this indirect effect cannot be exhibited unless there is a sufficient 
amount of salts in the deeper lying soil. But in a glaciated region 
such as this, there can be little doubt as to the presence of these rela- 
tively near the surface. 
Thirdly, the checking of air access, coincidént with the filling of 
the pores of the soil with water, must check the process of oxidation 
and thus accelerate the formation of humus. This seems to be a 
very important factor in determining vegetation. 
Fourthly, the growth of micro-organisms in the soil, bacteria, 
etc., takes place much more rapidly in a moist than in a dry soil. 
However, excess of water is deleterious to the growth of many of these 
organisms, so that a soil may be too wet for them; but flooding is not 
so fatal in sand as in finer soils.*?__ It is well known that soil bacteria 
and mycorhizal forms are very important in increasing the amount 
of nitrates in the soil, and thus it appears that a moist soil, even a wet 
soil, if it be sandy, will gain nitrates much more rapidly than a dry one. 
The comparative rapidity of temperature changes in moist and 
dry soils may have considerable importance in regard to plant growth. 
A dry soil is subject to much more sudden and rapid changes of tem- 
perature than is a moist one when the soils are exactly alike, as I 
have shown by actual tests. 
The points of difference between a moist and a wet soil may be 
tabulated as follows: 
; 
36 BOTANICAL GAZETTE [JANUARY " 
b. 
Re nae 
ee Re ae % 
Condition of soil Dry | _* Moist Wet 
Water for rocts. Too little Enough for most More than needed for 
am : plants most plants 
Soluble salts originally near sur- Sometimes . 
face. f leached farther down Still near surface | Still near surface 
- Soluble salts originally in lower é —— 2S - - ae co oe 
layers Lo. cen jeac farther down artly in upper layers Partly upper layers : 
Ems. content... 5024 2S None — hoch Gs i ei * 
Pepa io Puenben uae stwe = Some —— 
Miao eee ae ae 
‘eperaare change. 22. : er i “Les raid Slow 
Whether these points are Paar by nearness of the under- 
ground water level or by the capillary power of the soil, there appears _ 
| to be no doubt that the amount of water in the layers near the sur 
- practically determines the nature of the vegetation in this 
sabi ee ae 
:) - Besides the general discussion of this matter to be found in 
¥ Gams, E. Action de Veau du sol sur Ia végtation. Rev. Gén. et a 
