62 LIGHT, VEGETATION AND CHLOROPHYLL 



limit of the visible, and would gradually diminish in the 

 infra-red, to 8 per cent at 30,000 A. Unfortunately, this 

 result, at least for the near infra-red accessible to special 

 photographic plates, is contradicted by photographs of trees 

 taken in infra-red light, where the leaves appear as if they 

 were white. 



For the single wave-length of 11,000 A, one investigator 

 measured, certainly with a more correct method, the reflection 

 factor of a vine leaf and of a potato leaf; it is about 45 per 

 cent. This high value confirms the indications given by the 

 photographs. 



It is therefore extremely probable that, at least in this 

 region of wave-length, leaves reflect about half the infra-red 

 that they receive. 



It is precisely in this region that the sun sends most 

 radiating power and the more extreme radiations are less 

 important. 



We can expect, as Coblentz indicated, that the reflection 

 will become less and less intense towards the long wave- 

 lengths, for this diffuse reflection is caused by the thickness 

 of the leaf, and beyond 14,000 A the large quantity of water 

 contained in the vegetable tissues absorbs strongly. 



We shall therefore assume, until more precise measure- 

 ments have been made, that the reflection factor is 45 per cent 

 at the beginning of the infra-red and lower than this in the 

 more extreme range. 



As regards the transmission factors, all the data that 

 we have relate to some measurements made by John M. 

 Arthur by a very simphfied method, but he assumed that 

 the reflection factor was 20 per cent, which is certainly too 

 low. 



The only result to be retained is that the transmission 

 factor is notably higher in the infra-red than in the visible. 

 We can estimate that it is of the order of 20 per cent to 

 25 per cent. 



To sum up, in the near infra-red, which is the most 

 important region, half the incident radiation is reflected, a 



