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THE AMERICAN NATURALIST [Vol. XLIH 



would doubtless give the data needed, but it is not well 

 suited to field work and is at best too expensive. The 

 so-called photometers (such as those used in photog- 

 raphy) are unsatisfactory in the extreme, both theoret- 

 ically and practically. They are, of course, not photom- 

 eters at all, but actinometers, and for our purpose it is 

 to be remembered that the shorter light waves are at 

 least not the most important in plant activity. Here again 

 is a field that should prove wonderfully fertile to him who 

 has the courage and patience to cultivate it. 



The determination of the composition of the atmos- 

 phere is important in certain lines of investigation and in 

 the solution of general ecological problems in certain 

 regions. For this purpose the methods already at hand 

 are perhaps satisfactory enough, though we are unable 

 to obtain automatic records of fluctuations in these con- 

 ditions. 



Soil Factors.— If the factors active above the soil sur- 

 face present great difficulties, those active in the soil 

 present greater ones. What knowledge has been gained 

 by the agriculturists is seldom at the disposal of the 

 ecologist, perhaps partly from the nature of the agricul- 

 tural literature, and partly from a too common feeling 

 that agriculture and ecology are far apart. (It appears 

 to me that ecology is the legitimate and necessary meet- 

 ing-place for scientific botany and scientific and practical 

 agriculture.) But the agriculturists have not made any 

 very great progress in this field. Their measurements 

 of soil conditions are too often merely determinations 

 of the various amounts of inorganic salts which can be 

 extracted from the soil by one or another cleverly chosen 

 solvent. In some cases determinations are made of the 

 total amount of organic matter in the soil, but it appears 

 that these chemical results lack much in ease of inter- 

 pretation, so much that they are of little value to the 

 ecologist. 



Soils have been classified by various workers according 

 to the size of the component particles, but I have not 



