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THE AMERICAN NATURALIST [Vol. XLIH 



ponent factors usually need to be again summed in order 

 to express the environment as a whole. For the great 

 general problems of plant geography it seems inadvisable 

 to attempt too extended an analysis, rather is it better to 

 seek methods of measurement which will furnish in- 

 tegrated evaluations of groups of environmental condi- 

 tions. With our present lack of knowledge, the pressing 

 of the analysis too far often results in such a complex of 

 data that an interpretation is impossible. A fairly satis- 

 factory integration of the main air factors seems to be fur- 

 nished by the atmometer; as to the soil factors, we have 

 made hardly a beginning in this direction. In the fol- 

 lowing paragraphs I shall first consider the measurement 

 of the air factors, denoting by this term all factors which 

 are active above the soil surface, whether or not the air 

 is actually involved. 



Atmospheric Factors.— Atmospheric pressure can be 

 easily measured by means of the barometer, curves are 

 automatically constructed by the barograph. Consider- 

 ing the perfection of this instrument, it is rather unfor- 

 tunate for ecology that the plant is so little influenced by 

 the natural variations in atmospheric pressure. 



Temperature is very important in plant activities, and 

 we have practically perfect instruments for its measure- 

 ment and for the construction of its curve. Unfortu- 

 nately we have as yet no well-tested method by which 

 temperature can be interpreted in regard to its effect. 

 A beginning which promises much has been made by Dr. 

 MacDougal with his integration of the thermograph 

 record and Professor Lloyd has told me of a new method 

 devised by him for interpreting maxima and minima. 

 Here lies one of the best fields for the scientific ecologist 

 with a mathematical turn of mind. 



Wind velocity can be measured and recorded by means 

 of the ordinary forms of anemometer, but the instruments 

 are not well suited to field work, largely on account of 

 their expense. Perhaps improvement may be forthcom- 

 ing along this line. 



