CHAPTER V 



THE ROLE OF ULTRA-VIOLET RADIATION 



Ultra-violet 



If we asked a physicist the properties of ultra-violet rays, 

 he would reply that, since their quantum of energy is greater 

 than that of the visible and the infra-red rays, the absorption 

 of ultra-violet will more easily disturb the delicate chemical 

 structures of plants by provoking various "activations". We 

 must therefore expect intense chemical action to be stimulated 

 by the ultra-violet part of solar radiation, which is of vital 

 importance to man on account of its anti-rachitic potency. 



Now, this ultra-violet part is not very abundant and, in 

 our present state of knowledge, it seems that plants have 

 rather disdained the small proportion of solar ultra-violet, 

 since no really important action which could be attributed 

 to it has been completely proved ; they have adapted themselves 

 so that they are capable of using the visible rays, which are 

 reputed to be less chemically active but which are incompar- 

 ably more abundant in sunhght. 



The action of ultra-violet on plants becomes important 

 only if radiations of very short wave-length are produced 

 artificially — radiations which, owing to atmospheric absorp- 

 tion, are absent from the solar spectrum and have wave- 

 lengths lower than its Umit of 2,890 A. They have a clearly 

 injurious effect. 



But on the near side of this harmful region, it does not 

 seem that any chemical action caused by radiations situated 

 between it and the visible has been observed with certainty. 

 Perhaps suitable methods of observation have not been found. 

 In any case, it is unlikely that any very important effect will 

 be discovered in the future, since plants can be cultivated for 



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