ACCUMULATION OF ENERGY 99 



is most efficient in photosynthesis. If a leaf which readily 

 deposits starch is exposed to the light of a spectrum, it is 

 found that the starch is deposited first in the region corre- 

 sponding to the absorption band in the red and then to the 

 other absorption bands.* Another method of showing the 

 relation of photosynthesis to absorbed light is to place a 

 filament of spirogyra in a culture of bacteria which become 

 active only in the presence of oxygen. On exposing the 

 filament to a spectrum the bacteria first become active in 

 the neighbourhood of the absorption band in the red.f 



The latter method can also be used to show that the 

 chlorophyll is the portion of the cell which absorbs the energy 

 for the photosynthesis. If light is allowed to fall on a spirogyra 

 filament so that the chlorophyll is illuminated in parts and 

 some parts of the cells free from chlorophyll are also illumin- 

 ated, the bacteria become active only near those points 

 where the light falls on the chlorophyll (Engelmann). 



We have therefore found that the photosynthesis depends 

 upon the light being absorbed by chlorophyll, and that the 

 maximum effect corresponds to the absorption bands of 

 chlorophyll. The efficiency of the absorbed light does not 

 depend upon the region of the spectrum from which it comes, 

 as careful measurements indicate that the photosynthesis 

 in the regions where no absorption bands occur is proportional 

 to the energy of the small amount of light absorbed. Therefore 

 the greater synthesis is not due to the greater efficiency of 

 the absorbed rays, but merely to the fact that more rays are 

 absorbed, hence more energy is available for photosynthesis. 

 The chlorophyll acts as an energy trap. 



On p. 59 we mentioned Le Chatelier's theorem, and the 

 action of chlorophyll in photosynthesis seems a similar case. 

 The photosynthesis is an endothermic reaction, and the 

 equilibrium should be shifted in the direction of carbohydrate 

 formation by a rise of temperature. It is clear that the 

 temperature cannot be raised in the leaf to an extent which 

 would cause a marked shift of the equilibrium point, but if 

 we use the term addition of energy for rise of temperature 

 the cases are parallel. The only condition required is that 

 the energy be furnished in a form which will be taken up by 

 the endothermic reaction, and this seems to be the function 

 of the chlorophyll. 



* C. Timiriazeff, Proc. Roy. Soc., 1903, vol. 72, p. 424. 

 f Engelmann, Onderzoek. Physiol. Lab., Utrecht, 1882, vol. 7, p. 

 191, cit. nach Bayliss. 



