334 PHOTOSYNTHESIS 



TABLE 32 EFFICIENCY OF PHOTOSYNTHESIS IN CORN (dATA OF TRANSEAU, I926) 



Energy required for synthesis of i kg. glucose 37^0 kg.-cal. 



Total energy utilized in photosynthesis by an acre of corn plants 



in manufacture of 8732 kg. glucose 23 million kg.-cal. 



Total solar energy available on the acre during growing season. . . 2043 million kg.-cal. 

 Per cent of available energy used by corn plant in photosynthesis — 



i.e. its photosynthetic efficiency 1.6 per cent 



Of the total carbohydrate synthesized by the corn plant only about 25 per 

 cent is harvested as a grain crop. The hundred bushels of corn grain obtained 

 at the end of the growing season represents only about 0.4 per cent of the 

 total radiant energy which fell on the acre. IVlost crop plants are considerably 

 less efficient than this unusually productive acre of corn as converters of 

 radiant energy into chemical energy. 



The calculation of the efficiency of photosynthesis just presented was based 

 upon the total radiation incident upon the acre. This is the correct basis 

 of calculation for evaluations of the efficiency of any plant as a crop. For 

 calculating the efficiency of the process itself other bases of computation are 

 generally used. The efficiency of photosynthesis may be computed on the 

 basis of: (i) the total radiant energy incident upon the leaf, (2) the radiant 

 energy actually absorbed by the leaf, or (3) the radiant energy actually 

 absorbed by the chloroplasts. 



Discussion Questions 



1. What would be the effect upon plants of the disappearance of all animals 



from the earth? the effect upon animals of the disappearance of all plants? 



2. Is it true that no life could exist upon the earth in the absence of photo- 



synthesis? 



3. What are some possible explanations of the failure of leaves of some species 



to synthesize starch from the photosynthetic products? 



4. Under what conditions would you expect that exposure of a plant to inter- 



mittent light would not result in an increase in the quantity of photosyn- 

 thate produced per unit of light? 



5. When leaves of some species are submerged in water slow infiltration of the 



intercellular spaces occurs if they are kept in the dark, but not if they 

 are kept in the light. Explain. 



6. Masses of filamentous algae are often found floating near the surface of 



a pond after several days of clear weather, but the same algal masses are 

 often submerged after a period of several cloudy days. Explain. 



7. Most soils contain considerable quantities of organic matter in the form 



of humus and derivative compounds. How could you demonstrate that 

 green plants do not utilize any appreciable quantity of these compounds 

 as foods? 



8. What method would you recommend for measuring the rate of photosynthesis 



