CONCLUSIONS 



We have tried, in the preceding pages, to give the reader an 

 idea of what is known, and also of what is not known, of this 

 vast and varied subject of the relationship between Hght and 

 vegetation. As our knowledge advances, new problems arise, 

 as well as new methods of approaching them. The means at 

 the disposal of research today appear enormous; in fact, 

 methods of measuring radiations, formerly neglected, are 

 extending rapidly and will soon be in the hands of biologists; 

 organic chemistry is advancing and radioactive indicators pro- 

 vide a new and very promising tool for biological chemistry. 



Beside the recent methods available to laboratories, the 

 knowledge already acquired has practical applications of not 

 inconsiderable economic importance. Certainly, the cost of 

 electricity is too high to make one despise natural daylight, 

 but plants can be cultivated successfully in entirely artificial 

 hght. G. Truffaut was one of the first to do this from 1929- 

 1933 by illuminating oats with overrun incandescent lamps. 

 Generally speaking, the use of artificial light to extend the 

 natural day has the best chance of being economic, either for 

 bringing on early vegetables or for obtaining a photoperiodic 

 effect. Large lamp-manufacturing companies have studied 

 various special lamps for these purposes and have taken an 

 interest in the subject for many years. 



The action of natural light can also be modified by powders 

 or sprays, which appreciably change the optical properties of 

 leaves. This has been confirmed at the Institute of Optics, 

 and it is certain that agricultural washes do not have only a 

 chemical effect. 



Such applications will be further developed as science 

 enables us to acquire a better understanding of plant life — a 

 Hfe in which the importance of hght becomes more and more 

 evident. 



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