68 THE RELATION OF PLANTS TO TIDE-LEVELS 
Monostroma latissimum: This species is found on the wharves only at 
the relatively few points where small streams of fresh water trickle over or 
through the wall of the wharf. Just below the artesian-well outlet at 1,435 
north on the east shore, the wall is covered pretty thickly for a width of 0.5 
meter or more between the 5-foot and 2-foot tide-levels, with plants of Mono- 
stroma which are 4 or 5 cm. long. These plants are submerged in salt water 
from 5 to 8 hours each tide, and exposed to the dripping fresh water or its 
spray for the rest of each tide, about 8 to 5 hours. Another fresh-water outlet 
at 1,010 north on the east side has, in most summers, a colony of Monostroma, 
both on the rocks of the wall and on the pebbles of the stream-bed at the 7 or 
7.5 foot levels. Finally, Monostroma occurs on the brick-work about the fresh- 
water ram at 2,380 north by 990 west, from the 7-foot down to the 4-foot level. 
The factors influencing the distribution of Monostroma have been discussed in 
speaking of the mid-littoral beach or shore. 
Ulothriz: Of this genus U. (tmplexa Kutz?) has been found two or three 
times each summer in the neighborhood of fresh water, e. g., between the 6 and 
7% foot levels on piles under the mill at 500 north by 1,000 east, which are wet by 
spray from the mill-wheel. It is also found at the 7-foot level on pebbles in 
the fresh-water stream at 1,010 north on the east side. 
Ulothria flacca is a form which has not been found in the summer, but which 
was very abundant as an epiphyte and less frequent on logs and piles in April 
1911. The short, simple filaments of this alga at that time formed thin, 
grayish-green turfs over the woodwork of the wharves about the harbor. It has 
never been seen in these locations, or elsewhere in the harbor, in July or August. 
As an epiphyte on Ascophyllum or Fucus it forms a greenish coating of short 
filaments over that surface of the flat frond which is usually exposed to the 
light as the rockweed hangs down beside the wall at low tide. What is appar- 
enly the same alga also forms long, lubricous tufts on the fertile clubs of both 
-rockweeds in April. It was also frequent at this season as an epiphyte on 
Pylaella, which flourished in great numbers along the Inlet. The absence of 
this epiphytic Ulothria from the rockweeds in summer seems probably due to its 
inability to withstand the more severe desiccation during low tide at this season, 
though this has not been proven experimentally. 
Ulva lactuca: This is the only green alga of the wharves remaining for us to 
discuss. This species we have found to be abundant on the harbor bottom, and 
frequent also on the mid-littoral marsh, up to the 4-foot level, but it occurs only 
infrequently on the wharves. A few small plants have been found scattered 
about the wharf of the Research Laboratory, and still fewer on other wharves. 
Kven in April 1911, when young plants of Ulva were very numerous in the Inlet, 
they seemed to be no more frequent on the wharves. From observations thus 
far made it is impossible to decide whether the scarcity of Ulva-on the wharves 
is due chiefly to the competition of the rockweeds or is directly due to the ex- 
posure to desiccation at these levels. In view of the slow growth of the rock- 
weeds and rapid growth of the Ulva, it seems probable that the latter might, at 
least temporarily, occupy the spots bared of rockweed by the ice, were it not for 
the long exposure to desiccation it would experience on most of these walls. 
