2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



light of the sun. A direct experimental comparison between sunlight 

 and artificial light is, of course, difficult to make because of the great 

 number of variables entering into the problem. 



Numerous experiments have been made for the purpose of growing 

 plants under artificial illumination. The object of many such experi- 

 ments was to find a satisfactory artificial light which could be operated 

 economically on a commercial scale. In other experiments the technical 

 and scientific aspects were the main objectives. So far as is known, 

 there is no available light source which is like that of the sun in its 

 wave-length distribution. Plants have been grown fairly successfully 

 in a few instances under well-controlled laboratory conditions, but 

 the problem is by no means solved. It may even be found that plants 

 can be grown normally under greatly reduced intensities of light pro- 

 vided a proper proportion between the intensities of its component 

 wave lengths is worked out. 



The purpose of the present report is to discuss briefly some pre- 

 liminary experiments dealing with the question of a wave-length 

 balance of artificial light. 



EXPERIMENTATION 



In the experiments herein described, plants were grown between 

 two different light sources. Three or more types of lights could be 

 used, but for this preliminary survey it was thought best to limit the 

 wave-length distribution to two types. All the experiments were con- 

 ducted in a small room (approximately 15 x lo ft. x 8 ft. high) the 

 walls and ceilings of which were painted a flat black to minimize 

 scattered light effects. Both temperature and humidity were auto- 

 matically controlled. The plants were grown in i-quart jars contain- 

 ing nutrient solution. Each culture was placed on a small rotating 

 table and usually grown for 3 weeks with a daily light period of 12 

 to 18 hours. By constantly rotating the plants (3.4 r.p.m.) on an axis 

 parallel to their stems, the phototropism of these stems was reduced 

 to zero. The leaves in some experiments showed phototropic response. 

 The wave-length distribution depended upon the light source. The 

 intensity was regulated largely by the distance the culture was placed 

 from the light. 



In an earlier paper Johnston (1932) found that the excess of near 

 infrared of the Mazda lamp caused a distinct yellowing of tomato 

 leaves. If this region of the spectrum was not actually destructive to 

 chlorophyll, . it was of little or no benefit to its formation. It would 

 thus appear that more nearly normal color could be obtained by re- 



