96 THE RELATION OF PLANTS TO TIDE-LEVELS 
Enteromorpha clathrata is another alga which pushes up beyond the 6.5- 
foot level. Fragments of this species in good living condition are found matted 
in the looser felts of blue-greens and Rhizocloniums along the west shore, up 
as far as the 7-foot level. This alga has not been recorded growing at levels 
above 6.5 feet on the Marsh and Spit, though tangled masses of it are often 
thrown up by high tides. It seems probable that this species and the asso- 
ciated alge are able to persist at higher levels on this west shore than elsewhere, 
because of the greater dampness of the soil, due not only to the abundance of 
fresh water, but also to the shade from high trees which protect this shore from 
the drying effects of the sun during low tides occurring in the afternoon. 
Enteromorpha intestinalis is a species which is absent from the Spit, but 
which pushes up into the present belt on the other three sides of the harbor 
wherever there is a fresh-water rivulet for it to grow in. The upper limit of 
this alga is pretty constant at 7 feet, the uppermost individuals being relatively 
small plants of 6 or 8 cm. in height. It is an interesting fact that this species 
does not push up to a higher level along the shady western shore. This fact 
seems to confirm the suggestion offered above, that this species, which actually 
grows in the fresh-water rivulets, nevertheless needs contact with salt water 
for an hour or two each tide. Its ascent to high levels does not seem to be 
prevented by any inability to endure long exposure to the air at low tide. Any 
plants of this species that might locate above the 7-foot level would not be wet 
by salt water at all for several tides during each series of neap tides, since these 
minimum tides barely reach the 7-foot level. 
Ilea fulvescens, as we have seen, is a characteristic form in certain of the 
larger fresh-water streams, where its upper limit is in one or two cases as high 
as 7 feet (1,020 north by 470 west) (plate virr). It is very interesting to find 
what is evidently the same species, in active condition, matted with other species 
on the south shore of the Spit at 6.5 to 7 feet (680 east). It is hard to see what 
conditions present at this high level on the Spit can make life possible for an 
alga that is elsewhere accustomed to constant submergence, and, in fact, for 
most of each tide to submergence in fresh water. 
Monostroma latissimum is still another alga associated, as we have seen, 
with fresh-water inlets. Where these are present it often pushes up to the 
7 or 7.5 foot level, as in the Creek south of the harbor (plate vi1r) and in 
several larger rivulets along the west shore. It seems more tolerant of prolonged 
submergence in fresh water than Hnteromorpha intestinalis. 
The two species of Rhizoclonium, R. riparium and R. tortuosum, which we 
have found so generally distributed in the next lower belt, on wharves, and 
among the Spartina glabra, are also the most abundant and widespread green 
alge in the present belt. Not only are they widely distributed horizontally 
about the harbor, but they push far up the beach, often to the 7.5-foot level. 
In the shade of seed plants on the beach, or among the stones of the wharves, 
they may occasionally get up to the 8-foot level, or slightly above. In the lower 
parts of this belt the Rhizocloniums are scattered through the firm mats of . 
Lyngbya, Microcoleus, etc. At the higher levels they are found in sparse webs, 
with sometimes a slight admixture of Cladophora, clinging about the bases of 
the seed plants that shade them. The uppermost threads of Rhizoclonium seen 
were associated with a Rivularia about the bases of Iris versicolor at the 8-foot 
level (480 north by 1,070 west), or were growing among the Spartina patens at 
