) 1905] LIVINGSTON—RELATION OF SOILS TO VEGETATION 35 
L. To explain the distribution of the different types, either of two 
hypotheses may be resorted to. As has already been mentioned, 
bes the finer the particles of the soil, the greater its power to lift and hold 
Nie _ water above the underground level. It is well known, too, that some 
3 soils contain more of*certain salts than do others. Thus, the reason 
for the observed distribution on the uplands may be sought for 
either in the water-retaining power of the soil or in its chemical 
constituents.1° That the depth of the water table itself sometimes 
_ plays an important part in determining plant distribution is shown 
by the above table. Along a swamp margin the increased amount 
of water may influence the plant growth directly, but how much of 
the observed influence is to be considered as indirect is an open 
question. The presence of water alters a number of other soil 
factors. 
First, it checks free access of air. Thus, if jack pine roots need 
- more air than do those of Norway, this might explain why the former 
_ fails along the swamp borders and the latter takes its place. 
Secondly, with increase in water content there follows a more 
equal distribution of the dissolved salts, for these can diffuse only 
through continuous water films, and the greater the cross-section of 
\ the latter, the more rapidly will diffusion take place. As a corollary 
‘to this statement, it follows that ‘“‘leaching,” the washing down of 
| soluble salts out of the upper into the lower strata, cannot occur to 
/ any great extent in a soil which is constantly filled with water. More- 
‘ over the upward diffusion of salts during dry times would probably 
F more than counteract the downward washing during heavy rains. 
_. The upper layers of a wet soil are apt to have more soluble salts 
- \ after they have lain for a time than when first placed. This is of 
i course on account of the evaporation at the surface, which increases 
s [ the concentration of the soil solutions in the upper layers. Of course 
16 For A papers on this subject se 
THURMANN, J., Essai de ean appliquée 4 la chaine du Jura. Berne. 
849. 
NAGELI, C., Sitzungsber. Akad. Wiss. Miinchen. 1865. 
UNGER, Ueber den Einfluss des Bodens auf die Verteilung der Gewaichse. Wien. 
1836. 
\ more recent paper dealing with this penser of soil physics and soil chemistry 
,as influencing vegetation is Cow1Es, H. C., influence of underlying rocks on the 
,character of the vegetation. Bull. Amer. nr of Geog. 2:1-26. el 
i 
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