200 PLANT PHYSIOLOGY 



flax, cucumbers, tomatoes, strawberries, beans, buckwheat, and 

 others, thrive excellently in artificial light; while others such as 

 radishes, cabbage, salad, spinach, and especially sunflower are 

 extremely sensitive to the deficiency of blue-violet rays and 

 elongate excessively under such conditions. 



For the normal growth of plants, it is of major importance to 

 improve the composition of artificial light by the addition of 

 blue-violet rays. However, sufficiently powerful and convenient 

 sources of these rays are not yet available. Arc street lamps 

 are suitable as to the quality of their light if the high violet region 

 is absorbed by a glass mantle, but they are not convenient to use. 

 This led to the substitution of incandescent electric lamps. 

 Mercury arc lamps also could serve as a source of the rays 

 required. They contain mercury vapor, which with the passage 

 of electric current emits light rich in green, blue, and considerable 

 ultraviolet rays. Mercury arc lamps with a quartz tube allow 

 the free passage of ultraviolet rays, but for plants these rays are 

 injurious, and even a short exposure causes serious injuries to 

 the leaves. That is why in adding to the incandescent lamps the 

 light from mercury arc lamps it is necessary to provide them 

 with a glass tube that absorbs the harmful shorter ultraviolet 

 rays. Experiments carried out with such lamps have given 

 favorable results with only a few plants. This may have been 

 caused by a too low light intensity. 



During recent years, tests have been made with the cultivation 

 of plants in the light of neon lamps, which in the shape of tubes 

 are used for signboards. When an electric current of high poten- 

 tial is passed through neon, it emits a reddish-orange light well 

 utilized by plants in photosynthesis. Neon lamps have the 

 advantage that about 80 per cent of the wattage consumed by 

 them is transformed into light energy, while the efficiency of 

 incandescent lamps is not more than 15 per cent. But being 

 almost completely devoid of blue- violet rays, neon light does not 

 foster normal development in plants and should be added only 

 as a source of additional light to the normal daylight or to the 

 light of incandescent lamps. 



The growing of plants entirely in artificial light is undoubtedly 

 very expensive. Most of the plants require on the average 

 about 500 watts per square meter of the soil surface. With 24 

 hr. of illumination, this makes up 12 kw.-hr. per day. That is 



