LIGHT AND VEGETATION 75 



lower layers. But as the plant grows, this protective envelope 

 is torn away and the radiation can penetrate more deeply. 



Even inside the leaf, the growth of the cells appears to be 

 slowed down ; there are fewer small cavities and differentiation 

 is checked. 



Anthocyanins, the colouring matter found for example 

 in begonia leaves, resist the action of ultra-violet, but never- 

 theless they disappear in the living leaves submitted to this 

 radiation. 



Finally, it is worth noting that no beneficial effect of these 

 radiations has ever been proved. 



Fluorescence 



Ultra-violet light is more apt than any other to make 

 certain substances fluorescent. Fluorescence is the phenomenon 

 by which absorbed radiant energy is in part immediately 

 re-emitted in the form of Hght; the most striking case is that 

 in which the light is visible, for the fluorescent substance, lit 

 by invisible radiation, seems to be independently luminous 

 and sometimes shows a brilliantly coloured light. 



Many instances of fluorescence have been observed in the 

 vegetable world. Green leaves and some kinds of flowers 

 show intense fluorescence under the action of radiations 

 between 3,400 A and 3,800 A. Radiations from 3,000 A to 

 4,000 A are capable of producing fluorescence in certain 

 seeds during their germination, and it has sometimes even 

 been possible to distinguish breeds or varieties by the 

 differences in the light emitted. 



These phenomena are very striking to observe, but their 

 scientific interpretation is too complex to enable us to draw 

 practical conclusions from them. We know, however, that the 

 fluorescence of a substance is proof that it has absorbed ultra- 

 violet and is momentarily in an excited state; it returns to its 

 normal state by emitting its energy of excitation in the form 

 of hght. 



The addition of another substance may extinguish the 

 fluorescence; this proves that the energy of excitation has 



