120 BOTANICAL GAZETTE [AUGUST 
effect. With the decrease in the position of the water table, there 
is an increase in the oxygen supply, the number of soil organisms, 
and a rise in temperature. These produce a change in the compo- 
sition of the peat, making water more and more available for 
plants and making the habitat less and less xerophytic. In older 
parts where this process has been going on for years, there is a 
decrease in the volume of the peat and a lowering of the surface. 
In the areas studied around Ann Arbor, this seems to be the explana- 
Fic. 8.—Station under the tamaracks, showing ee almost entirely 
dead; the light value here was 0.033; photograph by STEE 
tion of the marginal ditch characteristic of bogs there. In other 
areas different reasons have been given, but they do not seem to 
explain the condition found in this region (16). During wet 
periods the width of this ditch is often greatly increased, as has 
been pointed out previously. In such an area only those plants 
can grow which can stand high water during a term of wet years 
and high water during the spring of every year. The vegetation 
which is found here is much the same as that found under these 
conditions along the borders of our streams. The peat found 
