Dixon and Poole — Photos;/ uthesis and the Electronic 'fhcort/. 75 



deduced theoretically, is almost certainly too low. The current due to the 

 actual photo-electrons emitted must then have been 4'5 x 10"'. As the electronic 

 charge is 4774 x 10"'° E. s. u., this corresponds to the emission of 940 elec- 

 trons per second from the ilhiminated surface. The effective area of this 

 vras about 12'5 square centimetres, allowance being made for the part shaded 

 by the grid, so the number of electrons per square centimetre per second 

 works out at 75. This, which is probably an over-estimate, corresponds to 

 the emission of '^7 x 10' electrons per square metre per hour. It is most 

 unlikely, in view of the small amount of energy available, that one electron 

 could cause the assimilation of more than one atom of carbon. In fact, the 

 energy quantum for red light of frequency 5 x 10" is about 3'3 x 10"'* erg, 

 while the energy required to decompose one molecule of carbon dioxide 

 is about 6'6 x 10"'-, i.e. equal to two quanta. This figure refers to gaseous 

 carbon dioxide, and may need modification in the case of dissolved carbon 

 dioxide. But if we assume that each electron causes the assimilation of 

 one atom, we are not likely to be \inder-estimating the quantity of carbon 

 assimilated. Taking the mass of the hydrogen atom as 1'65 x 10"", 

 this comes out as 5'3 x 10"'* gram per square metre per hour. This is an 

 utterly negligible figure in comparison with the quantity actually assimilated 

 in strong sunlight, which is of the order of half a gram per square metre of 

 leaf-surface per hour. 



The leaf-powder used was not treated chemically in any way. The leaves 

 were merely dried at a moderate temperature in an air-oven, and powdered. 

 "Willstatter and StoU (9) have shown that the chlorophyll is not altered 

 chemically by this treatment. 



As the energy of photo-electrons due to visible light would be so small, 

 it seems unlikely that their range in the leaf-powder would much exceed the 

 distance between two adjacent atoms, so we would only expect those chloro- 

 phyll molecules which are actually on the surface to contribute to the 

 photo-electric current. As, however, the diameter of an atom is of the order 

 of 10"^ centimetre, it seems incredible that only about one in 10'^ of the 

 chlorophyll molecules in a layer of powder one millimetre thick is on the 

 surface. Even then we would have to assume that the activity of the living 

 leaf is equivalent to that caused by all the chlorophyll in such a layer. 



Conclusion. 



The experiments recorded above show that those wave-lengths of light 

 which are effective in photosynthesis are unable, to any appreciable extent, 

 to expel electrons from the leaf-pigment complex, and hence cannot in this 

 way produce ionization, or bring about reactions external to the pigment. 



