ROCKWEED ASSOCIATION 65 
dance and in widespread distribution, though less noticeable because of their 
size. 
The two rockweeds together cover from two-thirds to nine-tenths of the sur- 
face of the walls of these wharves, from 8 to 12 inches above the bottom up to 
6.5 feet. The bottom of the harbor, near all of these wharves, except those by 
the Inlet and that of the Research Laboratory, is about 1.5 to 2 feet above mean 
low water. ‘The lowest rockweeds are attached at about the 2.5 or 3 foot level, 
and thus hang down at low tide until their tips lie on the mud, and so they 
actually cover the walls all the way down to the bottom. Hence, as one looks at 
the walls at low tide, he sees a brown band of rockweeds starting at the bottom 
and ending above with the thinned-out Fucus, quite constantly between 6.5 and 
7 feet above mean low water (plate xvi). 
While Fucus may occur throughout the whole width of this belt, Ascophyllum 
is usually confined to levels below the 5.5 or 6 foot levels. That is, the upper 
12 or 15 inches of the rockweed belt consists of Fucus alone, or of Fucus 
interspersed with a few plants of Ascophyllum, and with the relatively incon- 
spicuous Rhizoclonium and Bostrychia. In the lower 3 or 4 feet of the zone 
Ascophyllum makes up from 50 to 90 per cent of the conspicuous brown 
covering of the walls. Thus at 1,550 north on the east side, a vertical strip of 
the bottom and wall a foot in width showed the neighboring bottom to be well 
covered with Ulva. The portion of the wall between 1.5 and 5 feet bore 150 
plants of Ascophyllum, and above the 5-foot level there were 120 plants or 
plantlets of Fucus besides Rhizoclonium and a few tufts of Bostrychia. 
On some wharves a band of Rhizoclonium 0.5 to 0.7 foot high is found above 
the rockweeds. This band, which often includes some matted Lyngbyas and 
Calothrix, is frequently interrupted and not prominent when viewed from any 
great distance. 
In general then, the rockweed band is nearly continuous on the east side of 
the harbor from 500 to 2,800 north. (See plate xviir4.) On the south it 
extends from 0 to 370 east and on the west side it is found from 0 to 550 north, 
from 1,060 to 1,220 north, and from 2,090 to 2,240 north. This rockweed band 
differs little in general character on different wharves except that its lower 
boundary is higher on wharves where the neighboring bottom is high (as between 
200 and 300 east, at the south end of the harbor). The upper border may be 
unusually high on well-shaded walls. For example, in the southwest corner of 
the harbor from 0 to 20 south by 0 to 200 east the uppermost Fucus occasionally 
grows as high as 7 feet, or even slightly above this where attached to the under- 
sides of the stones and logs of the wharves or in the cracks between them. 
The density of the stand of these rockweeds on the wharves near the Biological 
Laboratory has been somewhat lessened by the removal of several boatloads 
each year to be used for the annual clambake at the Laboratory. The Mucus and 
Ascophyllum are pulled off, leaving only the holdfast and more or less of the 
basal part of the plant adhering to the wall. The renewal of the covering on 
the wall is due in large part to the development of new fronds from these 
stumps of the old plants. Such an annual cleaning off also, of course, gives 
sporelings a better chance to get started without being shaded out. 
Miss Streeter has discovered that a regeneration, similar to that mentioned 
above, occurs when the growing tips of either Ascophyllum or Fucus are eaten 
off by snails (Paludina), as they frequently are. 
5 
