[ 63 ] 



MAR- 5 1921 



PHOTOSYNTHESIS AND THE ELECTRONIC THED^Y,._ 



By HENEY H. DIXON, Sc.D., F.E.S., 

 University Professor of Botany, Trinity College, Dublin ; 



AJSTD 



HOEACE H. POOLE, Sc.D., 



Assistant to Erasmus Smith's Professor of Natural and Experimental 

 Philosophy, Trinity College, Dublin. 



Eead January 27. Published March 12, 1920. 



Introduction. 



The most fundamental question in photosynthesis is : " How are the raw 

 materials carbon dioxide and oxygen rendered reactive by chlorophyll when 

 exposed to light ? " The answer must be in some way connected with the 

 behaviour of the negative electrons, which, according to modern views, are 

 important constituents of all atoms, and are apparently entirely responsible 

 for the absorption of radiant energy from the aether. The energy absorbed 

 is generally converted into molecular energy, i.e., heat, or otherwise disposed 

 of by the electron, without reaching such a value as to cause the expulsion of 

 the latter from the atom. If, liowever, the frequency of the incident radiation 

 exceeds a certain limit, depending on the nature of the substance, electrons 

 are actually hurled out of the atoms with very high velocities, the velocity 

 of emission increasing with increase of frequency. This phenomenon, known 

 as the photo-electric effect, is shown most intensely by metals, and in a lesser 

 degree by most other substances. In the case of the alkali metals the 

 limiting frequency is such that the effect can be caused by visible light. 

 With less electro-positive metals and most other substances a higher 

 frequency is necessary, so that ultra-violet light is required. It is obvious 

 that, in order to cause the photo-electric effect, the radiation must be at least 

 partially absorbed. 



Following this train of thought, it would appear rational to attribute the 

 conversion of light energy into chemical energy in photosynthesis to the 

 intervention of electrons displaced by the light. The analogy of the 



SCIENT. PKOC. K.D.S., VOL. XVI., NO. V. I 



