8o THE HABITAT 



entirely upon mechanical analyses, and in some cases are too closely related 

 to be useful. The line between them can nowhere be sharply drawn. In- 

 deed, the variation within one class is so great that soils have frequently 

 been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per 

 cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely re- 

 lated to Oxnard sandy loam (26-37-18-12) and to Afton fine sandy loam 

 (28-43-18-8) than to Coral sand (61-29-3-4), Galveston sand (6-91- 

 i-i), or Salt Lake sand (84-1 5-1-0). Elsinore sandy loam (8-38- 

 35-10) is much nearer to Hanford fine sandy loam (9-36-33-14) than to 

 Billings sandy loam (1-60-22-11) or to Utuado sandy loam (48-23- 

 19-8). The soil types are much more confused, and for ecological pur- 

 poses at least are entirely valueless. Lake Charles fine sandy loam has 

 the composition, 1-34-52-9 ; Vernon fine sandy loam, 1-37-54-7? while 

 many other so-called types show nearly the same degree of identity. 



117. The chemical nature of soils. The effect of alkaline and acid sub- 

 stances in the soil upon water-content and the activities of the plant is 

 far from being well understood. It is generally recognized that salts and 

 acids tend to inhibit the absorptive power of the root-hairs. In the case 

 of saline soils, this inhibitive effect seems to be established, but the action 

 of acids in bogs and swamps is still an open question. It is probable that 

 the influence of organic acid has been overestimated, and that the curious 

 anomaly of a structural xerophyte in a swamp is to be explained by the 

 stability of the ancestral type and by the law of extremes. Apart from the 

 effect which excessive amounts of acids and salts may have in reducing 

 the chresard, the chemical character of the soil is powerless to produce 

 structural modification in the plant. Since Thurmann's researches there 

 has been no real support of the contention that the chemical properties of 

 the soil, not its physical nature, are the decisive factors in the distribution 

 and adaptation of plants. It is not sufficient that the vegetation of a 

 silicious soil dift'ers from that of a calcareous one. A soil can modify the 

 plants upon it only though its water-content, or the solutes it contains. 

 Hence, the chemical composition of the original rogk is immaterial, except 

 in so far as it modifies these two factors. Humus, moreover, while an 

 important factor in growth, has no formative influence beyond that which 

 it exerts through water-content. 



PHYSIOGRAPHY 



118. Factors. The physiographic factors of a definite habitat are altitude, 

 exposure, slope, and surface. In addition, topography is a general though 

 less tangible factor of regions, while the dynamic forces of weathering, 



