THE ORIGIN OF ORGANIC MATTER ZZ 



of corn (bushels of 56 pounds) per acre and consider the heat value of 

 the grain only. De Baufre ^^ reports the heat value of fully cured corn, 

 containing 10 per cent of water, as 6,700 B.t.u. per pound. The energy 

 of the crop of corn from an acre of land is thus equivalent to 0.325 of a 

 ton of anthracite coal. That is, by raising corn on a given area of land 

 about .13 per cent of the total amount of solar energy received is "fixed" 

 by conversion into potential energy of carbon compounds. 



If the yield of corn from one acre were fermented in order to obtain 

 alcohol, the energy obtainable would be still further reduced. Thus, from 

 25 bushels of corn at 2.7 gallons per bushel there would be obtained 67.5 

 gallons of alcohol. The heat of combustion of this amount of ethyl alcohol 

 corresponds to about 0.20 tons of anthracite coal or about 0.08 per cent 

 of the total energy of solar radiation per acre for a period of 90 days. 



On the basis of a conversion factor of 10 per cent, the yield- of corn 

 from one acre, when fed to steers, would produce meat corresponding 

 to about 0.033 tons of coal per acre. 



A similar calculation can be made of the production of material syn- 

 thesized by forest trees. ^^ We shall take one of the fastest growing trees, 

 the redwood {sequoia scmpervirens) . The average annual growth of 20-60 

 year trees is about 300 cubic feet, or 75 cubic feet for a period of three 

 months. This includes the stem and top but not the bark and limbs. The 

 heat of combustion of one cubic foot of redwood is 159,000 B.t.u. This 

 would yield an energy equivalent of 0.41 tons of anthracite from one acre 

 of redwood in a period of three months. While this does not represent 

 the entire yield of material synthesized by the trees, it is a liberal account- 

 ing of the merchantable timber. Another very rapid growing tree is 

 Eucalyptus globulus. This tree averages about 355 cubic feet of wood 

 per year for 20 year trees, or 89 cubic feet for three months. The heat 

 of combustion of one cubic foot of this wood is 268,000 B.t.u. On this 

 basis an acre of eucalyptus trees would yield energy corresponding to 

 0.826 tons of coal in a period of three months. This is about double the 

 quantity obtained from redwood ; the latter is notoriously poor fuel while 

 eucalyptus though fast growing, is a hard wood of high heat of combustion. 



The foregoing figures have been given in order to present an approxi- 

 mate idea of the plant as a converter of solar energy. They cannot be 

 taken as a true index of the efficiency of the photosynthetic process ; this 

 will be discussed in a later chapter. The values have a "practical" rather 

 than a strictly scientific meaning. For instance, in the determination of 

 the per cent of conversion in corn we did not include the stalks, which 

 would probably double the total value. Yet the use of such material for 

 fuel or for the production of alcohol presents many difficulties which 

 make them of questionable value. One of the practical questions of the 



"De Baufre, Pozver, 56, 212 (1922). 



^ Bruce, University of California, Agricultural Experiment Station. Bulletin 

 No. 361, Metcalf, Ibid., 380. U. S. Dept. of Agriculture, Agriculture Bull., 753 

 (1919). 



