60 THE RELATION OF PLANTS TO TIDE-LEVELS 
one other habitat in which Ulva occurs in this mid-littoral belt is along 
the edges of fresh-water streams and rivulets. Here the plants are attached 
to pebbles, shells, or stems of Spartina. For example, in the Creek the 
southern limit of Ulva is between 100 and 200 south. Here it occurs along the 
stream-edges at the 1-foot level, just above the surface at low tide, where the 
water is quite fresh. Northward from this point the alga becomes more and 
more abundant up to 200 north, where there is a dense growth of Ulva, especially 
on the east side of the channel. Only a few plants occur here, even in the 
channel where constantly submerged. In the small rivulets on the east and 
west sides of the harbor Ulva is found up to the 3.5 or 4 foot level, on immersed 
pebbles or on stalks of the neighboring Spartina where shaded by this grass. 
The Ulva is confined to those parts of the mid-littoral belt where the substratum, 
or perhaps the surrounding air, is unusually moist, and thus prevents the drying 
out of the plant at low tide. This view seems confirmed by the fact that in 
April 1911 numerous small plants of Ulva were found attached to the dead 
stubble of the Spartina all along the west shore, where, except near the rivulets, 
Ulva was wanting entirely in July, August, and September 1911. 
Vaucheria (thuretu?) is the last species of green alga to be mentioned as 
occurring on the shores of the mid-littoral belt. It is found either in nearly 
pure tufts or as a constituent of the compound felts referred to above, which are 
made up primarily of Rhizoclonwum, Enteromorpha clathrata, Cladophora, and 
Lyngbya estuaru. These composite felts including Vaucheria are found quite 
generally on all four sides of the harbor between the 5 and 7.5 foot levels. The 
Vaucherva is usually found in felts that are more or less protected from desicca- 
tion by shade or by unusual wetness of the soil. The pure tufts are most abun- 
dant on the south and west sides and have not been seen at all on the Spit. The 
only considerable patches seen are near fresh-water inlets, though never so near 
as to be submerged in fresh water for more than an hour or so at each tide. Good 
. examples of these turfs are the dark-green ones, a meter or more square, found 
on the banks of the Creek, at 300 south by 790 east at the 4-foot level; at 400 
south by 800 east at 5 to 6 feet, and that near 500 south by 750 east at 6 feet. 
A seeming exception to this constant association of the Vaucheria turfs with 
fresh water is found at 1,714 north on the west shore at 6 to 6.5 feet. But 
though no fresh water is running over the surface at this point, it 1s seeping 
through the soil in quantity sufficient to make the soil-water here at low tide 
decidedly less saline than the water of the Spit beach or the water of the harbor. 
In speaking of the occurrence of Lilwopsis at this point, we have already men- 
tioned the fact that small trickles of fresh water come out of the beach between 
the 4 and 5 foot levels. 
The turfs of Vaucheria are also found in wet places in the upper littoral 
levels of the Marsh south of the harbor, in wet areas near tide-pools, or where 
the water is apparently coming up from below. 
From the observations above given it seems evident that the presence of an 
unusual amount of water in the soil, even though it be fresh water, serves to 
protect the overlying Vaucheria from drying out. In the composite felt on the 
drier beaches, the other more resistant alge probably serve to protect the more 
delicate Vaucherta, and thus enable it to live higher up the beach than it could 
otherwise do. This same sort of protection is sometimes given to the Vaucheria 
at higher levels by the Spartina patens, as from 150 to 250 south and between 
