32 PHOTOSYNTHESIS 



Thus from day to day there have been extreme fluctuations observed rang- 

 ing over nearly 10 per cent. These short-period variations are apparently 

 associated with changes in the opacity of the outer solar layers.^" The 

 correlation between the variations in solar activity and climatic conditions 

 on the earth may be of great significance for the photosynthetic activity of 

 plants. It should be stated, however, that there is no direct correlation 

 between variations of the solar constant and terrestrial temperatures and 

 that the interrelations appear to be very complex. 



The foregoing is of necessity only a very brief consideration of the 

 factor of solar radiation. Many phases have not been touched upon but 

 the reader will find in the literature cited a wealth of material for this most 

 fascinating study. We have, however, sufficient data to enable us to 

 draw some general conclusions on the relation of solar energy to photo- 

 synthetic activity ; a more detailed consideration of the light factor in 

 photosynthesis will be found in Chapter 2. 



In order to form a conception of the amount of solar energy received 

 at the surface of the earth and the proportion of this energy which is 

 utilized by the plant and which thus represents the amount of energy now 

 available for use by man, let us make the following calculations. Instead 

 of 1.5 calories per square centimeter per minute as the amount of solar 

 energy received at the surface of the earth we shall take 1.35 calories. 

 Such a calculation should be based upon good agriculture conditions and 

 should exclude, naturally, extreme conditions of solar intensity such as 

 exist on the desert. In a six hour day at 1.35 calories per square centi- 

 meter per minute there would be received 486 calories per square centi- 

 meter. From Kimball's ^^ values, obtained with a Callender pyreheliom- 

 eter, at Washington, D. C, and Mount Weather, Va., the mean daily 

 normal for the 92 days of May, June and July, 1910-1914, is 522 gram 

 calories per square centimeter of horizontal surface. This somewhat higher 

 value is due to the fact that it represents the amount of radiation received 

 during the entire day instead of only six hours, but it serves to show that 

 we have not taken too high a quantity in the 1.35 gram calories. We 

 shall take 90 days as the growing season, and convert the total amount 

 of solar radiation received on an area of one U. S. acre into terms ex- 

 pressed by the energy derived from the combustion of coal. Thus, taking 

 the heat of combustion of anthracite coal equal to 8,000 kilogram calories 

 per kilogram, we conclude that the solar energy received on an acre of 

 land during a growing season of 90 days is equal to the energy contained 

 in 243 tons of anthracite coal. 



For the purix)se of comparison, we can calculate on the same basis, the 

 amount of this total energy which the plant fixes and which is found 

 in the jx^tential energy of the substances which have been elaborated by 

 the plant in the photosynthetic process. We shall take a yield of 25 bushels 



"Abbot, C. G., Smithsonian Inst. Ann. Astro physical Obser. 4 16 177 184 

 (1922). Humphreys, W. J., Astrophx. Jour.. 32, 97 (1910). 

 "Kimball, Monthly Weather Reznezv, 43, 108 (1915). 



