THE LIGHT FACTOR IN PHOTOSYNTHESIS 1045 



molecule to which the absorbed light energy is transferred and has 

 suggested that chlorophyll itself may utilize this energy through a 

 reaction with oxygen. A similar attitude has been taken by Gaffron (29) 

 on the basis of his interesting investigations on the activation of oxygen 

 by ilhuninated pigments. 



THE ENERGY RELATIONS OF PHOTOSYNTHESIS 



There are two ways in which the energy efficiency of photosynthesis 

 may be expressed: one is the apparent efficiency in which the amount of 

 energy stored by the leaf is stated as the fraction of the energy incident 

 on the leaf; the other is the real efficiency, in which the amount of energy 

 stored by the leaf is gi^'en as a fraction of the energy absorbed by the 

 leaf. While still a third type of efficiency might be determined, in which 

 the chemical energy stored is related to the light energy absorbed by 

 the photosynthetic pigments, no method has as yet been found by which 

 this estimation can be accomplished. It is only the radiant energy which 

 is absorbed by the leaf that can be used in photosynthesis. Of this, 

 however, only a small portion is used in the photosynthetic reaction, the 

 rest being dissipated by transpiration and reradiation. 



THE APPARENT EFFICIENCY OF THE PHOTOSYNTHETIC PROCESS 



The apparent efficiency of the photosynthetic process has been 

 determined in two different ways. In the first method the heat of com- 

 bustion of two analogous portions of a leaf are determined before and 

 after insolation. The increase in heat of combustion of the insolated 

 portion is then compared with the total energy incident on the illuminated 

 leaf during the period of exposure. 



By the use of this method, Puriewitsch (92) has made an estimate of 

 the energy storing efficiency of several kinds of leaves. The values found 

 range from 0.6 to 7.7 per cent. Even though the values show great 

 variation, they indicate that the efficiency of the leaf is very low in the 

 conversion of the energy incident on it into stored chemical energy. 

 This method has all the disadvantages of a half-leaf method. It lacks 

 in accuracy, but further development and improvement might make it 

 extremely useful in certain aspects of the photosynthetic problem. 



In the second method, the energy stored by the plant is calculated 

 from the idealized photosynthetic reaction on the basis of the amounts of 

 carbon dioxide absorbed or of oxygen liberated. The ratio of this 

 calculated energy to the total energy incident on the leaf during the 

 insolation period is designated as the efficiency of the process. The 

 photosynthetic reaction may be represented by the following idealized 

 equation : 



6CO2 + 6H2O -^ CeHiaOe + 6O2 - 674,000 cal. 



