LIGHT AND VEGETATION 123 



will vary as the cube, and the number excited four times as 

 the 4th power, of the illumination. This theory has been 

 confirmed by the study of phenomena of excitation by light 

 in gases. 



There is, then, in photosynthesis, owing to a process still 

 unknown, a new condition which this deduction does not take 

 into account. These various problems are of almost insur- 

 mountable difficulty if we merely consider chlorophyll as 

 being dispersed in the plant tissue, or even in granules, in a 

 medium which serves only to support it, and we are forcibly 

 reminded that these phenomena take place in living matter. 



How are these "grana" which are revealed under the 

 microscope organized? We can only form hypotheses which 

 will need to be checked by experiment. It is an attractive but 

 extremely difficult subject of study. The investigators who 

 attempt it must possess a profound knowledge, not only of 

 the biological phenomena of photosynthesis, and of the 

 organic chemical compounds of the leaf on which there is 

 much research still to be done, but also, and especially, of the 

 interactions between Hght and matter in soUds and particularly 

 in colloids. 



This physical study has scarcely begun even in simple 

 cases. It seems, therefore, that our scientific knowledge is still 

 insufficient to enable such a study to be successfully attempted, 

 but the interest that it presents cannot fail to be a stimulus 

 to the scientific, chemical and physical researches now in 

 progress the results of which will bring new methods of 

 exploring this mysterious and vitally important domain of 

 plant photosynthesis. 



