The Action of Light on Chlorophyll. 



405 



Conclusion. 



The experiments outlined in this paper indicate, so far as experiments con- 

 ducted on dead chlorophyll extracts can be taken as an indication of what 

 goes on in the living plant, that the bleaching of chlorophyll is not a result 

 of the activities set up by photo-synthesis, as suggested by Usher and Priestley, 

 but is the actual basis and starting point of the changes set up in the green 

 leaf under the influence of light. In other words, the aldehyde produced 

 under the conditions described in this paper is a product of the photo- 

 decomposition or photo-oxidation of chlorophyll and is not a result of the 

 direct photo-synthesis of carbon dioxide and water. 



The aldehyde appears to be in fact purely a product of the photo-oxidation 

 of chlorophyll. This modifies our conception of the changes which may 

 possibly take place in the living plant. We know that carbon dioxide is 

 necessary for the production of sugar and starch in the living cell. But 

 if the sugar and starch are produced as the result of changes taking- 

 place in an aldehyde, and if the aldehyde is a direct product of the decom- 

 position of chlorophyll, then we must conclude that the carbon dioxide before 

 it can be used is built up independently into the chlorophyll molecule, and 

 it is possible that the' production of sugars and starch may be initiated by 

 photo-oxidation of the chlorophyll rather than by the direct photo-synthesis of 

 carbon dioxide and water. 



Summary. 



1. An account is given in this paper of some of the effects produced by 

 light upon chlorophyll. When chlorophyll is exposed to the light at least 

 two substances are formed, one of which is an aldehyde or mixture of alde- 

 hydes and the other an active chemical agent, capable of bringing about the 

 liberation of iodine from potassium iodide. 



2. These products of decomposition can be very easily demonstrated by 

 means of strips of paper tinged with chlorophyll. When bleached in the light 

 -and placed in Schiff's solution, a deep pink colour is developed showing the 

 presence of an aldehyde ; but if placed in a 10-per-cent. solution of potassium 

 iodide, a reddish blue coloration, which becomes blue on washing in water, is 

 developed, showing the presence of an oxidising agent. The same products 

 are obtained when films of chlorophyll on glass are bleached in the light. 



3. The bleaching of chlorophyll is less at the blue end of the spectrum than 

 at the red end, with a corresponding variation both in the aldehyde and 

 potassium iodide reactions. But if the exposure to the blue light is prolonged 

 to about eight or ten times that of the red light, the reactions are just as 

 pronounced. The bleaching and the corresponding products of decomposition 



