LIGHT AND VEGETATION 73 



medical applications. They are particularly convenient, being 

 as easy to use as incandescent lamps. Their radiation includes 

 both the near ultra-violet found in sunUght and the extreme 

 ultra-violet. By interposing suitable filters, it is possible to 

 isolate any region of ultra-violet and submit the plants to 

 investigation. 



These artificial sources have enabled the region of the 

 near ultra-violet, between the wave-lengths 2,900 A and 

 3,130 A, which are present in sunlight with only a very small 

 quantity of energy, to be studied with stronger illumination. 

 The experiments of Withrow and Benedict (1931) were carried 

 out in well-defined radiation, but unfortunately on too small 

 a number of plants (coleus and tomato) and they have not 

 been repeated. Since these radiations are vitally important 

 for animal life, we may reasonably wonder whether they 

 are just as important for the vegetable kingdom. 



The conclusion reached by the investigators, which they 

 themselves give as only provisional, is that there are some 

 indications that wave-lengths between 2,900 A and 3,100 A 

 have a favourable effect on vegetation; the plants are taller, 

 the stems are thicker, the leaves larger and more numerous, 

 and the weight, in the fresh state and after desiccation, is 

 heavier. 



A very small proportion of ultra-violet of shorter wave- 

 length neutralizes the beneficial effect observed; we shall see 

 later that this extreme ultra-violet is, in fact, very injurious. 



The general illumination in these experiments was so 

 small that the conditions of normal vegetation were not 

 assured, so that the slightest difference between the illumination 

 in visible light was capable of reacting intensely on the 

 activity of growth. In fact, the individual differences between 

 plants cultivated under the same conditions were too great 

 for the results to be clear. 



We must therefore conclude that the near ultra-violet has 

 no perceptible action on vegetation. Numerous experiments, 

 both in sunUght and in artificial fight, give only contradictory 

 or rather unconvincing results. Some of them, conducted 



