56 THE RELATION OF PLANTS TO TIDE-LEVELS 
at the 5-foot level, chiefly aside from the deepest parts of the channel. It is 
most abundant in rapids of this stream near 300 south at the 3-foot level, where 
it occurs along with Monostroma and living barnacles. These facts seem to 
indicate that the movement and aeration of the water are of advantage to the 
Enteromorpha or of disadvantage to its competitors, and that it can endure 
long immersion in fresh water. The densest stands of H. intestinalis found 
were those of the rapids of the Creek and those about the outlets of artesian 
wells on the east side (1,440 north and 1,535 north). We have already men- 
tioned the Hnteromorpha growing on the bottom near these well outlets. It is 
also found less abundantly on the wall of the wharf below these outlets, from 
the 1.5 to the 3-foot level. Here it is mixed with a Monostroma that crowds out 
the Hnteromorpha from the part of the wall immediately below the outlets, 
which is actually flooded and not merely splashed with fresh water. 
Another interesting type of habitat for H. intestinalis is found in the fresh- 
water rivulets that run down across the shore from the high-water mark (plate 
vil). There are many of these on the west side of the harbor and several on the 
east side. The most interesting one on the latter side is the rivulet on top of the 
wharf at 1,000 north. The fresh water comes down through a ditch from a spring 
at some distance above the shore-line, and finally escapes between the big stones 
of the wall at the 5-foot level. In this stream H. intestinalis is found not 
merely among the Spartina on the bottom at 1.5 feet, but on the stones of the 
wall where washed by fresh water and also on the pebbles in the stream on top 
of the wharf, where at the 7-foot or 7.5-foot level, this alga is associated with 
Scirpus americanus, a characteristic plant of the upper littoral belt. This alga 
grows on pebbles and stones in the bottoms of streams on the west shore, where 
it is flooded with fresh water at low tide (1,050 north, 1,400 north, etc.). It 
often occurs here from the 7-foot level downward across the whole width of the 
mid-littoral belt. 
On the banks of the little streams which are at all exposed to drying out at 
. low tide, other alge take the place of this Hnteromorpha. In the pool below 
the flume of the mill at 500 north on the east side H. intestinalis grew at the 
5-foot level where immersed in pure fresh water for 8 hours or more at each 
tide. Even at high tide the flow of fresh water was abundant enough probably 
to prevent the pool from ever becoming really salt. In this pool Ascophyllum 
was the only alga growing along with the Hnteromorpha. In the fresh-water 
rivulets of the east and west sides of the harbor we find with Hnteromorpha the 
red blotches of Htldenbrandia and several green alge which form incrustations 
on the submerged pebbles. The association of EH. intestinalis with fresh water 
is so characteristic about this harbor that one not only expects it wherever fresh 
water is found, but suspects the presence of fresh water wherever the alga occurs. 
Thus at 1,850 north, between the 2 and 5 foot levels, #. intestinalis occurs in a 
rivulet of water starting from about the 5.5-foot level. This water proved to be 
really fresh, and not merely salt water draining out of the marshy shore, as was 
at first supposed. | 
In all of those cases mentioned above where the Hnteromorpha grows at or 
above the 7-foot level, it is evident that the alga must in the average tide be 
immersed in fresh water for 10 or 11 hours at each tide. Moreover, during 
each series of neap tides these plants may not be wet by salt water for several 
days. These facts, together with the fact that certain species of Hnteromorpha 
