-64- 
may approximately be measured by the oldest of these. 
After such periods of high water, pioneer species again 
come in, initiating a new course of succession. A patch- 
work results, which is especially evident in the shrubby 
vegetation. This patchwork is further modified by the 
varying ability of the different species to withstand the 
presence of excessive water about their roots. The 
shorter-lived herbaceous forms often show more definite 
zonation, especially along such restricted areas as mud 
flats or sandy beaches bordering the river or lake. They 
are able to adapt themselves more readily to changing con¬ 
ditions. 
In order to illustrate the tendency of the hydraroh 
successions in the valley, the associations found in the 
river, lake and flood plain, will be briefly described. 
a. Carp River (fig. 42;. 
Where this stream flov^s through the 
flood plain it is comparatively shallow with a muddy 
bottom. The slow-moving current makes the ecological 
conditions very similar to those found along the margins 
of many of the smaller lakes in the Upper Peninsula. Among 
the immersed aquatics may be mentioned, Potamogeton pusillus 
L., Potamogeton heterophyllus Schreb., Potamogeton zosteri - 
folius Schumacher, Potamogeton amplifolius Tuckerm., 
