LIGHT AND VEGETATION 59 



Here are some examples of these overall efficiencies : 

 At the observatory of Saint-Maur, the luminous energy 

 received per year from the sun is about 100,000 calories per 

 sq. cm., energy theoretically sufficient to effect the synthesis 

 of 25 grammes of carbohydrates. For one acre this quantity 

 becomes 1,000 tons. This would be the dry weight of the 

 crop if the efficiency were 100 per cent. Now, the annual 

 production of dry wood from a forest of beech trees is 

 1-6 tons per acre; the overall efficiency is therefore 0-16 per 

 cent. 



A field of wheat makes better use of the hght ; its efficiency, 

 calculated in the same way, is of the order of 0-3 per cent. 



