IV. FACTORS INFLUENCING THE DISTRIBUTION OF 
LITTORAL PLANTS. 
The factors which most directly condition the distribution of littoral plants 
in this harbor are: the character of the substratum, water-currents, tidal changes 
in level, salinity of the water, and (probably) the temperature of the water. 
We will consider these in the order mentioned. 
1. SUBSTRATA. 
The substrata supporting plants of the shore and harbor bottom may be: 
(A) living plants or animals; (B) non-living substrata. 
A. LIVING SUBSTRATA (PLANTS OR ANIMALS). 
The only animal of great importance in serving as an attaching place for 
plants is the abundant black mussel, Mytilus edulis. This has become increas- 
ingly abundant in recent years and now nearly covers the bottom over some 
acres, in the region between 1,600 and 2,000 north by 200 to 1,000 east, and the 
region between 1,200 and 1,800 north by 200 and 400 west. As we stated in 
Chapter III, the young mussels become attached to the large sheets of Ulva by 
thousands, so as to make the bottom covered by the Ulva and its burden look 
black. The erectly standing mussels serve to catch the silt and organic débris 
that is drifting along near the bottom and thus the mussels become partially 
buried. As the shells of the mussels become larger, firmer, and rougher, the 
spores of Ulva and Hnteromorpha clathrata, settling upon them, give rise to 
numberless young sheets or threads of these alge, which wave back and forth 
above the upturned edges of the shells of this mollusk. It thus, in turn, becomes 
a substratum for more plants of the species on which the mussel itself first 
settled. The Ulva and EHnteromorpha then serve to still further retard the 
movement of the water near the bottom and thus increase the rate of silting up, 
until large areas may thus become covered with fine black mud to a depth of 
2 feet or more. 
Other animal substrata supporting plants are the oyster Ostrea virginica, 
and the various gasteropod shells inhabited by hermit crabs. The oysters of the 
Inlet, for example, often bear plants of A gardhiella, Polysiphonia, or Ceramium 
in addition to the green alge found on the mussels. 
The plants that are most important in serving as substrata for other plants 
are Spartina glabra, Zostera, and Ulva. 
Most of the species found on Spartina, like the Lyngbyas, Microcoleus, 
Rhizoclonwum, and Vaucheria, are seldom attached by definite holdfasts, but 
simply tangled about each other over the stalks of this grass in mats. Rarely a 
few small plants of Ulva or Enteromorpha are found actually attached by 
holdfasts. 
Zostera really bears a more definitely specialized epiphytic flora than any 
other plant serving as a substratum. Any of the Zostera below mean low water, 
especially that subjected to the swifter tidal currents, may bear epiphytic tufts 
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