l'.»2 



THE AMERICAN NATURALIST [Vol. XLIII 



ever, seem to preclude the use of this method at the 

 present time. 4 



The method suggested by Zederbaur of using a spectro- 

 photometer in connection with colored prisms and a 

 standard lamp has been hailed by some as a solution of 

 this problem. 5 It also involves several important sources 

 of error, but these may not prove to be insuperable. 

 This method has the great advantage over the actinom- 

 eter in that it takes into account light quality. With 

 certain modifications, some of which are suggested by 

 Zederbaur, this instrument may prove to be a step toward 

 a better means of light measurement. 



4. Some Climatic Problems of Vegetation.— But even if 

 the time is approaching when it may be possible to obtain 

 both qualitative and quantitative estimates of the climatic 

 factors related to vegetation processes, we should regard 

 this merely as a preliminary to more important investi- 

 gations. 



We have as yet almost no observational or experimental 

 data from a modern point of view, on geographic varia- 

 tion as related to climate. The reports concerning the 

 results of the cultivation of certain varieties of agricul- 

 tural and horticultural plants suggest a large field for the 

 application of experimental methods. 



A fundamental consideration in experimentation along 

 this line is the use of pedigreed plants. It seems possible 

 that many of the discordant results obtained by physiol- 

 ogists may be traced to physiological races within the 

 species. The use of seeds from the same plant, branch, 

 or fruit is not sufficient caution unless the pollination has 

 been guarded. Where the growth of such large numbers 

 of plants as would be essential to an experiment in geo- 

 graphic variation is necessary, the results could have 

 little value unless the plants used had been analyzed by 



4 Wallace, E. J. "Studies in Sensitometry, ' ' Astrophysical Journal, 

 25: 116, 1907. 



'Zederbaur, C. "The Light Eequirements of Forest Trees and the 

 Methods of Measuring Light." English translation. Forestry Quarterly, 

 6: 254, 1908. 



