14 THE RELATION OF PLANTS TO TIDE-LEVELS 
sand and shell fragments which are shifted about by the swift current at each 
ebb and flow of the tide. The only plants discovered here are those of Ulva, 
Polysiphonia, Agardhiella, etc., which are drifting along and dragging with 
them the pebbles or shells to which they are attached. To the south and the 
east of this deep hole there is a considerable area of bottom below the 1-foot level 
that is covered with a decimeter or more of mud overlying the gravel and which 
is occupied pretty completely by Zostera. (See plate 1.) To the west of this 
depression lies the tide-channel that starts at the Research Laboratory. This 
channel has a bottom somewhat below —1 foot, with a mud bottom, which also 
bears more or less scattered Zostera. 
From the —1 foot level up to the lower margin of the Spartina at 1.5 feet 
practically the whole bottom is of soft brown or black mud. The only exceptions 
to this are the gravelly east shore of the Inlet, from 1,600 to 2,400 north, and the 
bed of the Creek, from 100 south to 400 or 500 north. The depth of the mud 
over the gravelly bottom which underlies the whole Inner Harbor, varies from 
a decimeter or two up to 1.5 or even 2 meters. The only plants really growing 
on or in this mud, aside from the diatoms coating certain areas, are Zostera, 
which gets above mean low water near 600 north by 500 east, and Ruppia, 
which ranges from mean low water up to +1 foot. The alge found growing here 
on scattered shells, pebbles, or sunken stakes, or on the living mussels, are 
really rendered thus quite independent of the nature of the bottom. The same 
thing is true of the floating tangles of Hnteromorpha and sheets of Ulwa. 
The character of the bottom from +1.5 feet up to about 6.5 feet is pretty 
constant about the whole harbor, except where changed by entering streams or 
artificially modified. These levels of the shore consist of a fine-grained, peat- 
like mud that is more or less firmly bound together by the living and dead 
rhizomes and roots of Spartina glabra, which forms a nearly continuous belt 
on all natural shores at these levels. The distribution of the Spartina, shown 
in plate 1, indicates that of this type of bottom. Only on recently formed 
gravelly shoals (e. g., near 200 north by 600 east) or at points where smaller 
entering streams have cut away this peat down to the underlying gravel, or 
where bathing beaches have been constructed, is this type of bottom wanting 
about the whole harbor. Along the south shore of the Spit from 800 west to 
400 east, it is true, as was noted earlier in the paper, that this peaty bottom does 
not reach quite down to the 1.5-foot level. The depth of this peat, which usually 
tapers out to nothing between the 6-foot and 7-foot levels, may be as much as 
from 3 to 7 dm. in the lower half of the Spartina belt. A series of soundings 
with an iron rod, along a north-and-south line at 10 west, showed a thickness of 
this layer which at first increased and then decreased in going shoreward from 
the 2-foot level, in the way indicated in plate v. Essentially the same thickness 
of peat covers the gravelly bottom on the east and west shores, as is shown by 
the actual sections of the peat cut by the rivulets entering the harbor over the 
upper beach (e. g., that at 1,650 north by 800 west). 
This layer of peat or peaty mud on which the Spartina flourishes is not of the 
same consistency throughout its thickness. The upper 2 or 3 dm. are firm and 
fibrous, while the portion below this is far more liquid, so that the upper layer 
shakes or quakes with the stamp of the foot. The living rhizomes never 
penetrate far into this less-solid lower layer, which is but little more firm than 
