THE ORIGIN OF ORGANIC MATTER 



35 



radiation raises the temperature of the leaf above that of its surroundings. 

 The leaf thus also loses energy by reradiation and convection cooling. 

 With a loss of energy by transpiration of 50 per cent, that lost by reradia- 

 tion can be placed at 19 per cent. As directly determined the amount of 

 energy used up in the endothermic process of photosynthesis is relatively 

 very small. It also varies considerably with external conditions and in 

 different plants, but a fair value for the present purpose is about 1 per 

 cent. This value of 1 per cent of the total incident solar radiation used 

 in photosynthesis represents the value for mature active leaves. 



In Figure 4 is shown the distribution of material which has been manu- 

 factured in the photosynthetic process. The plant can, of course, produce 

 carbohydrates only during the hours of illumination. For its life activities 



r 



PHOTO - 

 SYNTHESIS l/C 



Fig. 3. — What happens to solar radiation incident on a chlorophyllous leaf. The 

 values indicated give approximate disposal of the energy ; the ratio varies with 

 changes in external conditions. 



a certain amount of the materials elaborated are used up by the process of 

 respiration. This in a sense represents the operating cost of the manu- 

 facture and amounts to approximately 20 per cent. The material of the 

 plant can be divided into the crop, the leaves and stalks, and the roots. 

 Here again there are great variations in different species of plant. For 

 corn which has served as an example we can take the ratio as given in 

 Figure 4. Thus we can account for a further reduction in the ratio of 

 total incident energy to energy recovered in the plant crop. The 1 per 

 cent of the incident solar energy used in photosynthesis is the ratio of 

 the energy utilized in photosynthesis to the total radiation falling on the 



leaf. 



In our first analysis we concluded that about 0.13 per cent of the total 

 incident energy was recovered in the crop over a period of 90 days. The 



