72 LIGHT, VEGETATION AND CHLOROPHYLL 



of ultra-violet, it would therefore be necessary to check by 

 very precise and complete measurements the nature of the 

 conditions other than the ultra-violet irradiation, both for the 

 controls and for the subjects of the experiments. 



Here, however, are some of the results obtained from 

 different experiments. Ultra-violet tends to make plants 

 dwarf and hairy with small thick leaves and brightly coloured 

 flowers. A natural example of this is furnished by alpine 

 vegetation at high altitudes; there, the ultra-violet is more 

 abundant than it is at sea level, since molecular diffusion, 

 which disperses principally the short wave-lengths, has not 

 yet diminished its proportion. 



It may be remarked that Ught of all wave-lengths is also 

 more intense at high altitudes and that very intense Ught 

 tends to produce small plants. 



Very many experiments, intended to provide practical 

 information for the cultivation of vegetables or flowers, have 

 been carried out in glasshouses in which ordinary glass has 

 been replaced by special glass. The results obtained are con- 

 tradictory; sometimes the crops are more abundant, some- 

 times less, than in ordinary conditions. Chemical analysis 

 shows, in general, a higher percentage of lipide in plants 

 cultivated under glass which is more transparent to ultra- 

 violet. This is undoubtedly the clearest result, but it does not 

 appear to be established that ultra-violet is the only cause of it. 



On the other hand, a very marked effect is observed in 

 plants which are deprived of visible violet, and particularly 

 blue, radiations, i.e., those which are nearest to the ultra- 

 violet but which still appear to our eyes to be coloured. The 

 plants are clearly abnormal. The absence of the violet and 

 blue rays gives rise to etiolation and the growth, measured by 

 the weight of dry matter, is reduced. But this action is related 

 to visible and not to ultra-violet radiations. 



In experiments with artificial light, arcs between iron or 

 carbon electrodes are used as the source, or more often 

 mercury arcs in a quartz envelope; the latter are ordinarily 

 manufactured for numerous photographic, chemical and 



