THE ENERGY RELATIONS IN PHOTOSYNTHESIS 337 



calculate what portion of the energy of solar radiation incident on a unit 

 area of open field is utilized in the synthesis of organic material. As a 

 basis of his calculations he used the yields of various field crops. He 

 included in these calculations the original weight of the seed, and the loss 

 of weight of the seedling through respiration. The total production of 

 the crop was increased by 15 per cent to allow for the material consumed 

 in respiration. Other material losses of the plant, such as pollen, were 

 disregarded. The analyses of different crops grown in Germany were 

 used and the energy determined on the basis that 1 gram crude protein 

 was equivalent to 4.8 calories, 1 gram crude fat to 9.2 calories, 1 gram 

 nitrogen-free extract to 4.0 calories and 1 gram crude fiber to 4.0 calories. 

 The organic dry materials (ash free) remaining in the soil were taken as: 

 wheat 2669 kilograms, rye 4044 kilograms, barley 1802 kilograms, and oats 

 2111 kilograms, and the heats of combustion of these were calculated on 

 the basis of 3.6 calories per gram. The heats of combustion of the grain 

 calculated to 3.59 calories per gram and that of the straw to 3.45 calories 

 per gram. The incident solar radiation was calculated for a growing 

 season of 18-21 weeks. 



The values in Table 42 illustrate the results of Piitter's calculations 

 of the per cent utilization of solar energy. These show an average 

 utilization of about 3 per cent. 



TABLE 42 

 Utilization of Solar Energy of Crop Plants in the Field. (Putter.) 



Mean In- 

 cident Solar 

 Radiation, 

 cal. per 

 Plant sq. m. 



Summer wheat 221,500 



Summer rye 209,000 



Summer barley — 



2 rowed 195,000 



4 rowed 154,000 



Hooded 123,000 



Oats 218,000 



Potatoes 250,000 



Beets 300,000 



