SUBSTRATA LAT 
of green and blue-green alge. Since, however, this Spartina grows luxuriantly 
in this neighborhood in nearly pure sand, it seems more probable that the Spar- 
tina determines the character of the soil rather than that the latter conditions 
the occurrence of the grass at these levels in our harbor, that is, the thick stand 
of salt reed-grass, between the tidal limits endured by it, favors the deposit of 
organic as well as inorganic sediment on the surface and also adds considerable 
organic material by the decay of its own roots and rhizomes within the soil. 
(3) In the case of the higher levels of the Marsh there seems to be a definite 
dependence of the character of the plant-covering on the salinity of the soil- 
water and on the depth of the peat-like layer of top soil. (See fig. 3.) While 
Spartina patens may grow from the 7.5-foot to the 8-foot level, in peat with 
soil-water of a salinity or specific gravity of 1.017+- at this same level, and in soil 
of otherwise the same character, except that the soil-water has a shghtly lower 
specific gravity, the plant-covering consists primarily of Distichlis. If near the 
higher level mentioned the salinity of the soil-water gets below 1.006, these two 
grasses are often replaced by Scirpus americanus. (4) The soil differences 
of most importance in their effect on plant distribution in the two belts between 
the 6.5-foot and the 12-foot levels on the north, east, and west sides of the Harbor 
are those in salinity of soil-water, and, especially on the Spit, differences in the 
amount of humus in the soil. 
2. SoLIp SUBSTRATA (STONES, PEBBLES, SHELLS, PILES, AND Logs). 
The most important of the solid substrata in our harbor are the pebbles of 
the natural bottom of the Inlet and the stone walls and wooden piles and 
docklogs of the wharves. The scattered stakes and the shells and occasional 
stones of the bottom are far less important as plant substrata, the one exception 
to this latter statement being the shells of living mussels mentioned earlier, with 
their hundreds or thousands of young plants of Ulva and Cladophora. 
The pebbles of the Inlet consist of well-rounded bits of quartz, granite, gneiss, 
sandstone, or conglomerate, of all sizes up to 1 or 2 dm. in diameter (plate 
xviIr). No adequate evidence was obtained of a marked preference of any of the 
algee for one material among these pebbles rather than another. The Chlorophy- 
cee, Hnteromorpha clathrata and Ulva, and red alge, such as Agardhiella, Chon- 
drus, Gracilaria, Hildenbrandia, and Polysiphomia, are found more commonly 
on the smaller, smooth, quartz pebbles, which make up the larger portion of the 
possible attaching surface on the bottom of the Inlet. The Phzophycee, 
Ascophyllum and Fucus, on the contrary, are usually found on the larger, rough- 
surfaced bits of granite or sandstone. It is probable that with the growth of the 
plants of the rockweeds that happen to start on smaller pebbles the supports are 
ultimately dragged away by these plants and thus either washed upon the beach 
or buried in the mud of the bottom of the harbor. This may account for the few 
quartz pebbles found bearing Fucus or Ascophyllum. Ulva starting on these 
smaller pebbles may likewise grow and finally drag off the latter. In other 
cases if the pebble is firmly fixed among its fellows or is too large to be dragged 
away by the plant, the growing sheet of Ulva may be torn loose and float or 
drift about over the bottom. In fact, the incrusting alge, such as Calothriz, 
Ralfsia, and Hildenbrandia, which grow on the large stones as well as the small, 
the rough as well as the smooth ones, are the only species which may not finally 
drag off their supports if the latter happen to be small. 
9 
