LIGHT AND VEGETATION 61 



In practice, the chemical activity of infra-red radiation is 

 not strong enough to disturb chemical structures; it causes 

 oscillations of the atoms round their positions of equilibrium 

 without destroying the molecules. These oscillations are solely 

 heat manifestations and we may expect that the special action 

 of infra-red will be to heat up the bodies that absorb it. 



On the contrary, although, by the same process, the visible 

 and the ultra-violet also have a heating effect on the sub- 

 stances that absorb them, these radiations can, in certain 

 cases, particularly the ultra-violet, transmit to the molecules 

 enough energy of excitation to make chemical reactions 

 possible. 



We shall see to what extent experiments on the action of 

 infra-red confirm this estimate concerning the whole of the 

 chemical phenomena in the hfe of plants. 



Absorption by the Leaves 



Since only the absorbed radiation can have any action, 

 it would be interesting to know the absorption factor of leaves 

 for the various infra-red radiations. Unfortunately, scarcely 

 any measurements have been made on this subject. The 

 fraction absorbed is determined by the difference between the 

 incident energy on the one hand, and the energy not absorbed, 

 that is, reflected or transmitted, on the other. 



The reason for this lack of data is undoubtedly the diffi- 

 culty of measuring the radiation diffused and dispersed by 

 the leaf. The only instrument that can be used in all the infra- 

 red is the thermopile, which, not being very sensitive, is 

 inadequate to measure the small quantity of illumination that 

 it receives in diffused Hght, while the incident energy cannot 

 be increased to excess without drying up the leaf that is being 

 studied. Measurements are therefore difficult, but nevertheless 

 it is possible to make them with suitable methods. 



In 1913, Coblentz attempted to measure the reflection 

 factor up to 30,000 A. He found that the values were rather 

 small; the highest related to the leaves of red oak. The 

 reflection factor would be 18 per cent at 8,000 A, at the red 



