206 



LIGHT, VEGETATION AND CHLOROPHYLL 



not of 1 gramme, but of 8 grammes of dry weight per square 

 metre per hour. The plant, however, had been kept in an 

 artificially enriched atmosphere with a carbon dioxide content 

 of 5 per cent. The experiments reproduced in natural con- 

 ditions have rarely given as much as one third of this result. 



The measurement of the carbon dioxide absorbed by 

 apple leaves gives, per square metre and per hour, a maximum 

 of 2 to 3 grammes of gas absorbed, which represents in 

 glucosides 1-36 to 2-05 grammes. 



The increase of weight proceeding from photosynthesis 



Incident light 



\ 



lOO 



Reflected light 



Transmitted light 



Fig. II, 6. The distribution of the light as a percentage of the 

 incident light falling on a leaf 



is therefore capable of reaching 2 grammes per square metre 

 in an hour, but this figure represents a maximum which is 

 rarely exceeded. 



In 1905, Brown and Escombe made their well-known 

 experiments on the utilization of light by plants. They found 

 that, on an average, 30 per cent of the light falling on a leaf 

 is transmitted or reflected, 20 per cent is transformed into 

 heat and sent back by thermal radiation, 49 per cent is used 

 in evaporation and only 1 per cent (between 0-4 per cent and 

 1 '7 per cent) for photosynthesis. See Fig. II, 6. 



The energy used for photosynthesis can also be calculated 



