The Light Relation 



433 



at hand, it is impracticable to at- 

 tempt to compute equal energy inten- 

 sities of the colored lights. 



Light intensity. - - Determine the 

 relative value of light in the open 

 and contrast it with the light inten- 

 sity in the greenhouse and in the 

 shade of vegetation or buildings. To 

 make this study use the ordinary 

 photographic actinometer, the device 

 employed by Clements, 1 or strips of 

 solio paper. If the latter are em- 

 ployed, it is simplest to determine the 

 length of exposure in seconds neces- 

 sary to bring the paper to a certain 

 standard shade of brown. This may 

 be done by previously following the 

 changes in the paper while contrast- 

 ing it with a brown color scheme, 

 choosing some shade of color in the 

 color scheme as a standard which is 

 invariably one of those attained by 

 the paper in the process of darkening. 



Etiolation. - - Place in a perfectly 

 dark chamber water cultures of peas, 

 potatoes, and onions sprouting on 

 moist moss, and any potted plants 

 available. Make accurate observa- 

 tions of the conditions of the plants 

 or buds when placed in the dark, and, 

 if possible, arrange control cultures 

 exposed to the light, but under sim- 

 ilar conditions of moisture and tem- 

 perature. After ten days or more, 

 make comparative observations, not- 

 ing the effect (1) upon structures 



1 Physiology and Ecology, pp. 72-75. 



2F 



FIG. 122. Simple apparatus 

 for qualitative tests of the 

 effects of light of different 

 wave length. 



