During the growing period the 

 rate of formation is much greater, but the reverse normally 

 occurs in the autumn, or when a necessary substance such as 

 carbon dioxide, nitrogen or iron is lacking, or again when 

 the illumination is very strong; the plant then changes from 

 green to a more or less distinct shade of yellow because 

 chlorophyll no longer masks the carotenoids whose rate of 

 renewal is more rapid than its own. 



Chlorophyll disappears under the influence of light and of 

 oxygen; the products into which it is transformed are very 

 probably products of oxidation, although they have not yet 

 been isolated. 



In the absence of oxygen it seems to be stable, but, on the 

 other hand, it soon disappears when oxygen is plentiful or 

 when carbon dioxide is lacking. When a plant is subjected to 

 much more intense illumination than it ordinarily receives, 

 instead of a release of oxygen there is a fixation of oxygen 

 several times greater than that of respiration; the oxidation 

 seems first to affect the neighbouring oxidizable material but 

 soon attacks the chlorophyll, which rapidly disappears. 



Light acts on chlorophyll through the wave-lengths that 

 the latter absorbs. Dangeard demonstrated this very clearly; 

 he illuminated chlorophyll by a spectrum which was active 

 only in the absorption bands and also by rays which had 

 already traversed a green leaf and were shown to be totally 

 inactive. See Fig. II, 2. 



It is difficult to calculate the average hfe of chlorophyll, 



