74 LIGHT, VEGETATION AND CHLOROPHYLL 



J 



with the greatest care and in well-controlled conditions, as 

 they were at the Boyce Thompson Institute in New York, 

 showed no observable difference between cultivations under 

 ordinary glass and under glass specially transparent to ultra- 

 violet. Lamprecht, in Sweden, drew the same conclusion from 

 his practical experiments. 



There is therefore no reason to recommend other than 

 ordinary glass for the construction of glasshouses, since the 

 much more expensive special glass transparent to ultra- 

 violet would not have any indisputable advantage. 



The Extreme Ultra-violet 



If we pass on to consider the influence of the extreme 

 ultra-violet of wave-lengths shorter than 2,890 A, we find 

 unanimous agreement among all the experimenters. Extreme 

 ultra-violet causes severe damage, especially to the superficial 

 parts — a fact that has been known for a long time. The outer 

 layers of cells become discoloured and die ; the depth to which 

 they are affected depends on the nature of the radiation and 

 on the quantity received. Different tissues are sensitive in 

 different degrees to this action. 



Since radiations of wave-length very Httle shorter than 

 the limit of the solar spectrum begin to be dangerous, we see 

 the importance of the thin layer of atmospheric ozone which 

 suppresses by absorption all the solar radiations on the far 

 side of this limit of 2,890 A. The shorter the wave-length of 

 the radiations used, the more intense is their destructive action. 



A prolonged action may kill the whole plant. Otherwise, 

 the plant recovers after a length of time depending on the 

 seriousness of the injuries. 



A daily exposure of ten minutes to radiation from a 

 mercury arc is sufficient to cause the death of young plants. 

 Subjects raised under a subdued light are all the more sensitive, 

 which shows that light makes the epidermis of the leaves 

 more robust and more opaque to ultra-violet. 



When the first layers of cells are dead as a result of these 

 destructive radiations, they form a screen which protects the 



