ALG OF MID-LITTORAL MARSH 59 
curly mats and tangles from the 2-foot level (e. g., 35 south by 575 east) upward, 
to and beyond the upper limit of this zone, as, e. g., along the west shore, where 
it reaches to 5, 6, or even 7 feet. These tangles include, beside the Rhizoclonium, 
smaller amounts of each of a few species of green alge and of many species of 
blue-green alge, all of which we have referred to above. The tangles are some- 
times twisted more or less tightly about the stalks of Spartina glabra and 
sometimes form rather closer mats on the mud, at the base of the Spartina 
stalks, or in tide-pools from which Spartina is absent. In still other places, 
e. g., along the steep eastern bank of the Creek at 200 south, between the 
3 and 5 foot levels, these alge occur either in an almost pure growth, or 
mingled with Hnteromorpha crinita, and are woven into almost continuous 
sheets, sometimes 2 or 3 dm. wide, a millimeter or two in thickness, and often 
a meter or two in length. These algal curtains probably start as a coating 
over the surface of the vertical or overhanging bank, which is formed by the 
caving off of the peaty mud of the high Marsh east of the stream. The filmy 
covering formed at first in contact with the mud apparently peels off below and 
hangs vertically, supported by the adherent upper edge. Growth may still con- 
tinue in these pendant sheets, which are immersed from 4.33 to 6.5 hours each 
tide, and are kept moist during low tide by the water dripping from the bank 
above. 
A very marked peculiarity of this alga is its evident inability to endure long 
immersion in fresh water, although it exists in brackish water. Along each 
fresh-water rivulet the lower edge of the mat of Rhizoclonium remains a few 
centimeters above the level of the fresh water at low tide. The same thing is 
shown by its absence from those areas of the main stream which are covered 
by fresh water at low tide, though it does occur on the edges of the tide-pool at 
35 south by 575 east at 2 feet. This pool has 6 inches of brackish water in it 
at low tide. 
In April 1911, Rhizoclontum seemed to be about as abundant and in the 
same places as in July and September 1911. This seems to show clearly that 
Rhizoclonium is a perennial alga. It is the only monosiphonous cellular form 
on shore and wharf in the summer, but in April another somewhat similar 
alga (Ulothria flacca) was abundant on wharves and on branches of the 
Ascophyllum, Fucus, and Pylaiella that are attached to the wharves. 
Ulva lactuca latissima, as we have seen, is most characteristic of areas below 
1.5 feet, that is, of the harbor bottom. It occurs, though much less abundantly, 
in the present belt also. A few plants grow on wharves or stakes, but it is 
found chiefly along streams or in tide-pools, where it is not dried out at low 
tide. Up to the 3 or 4 foot level, considerable numbers of detached plants are 
found in tide-pools. In the pool at 35 south by 575 east it occurs at the 2-foot 
level, in water that is brackish at low tide. At 2,800 north by 650 east, at the 
4-foot level, is a tide-pool with hundreds of detached plants of Ulva, which are 
but a few decimeters broad, very much curled and perforated by numerous 
roundish holes a centimeter or more across. This type, which somewhat resem- 
bles var. mesenteriformia of Collins, occurs in a few other places where the water 
becomes much heated by the sun during low tide. It is found, e. g., in small 
pools along the tide-stream through the Spartina at the east end of the Spit near 
800 east, and less-developed examples of this variety occur along the northern 
border of the Spartina zone near 0 to 150 north by 400 to 500 east. The 
