1905] LIVINGSTON—RELATION OF SOILS TO \ °C’ ATION 33 
type of the forest. Very few determinatior: \..~ ‘cen made in this 
regard. Mayr" states for northern Wise. ns.) \i.t where the water 
level is less than 2.5°™ below the soil suriacc, ‘\« vegetation is of the 
swamp form; while if it is lower than est to. § “ the soil bears white 
pine or some other upland type. This » no sandy soil. Warm- 
ING’? has determined the depth of water eee in various soils in 
Denmark. He finds Juncus and Carex forms holding the ground 
until the water level is about 22.5°™ below the surface; with water 
at a depth of 30 to 37.5°™, grasses grow well, forming what we should 
term a moist meadow. With the water from 45 to 60°™ below the 
surface, all grains grow well; this seems to represent our fertile 
uplands. With the water still lower the soils becomes poor for 
grains. Data from natural vegetation, so far as I know, have not 
been gathered. 
The more water there is in a soil, the less is the access of air to 
the roots of plants growing therein. This is because air diffuses 
much more slowly when in aqueous solution than when in the form 
of a gas. Gas diffusion is checked by the filling of the interstices of 
the soil with water, and hence most of the oxygen which reaches 
roots in wet soil must do so by diffusing as a solute in the water. 
Some ordinary plants cannot grow without rather free access of oxygen 
to their roots, and so it follows that a soil saturated with water is 
very poorly adapted to their growth.ts This is probably the main 
reason why saturated soils are usually occupied by a vegetation 
differing entirely in aspect from that found on soils which are drier; 
thus we have swamp or lowland types of vegetation contrasted 
with upland types. Swamp plants are able to live with a scanty 
supply of air to their roots; but since upland plants cannot, it is 
possible to have too much water in the soil for the well-being of the 
latter. Thus areas with much water are occupied by typical swamp 
plants, often perhaps because they alone are able to live in ap 
situation. 
™ Mayr, H., Die Waldungen von Nordamerika. Miinchen. 1890. 
12 WARMIN' , Excursionen til Skagen i Juli 1896. Bot. Tid. 21: 59-112. 
figs. 4. 1897. : am A to G. H. Jensen for an abstract of the article. * 
13 WoLLNY, E., U. S. Dept. Agric. Exp. Sta. Record 4:528-543, 627-641. 1893. 
