LIGHT AND VEGETATION 65 



do in an oven. But drying in an oven, for the same maximum 

 temperature attained, takes five to ten times longer, because 

 the heat has to be transmitted from the surface towards the 

 interior, while the infra-red radiation more or less penetrates 

 before being absorbed and produces heat at a depth. 



It is possible also that evaporation is favoured by the 

 fact that the molecules of water are themselves capable of 

 absorbing the infra-red energy. 



The leaf submitted to infra-red radiation finds itself in 

 exactly the same conditions as the substances exposed to the 

 drying lamps, and one of the results must certainly be an 

 evaporation of water or a transpiration in proportion to the 

 illumination received or, more exactly, absorbed. We must 

 not forget, however, that visible radiation can also have the 

 same effect. 



The precise experiments made by Arthur and Stewart at 

 the Boyce Thompson Institute for Plant Research in New 

 York showed that this is the case. 



Potted tobacco plants were used for the tests; the pot was 

 watertight and covered with an impermeable paraffin paste 

 so that the measured loss of water was entirely due to the 

 leaves and stems. 



For example, under the infra-red Hght of incandescent 

 lamps fitted with filters opaque to the visible and giving a 

 power of 0-65 cal./sq. cm./minute (compare this with the 

 maximum power of sunHght on the ground, which is about 

 1-8 cal./sq. cm./m.), the loss of water was 0-34 grammes per 

 sq. cm. in twelve hours. The heat necessary to evaporate this 

 quantity of water is 585x0-34=200 cal., so that in one 

 minute the evaporation uses 0-28 cal./sq. cm., i.e., 43 per cent 

 of the incident energy. 



It would be of great interest to know what this energy 

 represents in relation, not to the incident energy, but to the 

 absorbed energy. From experiments made with infra-red 

 hght from incandescent lamps, we know that there is a 

 considerable absorption of these radiations and that most of 

 the absorbed energy serves to stimulate transpiration. 



E 



