36 



PHOTOSYNTHESIS 



1 per cent used in photosynthesis represents a value determined for the 

 absorbing leaves and not for an entire plant. Moreover, in a growing 

 period of 90 days we include the plant from the seedling stage to the time 

 of harvest. During this period the photosynthetic activity of the entire 

 plant varies from virtually nothing to maximum and then decreases agam. 

 Also the entire acre of land on which we based our calculation is not 

 covered by leaf surface. These various factors, then, contribute to reduc- 

 ing the net conversion by the plant to about 0.1 per cent of the total 

 incident solar radiation. 



The foregoing brief discussion gives a general idea of the efficiency of 

 the green plant as a converter of solar energy. Under natural conditions 



Pig 4_ — Approximate distributiuii ui material manufactured in the 



photosynthetic process. 



the rate of photosynthetic activity undergoes wide fluctuations according 

 to the changes in external conditions to which it is subjected. Before 

 entering upon an analytical study of the influence of each of these various 

 factors we shall consider the function of green plants on our earth in its 

 more general aspects. 



The composition of the atmosphere is, of course, of very great impor- 

 tance in relation to the photosynthetic activity of plants. While there is 

 still considerable discussion regarding the composition of the atmosphere 

 to its outer limits, we are concerned primarily with the composition at the 

 surface of the earth. Here the different gases are present in the following 

 volume percentages : 



Nitrogen 78.03 Hydrogen 0.01 



• Oxygen 20.99 Neon 0.0012 



Argon 0.94 Helium 0.0004 



Carbon dioxide 0.03 



