58 THE RELATION OF PLANTS TO TIDE-LEVELS 
Monostroma latissimum is a third green alga which is associated very con- 
stantly with fresh-water inlets about this harbor. This alga forms small sheets 
from a few millimeters to 5 or 6 cm. broad attached to pebbles of the bottom in 
most of the fresh-water streams about the shore, to rocks of the wharves below 
the fresh-water inlets, and more rarely to the stalks of Spartina glabra growing 
near fresh water. In the Creek the upper limit of Monostroma is found at 560 | 
south by 820 east, at about the 6-foot level. It occurs here in the more quiet 
nooks along the banks, just out of the swiftest currents. The upper limit of 
its range is also at 6 feet in the small, very cold stream at 500 south by 860 
east. A little farther north in the Creek it grows, across its whole width, on 
pebbles at the 4-foot level. From this point northward the Monostroma grows 
in midstream on shelly or pebbly bottom, down to the 1.5 or 1 foot level, at 140 
south. In some of the small streams of the west shore, with colder water and 
more shade, this alga is found as high as the 7.5-foot level. The densest growth 
of Monostroma found is probably that near the overflow pipe of an artesian well, 
which penetrates the wall of the wharf at 1,440 north by 1,080 east, at the 5-foot 
level (plate vi11). This wall from 2 to 4.5 feet, where wet by the dripping or 
the splashing of the water, is covered by hundreds of the delicate sheets of this 
alga mixed with smaller numbers of Hnteromorpha intestinalis. Similar though 
less dense growths of this alga are found at other points where fresh water flows 
over or through the wharf (1,025 north by 1,060 east and 2,075 north by 
1,140 east). 
A reference to the tide-chart (plate v1) will show that at the lowest limit men- 
tioned above (1 to 1.5 feet), this alga will be submerged in salt water for 9 hours 
or less, according to the magnitude of the tide, and in fresh water for 3 hours or 
more. At the extreme upper limit (7.5 feet), on the contrary, it is evident that 
this alga is exposed to fresh water for several days continuously during each 
series of neap tides. 
The chief competitors for standing room on substrata suitable for Mono- 
stroma are evidently Enteromorpha intestinalis and Ilea fulvescens. The former 
is abundant only near the lower limit of the Monostroma where the latter is 
sparse. ‘The Jlea, on the contrary, is densest at just about the same levels in 
the stream, and under the same conditions of salinity as the Monostroma. In 
this region of approximation of these two species the rocks in the swiftest cur- 
rent are occupied by Jlea, for which habitat its filamentous form and lubricous 
surface especially fit it, while Monostroma, with its broad and relatively deli- 
cate sheets, lives in less turbulent water. The two species are actually found 
together in considerable numbers, chiefly in regions of moderately swift cur- 
rent. Shade, from plants along the banks of the stream, is apparently a factor 
less endurable by Jlea than by Monostroma. 
The propagation of both species is evidently accomplished by zoospores, 
which must be capable of enduring immersion for some hours in salt water, 
while being transported about the harbor. This seems necessarily true, since 
Monostroma was practically wanting in April 1911, and yet had spread to most 
of the fresh-water rivulets in July. In September 1911 Monostroma was — 
present in about the usual numbers in streams about the harbor and a few 
plants were found on the wharf of the Research Laboratory. 
Rhizoclonium is a genus represented in the mid-littoral belt by two species, 
R. ripartum and R. tortwosum, which occur, usually together, in somewhat 
