226 JOURNAL OF FORESTRY 



1. Total Solar Radiation 



It is generally accepted that plants need less light as the temperature 

 rises and more light as it falls. This is seemingly true not only for 

 changes of temperature with latitudes but also for those due to differ- 

 ences of altitude. Hann's example of the difference in the minimum of 

 light necessary for poa annua in Cairo and in Vienna, due to the differ- 

 ences in the solar radiation received at the two places, is well known. 



Hann emphasizes the need of measurements of the strength of the 

 sunhght and of the heating eft'ects of direct solar radiation, which 

 should be supplemented by measurements of the chemical activity of 

 this radiation. These climatic elements have thus far been sadly neg- 

 lected. He expresses the hope that when the seriousness of this de- 

 ficiency becomes known, the difficulties that still stand in the way of 

 such measurements will speedily be overcome. 



On the other hand, the forest investigator is concerned chiefly 

 with the weaker diffused light actually utilized by the trees than with 

 the total solar and sky radiation. Most of the life processes, such as 

 the formation of chlorophyl, photosynthesis, etc., take place in very 

 weak light intensities. Direct sunlight in many cases is actually in- 

 jurious to the chlorophyl grains, and to transpiration, and other func- 

 tions of the tree. There can therefore be but little question as to the 

 total solar radiation being anywhere at the limit or even near the 

 actual requirement of forest trees, since even in the shade of the 

 densest forest there is enough light to cause the formation of chloro- 

 phyl bodies. 



Even should there be any need for determining the total solar 

 radiation at any geographic latitude, could it not be determined mathe- 

 matically from the observations carried on at a few places in the 

 United States instead of undertaking such complicated observations 

 at a large number of field stations? 



Since, however, there are some investigators who lay great em- 

 phasis on solar radiation and insist on its measurements at nearly all 

 of the experiment stations, this question, in my opinion, is one upon 

 which further investigations are needed. 



2. Spectro-photometric Measurements 



Are spectro-photometric measurements essential for studies of 

 plant growth? According to a number of investigators, such as Reinke, 

 Fr. Czapek, Bonnier and Mangin, the quality of light has not such an 



