34 PHOTOSYNTHESIS 



photosynthesis problem is whether the production of industrial energy 

 from solar radiation through the intermediary of the plant seems feasible. 

 In Figure 2 is shown graphically the relation of the total energy of 

 solar radiation to that portion which is fixed or transformed into usable 

 form by the plant. The great difference in these quantities is very evident. 

 We are now utilizing about 0.1 per cent of the solar energy falling on a 

 given area of the earth's surface, and this only during a fraction of the 

 year. For this utilization we are still wholly dependent upon the inter- 



0.41 0.83 



E F 



0.33 0.20 0.033 



BCD 



Fig. 2.— a, amount of sol^'r energy received on an area of one acre in a growing 

 season of 90 days expressed in tons of anthracite coal. B, 25 bushels of corn 

 from an acre of land, energy obtainable therefrom expressed in tons of coal. 

 C, energy of alcohol obtained'by fermentation of 25 bushels of corn. D, energy 

 from meat when corn obtained from an acre is fed to steers. E, energy m 

 terms of coal from an acre of redwood in 3 months. F, energy in terms of 

 coal from an acre of Eucalyptus. 



mediary of the chlorophyllous plant. The causes which account for this 

 low order of efficiency are rather complicated. Moreover, dififerent species 

 of plants vary greatly in the various factors which must be taken into 

 account in analyzing the effectiveness of plants as storers of solar energy. 

 We have prepared what may be termed a composite picture of the relation 

 of plants to solar energy. 



In Figure 3 is shown the disposal by the leaf of the energy incident 

 on it. All of this energy is not absorbed by the leaf ; approximately 70 

 per cent is absorbed, 30 per cent being transmitted or reflected. In land 

 plants a very considerable portion of the energy absorbed by the leaf is 

 dissipated through evaporation of water from the leaf. This has the effect 

 of cooling the leaf. The quantity of energy thus lost naturally varies 

 greatly with external conditions. We can place this amount at 50 per 

 cent. When there is relatively little evaporation the absorption of solar 



