SUN RATS AND PLANT LIFE— JOHNSTON 



369 



effect. Under these rays a typical wrinkled, necrotic area soon 

 develops. 



Much progress has been made in our knowledge of sunlight and 

 the manner in which it affects plants. A considerable amount of 

 this information, covering the general field of the biological effects 

 of radiation, has recently been compiled by the National Research 

 Council in two volumes edited by Dr. Benjamin M. Duggar (1936). 

 In order to simplify the problem we have considered sunlight under 

 the effects of its duration, intensity, and wave length. As our ex- 

 perimental science has improved, artificial light sources were used be- 

 cause their variables could be controlled better and the conditions of 

 the experiment repeated fairly accurately. In a last analysis, artifi- 



I 1 I I — r 



INFRA-RED 



ULTRA-VIOLET 



Figure 8. — Diagram of relative energy emission curves from a body at 1,000° K. (dull- 

 red therapeutic lamp) and at 3,000° K. (high-temperature tungsten lamp) compared 

 with that from the sun and from a mercury arc in quartz. The type of radiation from 

 a tungsten lamp equipped with a 1-cm water cell is also shown. 



cial light is modified sunlight. Duration experiments can be con- 

 trolled better with artificial light than with sunlight. Although the 

 intensity of artificial light is less than that of full sunlight, yet for 

 many purposes it is sufficiently great. The most vital difference be- 

 tween sunlight and artificial light lies in the quality or wave length 

 distribution. An examination of the curves shown in figure 8 may 

 help make this point clear. Compared with the energy distribu- 

 tion curve of solar radiation are similar curves constructed from 

 tungsten filament radiations at two different temperatures. Atten- 

 tion is called to the position of the maximum of each curve. In the 

 tungsten this is in the infrared, whereas in sunlight it occurs in the 

 yellow. Other artificial lights may likewise be compared with that 

 of the sun. Each shows a characteristic departure from a perfect 

 or even fairly good match. Even filters combined with artificial 

 lights have failed to give the desired similarity. It will be a dis- 

 tinct step forward in this type of research when an artificial light 

 source is developed that has an energy distribution curve similar 

 to that of solar radiation. 



