ROCKWEED ASSOCIATION 67 
plants of the harbor, within definite and quite restricted tide-levels, though 
here, as with other forms, it will take further study to distinguish the exact 
factors determining distribution. 
CHLOROPHYCEZ. 
The only species of this group which are at all prominent on the wharves 
are Rhizoclonium tortuosum and R. riparium, except for Ulothria flacca, which 
is abundant in winter. The first two alge are more abundant and widely dis- 
tributed on the wharves than any other alge save Ascophyllum and Fucus (see 
plate viir). 
Rhizoclonium: The two species of this genus form a distinct green band 
above the upper margin of the rockweed zone, including some of the wall sur- 
face within the mid-littoral belt, and often extending some inches above this. 
The upper limit of Rhizocloniwm in more exposed places may be at 6.75 or 7 
feet, while in more protected places between stones of docks, on the north side 
of a wharf, or on projecting stones or piles, it may get up to 7.5 feet. In rare 
instances, when the places are very well protected, it may go even to 8 feet. 
The lower limit of this alga, where there are otherwise unoccupied spots on 
stones or piles, is at 3 or 3.5 feet. In those rare and small areas about the 
wharves where Fucus or Ascophyllum are nearly or entirely wanting over the 
whole vertical height of the wharf, the Rhizoclonwwm may form a distinct 
though thin green band on the wall 3 or 4 feet in vertical width. 
Any of the various kinds of rock in the walls seem to furnish a suitable 
substratum for the Rhizocloniums. The upper limit of distribution of Rhizo- 
clonium is apparently determined chiefly by the amount of desiccation which 
it can endure. ‘This is evidently dependent not only upon the level at which the 
plant grows, but also on the direction in which the wall faces and the presence 
of damp and shaded crevices between the stones of the wharf. The usual 
absence of Rhizoclonium below 6.5 or 6 feet is due to the presence there of its 
more vigorous competitors, Fucus and Ascophyllum. In those places where 
Fucus and Ascophyllum are absent and Rhizoclonium present, the lower limit 
of the green alga is perhaps determined by the long submergence of habitats 
below the 3-foot level. This may well be of considerable importance in a harbor 
where the water is usually decidedly turbid near the wharf. This water is 
relatively opaque to light, and thus would prevent, or diminish, photosynthetic 
activity in the Rhizoclontwm during some 6 or 7 hours each tide. We shall 
note facts later which indicate that these same factors are concerned in 
determining the limits of this alga on the beach and Marsh. In April 1911, 
Rhizoclonium seemed just about as abundant as in July, and with the same 
distribution. 
The other species of green alge found on the wharves, save the epiphytic 
Ulothriz flacca, occur either singly and widely scattered or in strictly local and 
usually small groups. We will take these up for brief discussion, in alphabet- 
ical sequence. 
Bryopsis plumosa: This alga has been seen but three times, and then it was 
on the east side of the Inlet at the 3-foot level. Only two or three tufts of it were 
found here, though it is scattered generally in the Outer Harbor, and scores of 
dense clumps of it are found each summer in a tide-stream entering the Outer 
Harbor 3 miles north of the Inlet. 
