6 CHAP. I. INTRODUCTION 



There is also the important question of the relative 

 photosynthetic effectiveness of the different rays of the 

 spectrum ; the problem to be solved is whether the differing 

 effects are solely due to the quality of the light, or if they 

 are also due to the differing energy of the various rays of 

 the spectrum. In this latter investigation the energy of 

 the rays has been measured directly by a new Magnetic 

 Radiometer. Evidence has been adduced that some of the 

 energy absorbed by the plant is utilised in doing work other 

 than photosynthesis. 



The steam-engine converts the potential energy of coal 

 into kinetic energy of movement, and we can determine 

 the efficiency of this method of transformation. The 

 efficiency of the reverse process of conversion of the kinetic 

 energy of solar radiation into the potential energy stored 

 in plants is of very great interest. The results of the in- 

 vestigation which I have carried out on the subject show 

 that the efficiency is much higher than has been generally 

 supposed. 



Photosynthetic activity is changed under the action of 

 not one but many factors. The effects of simultaneous 

 variation of these are unknown ; a general law of photo- 

 synthesis has, however, been formulated by which the 

 effect of anv combination of different factors can be found. 



These and other questions are treated in detail in the 

 succeeding chapters. 



