ROLE OF LIGHT IN NATURAL AND ARTIFICIAL REFORESTATION 239 



Other factors may be fully controlled. But the application of this idea 

 in the forest should tend to clarify empiric ideas of tolerance, as well 

 as to furnish measurements which will be logical and hence convincing. 

 Photochemical measurements, and measurements of total radiant 

 energ}^ are highly to be desired, but to be useful for correlation with 

 tree growth must be integrated with measurements of air temperature 

 and air movement. The ideal will have been reached only when we 

 have an instrument which integrates these several factors in the same 

 way as the plant does it. For the present, the nearest approaches to 

 the measurement of the effectiveness of radiation would appear to be 

 had in evaporimeter readings and in comparisons of blackened and 

 shaded thermometers. The former should be given preference because 

 they express the result in terms comparable to transpiration. The 

 latter may be useful in indicating plant temperatures and the possi- 

 bility of physiological activity, when these cannot be determined 

 more directly. 



