CHLOROPHYLL AND ENERGY 207 



from the quantity of material synthesized. In an hour of full 

 sunUght, 1 square metre of leaf receives 720,000 calories. If 2 

 grammes of glucose are synthesized, this area has accumulated 

 6,500 calories, which represents 0-905 per cent of the incident 

 energy and nearly reaches the average of 1 per cent. 



In considering the luminous energy normally received on 

 the ground and calculated by the meteorological observatories, 

 it may be observed that leaves have, in fact, a larger useful area 

 than the ground for receiving the light ; they partly cover one 

 another and are more or less inclined to the rays of the sun. 

 A small quantity of light passes through the highest and a 

 small quantity is sent back on the others; in addition, the 

 maximum for assimilation is ordinarily below the maximum 

 of the Hght received from the sun. Thus, leaves receiving 

 oblique light may assimilate to the maximum, and that 

 is why in a plant or stem of maize sHghtly higher yields may 

 be found than the maximum calculated for an isolated leaf. 



Maize is one of the plants which seems to make the best 

 and most rapid use of the sun's energy. Transeau took as a 

 basis for his calculations a good field of maize yielding a crop 

 of two tons of grain per acre. The growing period is about a 

 hundred days and the dry matter formed reaches five and a 

 half tons, which corresponds to six and a half tons of 

 glucosides assimilated. A certain quantity of glucosides, which 

 may be estimated at nearly two tons, has been expended in 

 respiration; this brings the quantity of glucosides elaborated 

 by photosynthesis to eight and a half tons. 



Each pound of glucosides represents 1,710 kilo-calories. 

 The acre of maize has therefore stored 32,000,000 kilo- 

 calories, i.e., 1-6 per cent of the calories received from the sun 

 during the hundred days of growth. At best, the figure 

 scarcely exceeds the average given by Brown and Escombe. 



Studying the plankton of Lake Erie, Verduin nevertheless 

 found that the yield could be as much as three times higher, 

 but he took as his term of reference not the area but the 

 volume, and it is very probable that, for photosynthesis, these 

 small algae use their volume better than maize leaves. 



