LIGHT AND VEGETATION 111 



formol or of its starch equivalent (which in total corresponds 

 to the fixation of 112,000 calories), the chlorophyll had to 

 absorb 112,000^0-535=209,000 calories. 



On the other hand, the absorption of a photon per 

 elementary molecule is equivalent to the absorption of 49,200 

 calories per gram-molecule. As a result, a little more than 

 4 photons per molecule are necessary to effect the photo- 

 synthetic elementary action expressed by the equation 



CO2+H2O ^CH^O+Oa 



Similarly, it would be possible to calculate, according to 

 Warburg's measurements, the number of quanta absorbed 

 per molecule of oxygen liberated at the other wave-lengths; 

 these are: 



The measurement of the energy absorbed does not appear 

 to take into account the loss due to absorption by parts of the 

 plant other than those containing chlorophyll; the real 

 eflficiency is therefore a Uttle higher, especially in the blue 

 where the carotenoids may have a considerable influence. 

 It has been agreed that the minimum number of quanta 

 necessary for the whole of the visible spectrum is 4. 



This simple result, which is attractive because it accords 

 rather well with the idea that has been formed of the process 

 of a photochemical reaction, confirmed by other experiments 

 hke those of Schmucker (1930) on other plants {Cryptocoryne 

 ciliata and Cabomba caroliniana) and those of Rieke (1939), 

 has always had a great influence on the theories of photo- 

 synthesis which have been propounded. 



Nevertheless, these measurements are very difficult, and 

 considerable uncertainties still exist, so that other investi- 

 gators, repeating Rieke's experiments, have found that 

 20 quanta, rather than 4, are necessary to the reaction. 



A variation of this kind in such an important result, 



