LIGHT AND VEGETATION 63 



quarter is transmitted, and therefore only a quarter is absorbed 

 by the leaf. 



Although this conclusion cannot be final, it seems certain 

 that the proportion of infra-red energy absorbed by leaves 

 is lower than it was at one time supposed, being relatively 

 small — smaller than the proportion of absorbed visible 

 radiation. 



In the more extreme infra-red, i.e., at wave-lengths greater 

 than 14,000 A, it is probable that all leaves become very 

 opaque, reflecting very Uttle, and absorbing the greater part 

 of the radiant energy. 



It is necessary then to distinguish two principal zones in 

 the infra-red: the first, of wave-lengths shorter than 14,000 A, 

 which represents the major part of the solar infra-red and is 

 relatively little absorbed by the leaf; the second, of wave- 

 lengths greater than 14,000 A, relatively less abundant in 

 solar radiation but, proportionately to the visible, very 

 abundant in the radiation from incandescent lamps. This 

 second zone is strongly absorbed by leaves. 



As a result, when plants are cultivated under incandescent 

 lamps, the action of the infra-red is inordinately increased for 

 two reasons: first, because the incandescent lamp supplies, 

 for the same power in the visible, much more infra-red than 

 the sun ; secondly, because this infra-red is situated particularly 

 in the region where it is most strongly absorbed by the water 

 vapour in the leaf (see Fig. I, 13). 



Effect on Transpiration 



What becomes of the absorbed energy? Before answering 

 this question, it is interesting to note a fairly recent industrial 

 application of incandescent lamps. Special lamps, which are 

 very Uttle different from fighting lamps, except that the 

 filament is run at a sfightly lower temperature so that they 

 radiate particularly in the near infra-red, principally between 

 1 /x and 2 ^x, are sold for drying purposes. In fact, experience 

 shows that the substances exposed to this radiation become 

 heated and lose moisture, if they contain any, as they would 



