Ii]. PLANT ASSOCIATIONS; CHARACTERIZATION OF THE 
BELTS OR ZONES OF VEGETATION AND THEIR 
DISTRIBUTION. 
We will begin our discussion of the vegetation with a descriptive account of 
the plant associations found in the harbor, noting their composition and distri- 
bution. Then will follow a general discussion of the factors determining 
distribution, and a list of all plants found, showing the distribution of each 
species and the relation of each to the factors affecting its distribution. 
In defining the limits of the zones of vegetation in this harbor we shall use 
the term “ littoral” in a more definite way than we have found it used by other 
writers on the distribution of marine plants (see Lorenz, 1863; Kjellman, 
1877; and Warming, 1906). We shall designate as littoral the zone extending 
from mean low water up to mean high water, which, for our harbor, is a zone 
8 feet, or more exactly of 7.8 feet in vertical width. The littoral zone of this 
harbor is distinguished, in a manner determined by the vegetation itself, into 
lower littoral, mid-littoral, and upper littoral subzones. The region just below 
mean low-water level, of which 1.5 feet may be exposed at extremely low tides, 
will be referred to as the sublittoral zone. The zone of 3 or 4 feet above mean 
high water, which may be flooded by occasional high tides or storm tides, will 
be called the supralittoral zone. 
The most clearly marked primary vegetational groups or plant associations 
in the harbor are these: 
(1) Plankton, made up of the plants floating at the surface in a living 
condition, chiefly diatoms and Peridinez. 
(2) Plants at the bottom of the harbor, between the —5-foot and the +1.5- 
foot levels. These occupy the soft, muddy, sandy, or pebbly bottom, or are 
epiphytic upon plants which grow on this bottom. The shifting pebbly or 
shelly bottom in the deepest part of the harbor is bare of fixed vegetation up to 
the —5-foot level mentioned, which thus marks the lower boundary of the belt 
under discussion. The upper limit of this belt is at the +-1.5-foot level, where 
the soft mud, forming most of the bottom, is succeeded by a soil that is rendered 
firm by the close network of rhizomes and roots of Spartina glabra, which 
occupies the next succeeding belt on shore. This lower limit of S. glabra, with 
its sudden change in elevation of the bottom, in character of soil, and in the 
vegetation covering it, forms a very prominent topographic boundary. It is 
therefore indicated on our topographic map. 
(3) The mid-littoral belt of vegetation occupies the shore between the 1.5- 
foot and 6.5-foot levels. On muddy shores a fringing marsh of Spartina glabra © 
dominates between these levels to the exclusion of all other seed plants, save a 
few small groups of the inconspicuous umbellifer Lileopsis lineata. Numerous 
alge, however, are found on the mud between the stalks of the Spartina. 
Many of them are species not found elsewhere, and most of them are associated 
in mats or felts of a sort not occurring in other locations except in the next 
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