LIGHT AND VEGETATION 71 



part is rather small and that none, or very little, can penetrate 

 inside the thickness of the tissues. 



The immediate action of ultra-violet cannot be other 

 than principally superficial, but it is possible that profound 

 repercussions are ultimately manifested. 



The effects observed differ widely according to whether 

 the wave-length is longer than 2,890 A, or shorter. This is 

 precisely the limit of the solar spectrum; the solar radiation 

 received at ground level does not contain wave-lengths shorter 

 than 2,890 A. 



The Near Ultra-violet 



We shall call the spectral region between 2,890 A and 

 4,000 A the near ultra-violet and all the radiations of wave- 

 length shorter than 2,890 A the extreme ultra-violet. 



Experiments bearing on this question have been made, 

 both with sunhgbt and with sources of artificial Hght. 



In the first case, the vegetation directly exposed to the 

 sun is compared with that protected by glass screens; the 

 screens used are made of special glass, chosen for its trans- 

 parency or the degree to which it is opaque to wave-lengths 

 in the ultra-violet. Obviously only the possible influence of 

 the near ultra-violet can be shown in experiments of this 

 kind, since the extreme ultra-violet is not present in the solar 

 radiation, which has already been filtered by the atmospheric 

 ozone. 



We shall describe some of the observations that have been 

 made with this method, but the results obtained cannot be 

 accepted without a good deal of reserve. Most of the experi- 

 menters omitted to examine the properties of their screens in 

 the infra-red. The conditions of ultra-violet irradiation were 

 considerably aff'ected, in the way that one would expect them 

 to be, by the different screens, but other conditions, such as 

 infra-red irradiation, and even visible irradiation and tempera- 

 ture, varied at the same time. These last factors are extremely 

 active and greatly modify the manner and rate of growth. 

 To prove that an observed eff'ect is attributable to the action 



