LITHOPHILOUS BENTHOS 29 
main stream, at 150 south by 780 east. Just upstream from this depression 
(160 south), the pebbly bottom slopes sharply down from a 1.5-foot level to 
—0.5 foot in the deepest parts of the depression, and then rises to about 
1 foot at the northern end of the hollow. In the bottom of the deepest part of 
this depression, and still more abundantly in the little rapids above it, the 
Ectocarpus grew in luxuriant tufts, with numerous gametangia, in the early 
summer of 1911. In late August of this same year it had practically disap- 
peared. The water flowing over this alga during 3 to 5 hours of each tide, or 
6 to 10 hours per day, is entirely fresh, while for the remainder of the tide the 
plants are surrounded by salt water. Moreover, the change from one to the 
other is quite rapid, which shows that the alga is capable of withstanding 
marked and sudden changes in the osmotic quality of the surrounding medium. 
Scytosiphon lomentarius: This alga occurs in the Outer Harbor all summer 
and unattached small tangles of it are found scattered over the bottom of the 
Inner Harbor at this same season. Only in early April 1911 did we find it 
attached in the Inner Harbor. At that time it was sprinkled in frequently 
among Ulva, Enteromorpha clathrata, Porphyra, and Pylaiella, on the stony 
bottom east of the channel of the Inlet, at 1,900 to 2,100 north and from mean 
low water up to the 1-foot level. These plants were 30 cm. long, about 1 mm. 
in diameter, and were fruiting. 
Pylaella littoralis: This densely branched filamentous brown alga has been 
found nearly every summer at one or more spots about the harbor. It is 
usually present, for example, in the deeper part of the Creek at 150 south at 
—0.5 to +1.5-foot levels. It was abundant in the Inlet in July 1912, but had 
largely disappeared by September. It is sometimes found also on the shady 
sides of piles or stones on the wharf of the Research Laboratory, chiefly, though 
not wholly, above 1.5 feet. In 1912 large tufts grew where splashed by fresh 
water at 150 north and 500 north on the west shore. In April 1911 this alga was 
probably the most abundant species about the harbor from mean low water 
up to 3 or 4 feet. At half tide its dense tufts could be seen everywhere, often 
as much as 1.5 dm. long. They were attached to pebbles, shells, stones, wood, 
and even tangled among the stalks of the Spartina glabra at its lower levels. 
All of these plants were either sterile or had chains of zoosporangia. On 
September 29, 1911, this same alga was found on pebbles in the deep part 
of the main stream at 150 south and in the rapids just above this at 200 south. 
In the plants collected at this time the only reproductive organs seen were 
gametangia, which had not been seen at all on plants collected in April or in 
midsummer. 
From what we have noted it is clear that Pylaiella, like Hctocarpus stliculosus, 
is capable of enduring the rapid change from salt to fresh water and the reverse 
which occurs in the main stream with each change of tide. It is also noteworthy 
that while this alga is very abundant and widely distributed about the harbor 
in April, it is represented in summer by only a few groups of plants in areas 
protected from high temperature and desiccation. The fact that the plants 
found in spring and early summer bore zoosporangia only, while those found in 
September bore only gametangia, suggests the probability that this alga really 
has a distinct seasonal alternation of a spore-bearing period with a gamete- 
bearing period. It may even prove to be an alternation of distinct asexual 
