84 THE RELATION OF PLANTS TO TIDE-LEVELS 
south of the harbor, where, as noted above, this species attains its best develop- 
ment. Here it grows densely on soil at the 8.5 or 9 foot level, and at one place 
it is found at 9.25 feet. On the sunny east shore at 1,000 and 1,040 north this 
rush forms rather dense patches near the 8-foot level, and ascends still higher 
near the inflowing fresh-water rivulets. 
In the preceding paragraph reference is made to the occurrence of this Scirpus 
near fresh-water streams, and to its complete absence from the Spit, which is 
devoid of fresh water. After studying the entire shore of the harbor, it is clear 
that Scurpus americanus is found only in soils where fresh water is present, 
either running over the surface or barely saturating the mud as the water seeps 
through from little springs in the underlying gravel. This latter seems to be 
the case, for example, on the west shore, near 800 north, 1,800 north, and 
1,900 north, where, though no fresh water is found running over the beach 
between the 7 and 8 foot levels when the tide is out, yet the soil is constantly 
saturated even after many hours exposure. Moreover, the water collected in 
holes dug in this soil is entirely fresh to the taste and, at low tide, fresh water 
is also found trickling out of the beach at the 3 and 4 foot levels, directly below 
these Scirpus areas. On the Marsh also this Scirpus often grows in spots where 
no fresh water is visible on the surface. The soil-water, however, proves to be 
fresh when its specific gravity is taken. This is true, for example, of the large 
area between 400 and 500 south and 900 and 1,100 east, which is dominated by 
S. americanus. The soil-water here is practically fresh during the growing 
season, except just after the very high storm-tides mentioned above. 
In brief summary of the conditions under which Scirpus americanus grows, 
we find that it is a plant of sunny situations. It is absent from shaded soil on 
the west side, though this be wet. In fact, the Spartina glabra from the belt 
below has been found to replace this rush in such wet, shady spots (see p. 45). 
Scirpus grows chiefly on soil between the 6-foot and 8-foot tide levels, except 
on the Marsh and at one or two spots on the east side, where it may reach the 
9-foot level in wet, sunny places. This gives the plants an exposure of 15 or 
16 hours per day in the case of the lower ones and of 24 hours per day for the 
higher ones, except on the 5 or 6 days of each month when high spring or storm 
tides occur. The submergence varies from 3 to 10 hours per day. 
The critical factor determining the lower limit of this Scirpus is probably 
the high salt-content of the soil. It never gets far below the 7-foot level, 
except where fresh water is abundant enough to wash the salt out of the soil. 
It is, of course, possible that this fresh water really acts indirectly, by preventing 
the growth of its competitors in these soils (e. g., of Spartina glabra). It is 
conceivable, for example, that this Scirpus might occupy any soil between the 
5.5 and 8 foot levels, if only its competitors are kept out. This has not as yet 
been experimentally proven. The upper limit of distribution is probably 
determined by shade, or by the lack of sufficient soil-water on some parts of the 
beach. Tlsewhere, on the contrary, even in wet soils, its upper mit seems 
fixed by the competition of other species which can not follow the Scirpus down — 
to levels that are flooded by salt water, but can successfully compete with it when 
they have not this adverse condition to meet. 
Scirpus robustus on the upper littoral beach: This large, more leafy rush 
resembles S. americanus in its distribution, but is less abundant and less widely 
* Tests made at a few points show that this species can grow in soil water with a 
specific gravity of 1.006 or even of 1.017. 
