TJie Action of Light on Chlorophyll. 387 



not experiment with chlorophyll in a state of purity, owing to the nicety 

 required in its preparation. 



There is evidence to show that, under the influence of light, the chlorophyll 

 in a living cell is constantly being destroyed, hut that under normal conditions 

 the leaves remain green, the chlorophyll being reconstructed as fast as it is 

 destroyed. Thus when leaves are exposed to a stronger light than usual, they 

 become paler in colour, probably owing to the fact that under these conditions 

 the chlorophyll is destroyed at a more rapid rate than it is reconstructed. 

 This is frequently observed in the leaves of shade plants when exposed to 

 bright sunlight, and is also observed in Algae such as Spirogyra which 

 accumulate near the surface of water in the intense light of the sun during 

 the summer months. 



Thus Ewart* states that " when green leaves are exposed to sunlight, the 

 •decomposition of the chlorophyll goes on more rapidly than its production, 

 though the amount of chlorophyll decomposed is insufficient to cause a 

 change in the coloration visible to the eye." 



Stahl came to the conclusionf that the exposure of leaves to direct sunlight 

 for several hours gave no indication of the decomposition of chlorophyll. 



Keeble showed, however, that leaves exposed to bright sunlight gave a 

 weaker solution of chlorophyll in alcohol than similar leaves kept in the 

 shade.j Many other experiments support this view, notably those of Ewart§ 

 ■conducted on plants both in temperate and in tropical regions. Lubimenko|| 

 has also shown that the quantity of chlorophyll in a leaf varies with the 

 intensity of the light. 



It is usually assumed that the decomposition of chlorophyll is bound up in 

 some way with the photo-synthesis of CO2 and water, but as it is usually 

 considered to be more or less indirectly one of the results of photo-synthesis, 

 ■a sort of by-product as it were, very little attention has, so far as I know, 

 been paid to the products of its photo-decomposition. 



Timiriazeff pointed outlT that chlorophyll is a true optical sensitiser in that 

 it absorbs radiant energy, and is at the same time an absorbent of one or more 

 of the products by which the bleaching is then brought about. The function 

 of chlorophyll is to decompose carbon dioxide ; the chlorophyll absorbs the 



* ' Linn. Journ. Bot.,' vol. 31 (1895-97). 



t 1 Ann. du Jard. Bot. de Buitenzorg,' vol. 11 (1893). 



1 'Ann. Bot.,' vol. 9 (1895). 



§ 'Journ. Linn. Soc. Bot,,' vol. 31 ; 'Ann. Bot.,' vol. 11 (1897) ; 'Ann. Bot.,' vol. 12 

 (1898) ; see also references in Pfeffer's 'Physiology,' Eng. Ed., vol. 1, p. 334. 

 || ' Ann. Sci. Nat. Bot.,' 1908. 



IT ' Comptes Bendus,' 1885, and 1 Ann. Sci. Nat. Bot.,' 1885 ; see also ' Roy. Soc. Proc.,' 

 1903. . 



