PHYSIOGRAPHY 8 1 



erosion, and sedimentation play a fundamental role in the change of habi- 

 tats. It is evident, however, that these, except where they affect the de- 

 struction of vegetation directly, can operate upon the plant only through 

 more direct factors, such as water, light, and temperature. While they are 

 themselves not susceptible of measurement, they can often be expressed 

 in terms of determinable factors, i. e., slope, exposure, and surface. Fun- 

 damentally, they constitute the forces which change one habitat into another, 

 and, in consequence, are really to be considered as the factors which pro- 

 duce succession. The static features of physiography, altitude, etc., lend 

 themselves readily to determination by means of precise instruments. These 

 factors, though by no means negligible, are remote, and consequently their 

 mere measurement is insufficient to indicate the nature or extent of their 

 influence upon the plant. It is necessary to determine also the manner 

 and degree in which they affect other factors, a task yet to be done. 

 Readings of altitude, slope, and exposure are so easily made that the stu- 

 dent must carefully avoid the tendency to let them stand at their own 

 value, which is slight. Instead, they should be made the starting point for 

 ascertaining the differences which they produce' in water-content, humidity, 

 wind, and temperature. 



Altitude 



119. Analysis into factors. Of all physiographic features, altitude is the 

 most difficult to resolve into simple factors. Because of general geographic 

 relations, it has a certain connection with rainfall, but this is vague and 

 inconstant. Obviously, in its influence upon, the plant, altitude is really 

 pressure, and in consequence its effect is exerted upon the climatic and 

 not the edaphic factors of the habitat. Theoretically, the decrease of air- 

 pressure in the increased altitude directly affects humidity, light, and tem- 

 perature. Actually, while there is unquestionably a decrease in the ab- 

 sorption of the light and heat rays owing to the fact that they traverse 

 less atmosphere, which is at the same time less dense, this seems to be 

 negligible. Photometric readings at elevations of 6,000 and 14,000 feet 

 have so far failed to show more than slight differences, which are alto- 

 gether too small to be efficient. The effect upon humidity is greater, but the 

 degree is uncertain. Continuous psychrographic records at different ele- 

 vations for a full season, at least, will be necessary to determine this, since 

 the psychrometric readings so far made, while referred to a base psychro- 

 graph, are too scattered to be conclusive. Finally, the length of the sea- 

 son, itself a composite, is directly dependent upon the altitude. This rela- 

 tion, though obscure, rests chiefly upon the rarefaction of the air which 

 prevents the accumulation of heat in both the soil and the air. 

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