MAGNITUDE AND EFFICIENCY OF PHOTOSYNTHESIS 333 



The final stage in the photosynthetic process is presumed to be the poly- 

 merization of the formaldehyde resulting in a hexose sugar: 



6 HCHO -^ CeHioOo 



Various other theories have been proposed to account for the mechanism 

 of photosynthesis (see for example Emerson and Green 1937, and Franck 

 and Herzfeld 1937), but none of them should be entertained too seriously. 

 They are of value in so far as they present current or recent concepts of the 

 mechanism of the process, but they are subject to more or less continual change, 

 as new facts regarding photosynthesis are discovered. 



Magnitude and Efficiency of Photosynthesis. — Transeau's (1926) cal- 

 culations of the "energy budget" for an acre of corn (maize) plants illustrate 

 very clearly the material and energy relations of photosynthesis. This energy 

 budget was prepared for an hypothetical acre of corn (10,000 plants) grow- 

 ing in north central Illinois and yielding the very good return of lOO bushels 

 per acre. The growing season is assumed to be lOO days. The magnitude 

 of photosynthesis for such a field of corn is shown in Table 31. 



TABLE 31 — QUANTITV OF PHOTOSVNTHATE PRODUCED BY ONE ACRE OF CORN IN A GROWING 



SEASON (data of TRANSEAU, 1 926) 



Dry weight of average corn plant. 



Stalk. . 

 Leaves. 

 Roots. . 



Grain 216 g. 



200 



I40 



44 



600 g. 



Total dry weight of an acre of corn (10,000 plants) at end of season 6000 kg. 



Total ash (5.37 per cent of dry weight) 322 kg. 



Total organic matter in the plants 5^7^ ^E- 



Total carbon accumulated (44.58 per cent of the organic matter) 2675 kg. 



Glucose equivalent of accumulated carbon (CsHi^Oe : Ce = 180 : 72) 6687 kg. 



Glucose equivalent of respired carbon (calculated from estimated rate of 



CO2 release = i per cent of dry weight per day) 2045 kg. 



Total sugar manufactured in terms of glucose 8732 kg. 



The quantity of radiant energy falling on an acre of land surface in north 

 central Illinois during the growing period of lOO days is known from measure- 

 ments made at IMadison, Wisconsin, not far from the region of north central 

 Illinois. Using these data together with his own estimates of the total glucose 

 produced by the photosynthetic activity of the corn plants, Transeau was able 

 to calculate the photosynthetic efficiency of corn (Table 32). 



