76 LIGHT, VEGETATION AND CHLOROPHYLL 



been transferred to the other substance instead of being used 

 for the emission of Ught. It is therefore possible in a few 

 exceptional cases to follow part of the mysterious destiny of 

 the energy of the radiation absorbed by a plant. 



It is obvious that fluorescence cannot be observed under 

 the action of sunhght, for the emission of Ught that results 

 from it is extremely small and can be seen only in complete 

 darkness; it is necessary to work with an artificial source, a 

 mercury arc, for example, and filters opaque to the visible 

 and transparent to uhra-violet, or with a monochromator. 



To summarize this chapter briefly, we may say that ultra- 

 violet radiations, according to their wave-length, are either 

 harmful or inactive. In the first case the wave-lengths are 

 shorter than 2,890 A; in the second they are longer and there- 

 fore nearer to the visible. The first are absent from solar 

 radiation, which is therefore not harmful; the second are 

 present with a power rapidly increasing as a function of the 

 wave-length, but at the same time the entire solar ultra-violet 

 represents only 1-3 per cent of the total radiation. 



The chemical activity of ultra-violet is well known. We 

 should expect to see it used by nature for many syntheses of 

 vegetable hfe. We see, on the contrary, that plants cover 

 themselves with cells more or less opaque to these radiations 

 as if to protect themselves from them. Their activity would 

 undoubtedly be too intense and dangerous for the complex 

 substances of the cells. 



Plants are adapted, on the other hand, as we shall see in 

 the following chapters, to make use of less active radiations, 

 situated in the visible spectrum and abundantly provided by 

 the sun. 



