42 



TEXTBOOK OF PLANT PHYSIOLOGY 



The exact numerical expression of the amount of radiant energy 

 required by the plant for its normal development will be obtained 

 only when it will be possible to grow normal plants entirely in 

 artificial light under controlled conditions. Several such attempts 

 have been made (Harvey, Crocker, and Maximov). By using the 

 light of strong electric lamps of several thousand candlepower, it 

 is possible to grow plants and to make them bloom and mature 

 fruits (Fig. 17) in a room entirely deprived of daylight, for instance, 

 in a cellar. But because of the differences between the spectral 



composition of sunlight and 

 that of electric light (in the 

 light of an electric lamp there 

 are considerably less blue- 

 violet rays than in sunlight), 

 the plants grown in artificial 

 light differ somewhat in their 

 form from normal ones. Their 

 stems stretch too much in 

 length, while the leaf blades 

 remain somewhat abortive. 

 Therefore, it is impossible to 

 apply directly the results of 

 these experiments to plants 

 growing in natural conditions. 

 Some data regarding the 

 strength of illumination (in 

 meter-candles) sufficient to 

 induce flowering and fruiting 

 of plants may be given : peas, 

 1,100; beans, 2,400; barley and wheat, 1,860 to 2,200; radish, 

 4,000; tobacco, 2,200 to 2,800; corn, 1,400 to 8,000; buckwheat, 

 850 to 1,100. To permit a comparison, it must be mentioned 

 that direct sunlight, at noon, gives about 30,000 to 40,000 meter- 

 candles. It is interesting to note that plants grown under the 

 conditions of artificial lighting develop much better when con- 

 tinuously illuminated during day and night, than when subjected 

 to an 8- or 12-hr. period of darkness. Hence, the "night's rest" 

 is not necessary for plants. The possibility of work without 

 rest for several months seems an amazing fact. But it must be 

 kept in mind that in the process of photosynthesis the chief part 



Fig. 



17. — Buckwheat grown entirely in 

 artificial light. 



