2? THE RELATION OF PLANTS TO TIDE-LEVELS 
of square yards of the bottom. In the more quiet areas the Hnteromorpha may 
form tangles of considerable size while remaining attached to the Zostera on 
which it germinated. The later history of these tangles of Hnteromorpha is 
similar to that of the sheets of Ulva. They are often floated up by gases accu- 
mulated in the cavity of the tubular thread. They then drift about the harbor, 
to lodge on some new part of the bottom and continue active growth, or they may 
settle on the beach or on top of Spartina glabra. On the south shore of the Spit 
masses of H. clathrata with some intermingled Ulva may crush down 75 or 100 
square yards of Spartina, in some cases smothering out the rhizomes also and 
leaving a bare strip. Such tangles as lodge on the beach or on the salt reed-grass 
die and then break up and wash away or settle down to form part of the soil 
among Spartina stalks. Smaller bits of the living alga may settle on the mud 
between the Spartina stalks, or among pebbles on the beach above the Spartina, 
and there take part in the formation of the composite mats or felts of which 
other green or blue-green alge form the major part. Of its distribution there 
we shall have something to say when discussing the other alge of these higher 
levels. Despite the fact that it floats more readily because of the gases within 
its filaments, the freedom cf movement of Hnteromorpha is on the whole less 
than that of Ulva. Because of its filamentous form Hnteromorpha becomes 
more readily entangled with other objects on the bottom, and it is also more 
often weighted down by young mussels which become attached to it in 
thousands. On the bottom from the 6-inch to the 1.5-foot levels, where 
Zostera and mussels are usually wanting, the tangles of Hnteromorpha are also 
rare. The usual absence of Hnteromorpha lower than about 3 feet below mean 
low water seems also determined by the absence below this level of organisms 
such as Zostera and mussels, with which it may become entangled or weighted 
down. It is true that small mats of this alga are sometimes seen below the 
limit of the Zostera in the deep hole at 1,400 north, but these are apparently all 
in transit to or from the Inlet. Younger plants attached to stones are found all 
along the Inlet from 3 feet downward, some being found even in the deep hole. 
These plants must form an additional though relatively unimportant supply of 
free plants which may grow into tangles like the more numerous plants starting 
on the Zostera. 
Zostera marina: This “ eel-grass,’” because of its abundance, gives character 
to large areas of the harbor bottom at low tide, and also forms an important 
substratum on which grow several species of epiphytic alge. For this reason 
the region covered by a dense growth of Zostera has been indicated by a wavy 
outline on the map showing the topography of the harbor (plates 1 and xmir). 
The area so indicated is not by any means evenly covered with Zostera. In fact, 
many areas within this boundary which are several yards across may have but 
the barest sprinkling of this plant. Moreover, as is indicated on the map, there 
are numerous scattered or clustered and usually small plants of Zostera outside 
of this boundary. 
In the more vigorous patches of Zostera found in the Inner Harbor in July, 
the individual plants often have a rhizome 0.33 meter long and from 2 to 3.5 
or 4 mm. in diameter. It is made up of 15 or 20 internodes, and runs along 
horizontally 1 cm. or more below the surface of the mud. Each rhizome 
terminates in a floral shoot, and bears from 2 or 3 to as many as 5 or 6 leafy 
lateral shoots. These lateral shoots may branch two or three times above the 
