12 THE RELATION OF PLANTS TO TIDE-LEVELS 
1%0 south by 380 east. From here northward to 20 south, then west to 0 east, 
and from thence northward and westward to 550 north and 230 west, the harbor 
is bounded by the stone walls of a tide-pool, then further on by the walls of the 
wharves along the south and west sides of the harbor. 
The western shore of the harbor, from 550 north to 2,250 north, is a rather 
steep, gravelly beach. The only exceptions to this are the stone wharf, with 
wooden piles, from 1,070 north to 1,220 north, a stone wall from 1,410 north to 
1,560 north, and two stone and wooden wharves from 2,080 north to 2,250 north. 
From 2,250 north this shore is muddy and is covered rather completely with 
Spartina glabra alterniflora up to the extreme northwest corner of the harbor 
near 2,600 north by 1,000 west. 
The Spit, which is 100 to 175 feet in width, and has an elevation of from 
10 to 12 feet above mean low water, forms the northern boundary of the harbor. 
The south beach of this Spit, between the 6.5 and 8 foot tide-lines, differs in 
character in different parts of its extent. From 1,000 west to 700 west the 
shore is of sandy loam, very flat, and covered with a dense growth, made up 
almost entirely of Spartina patens and Distichlis spicata. From %00 west 
to about 300 east, the beach is steeper and often nearly bare for considerable 
stretches (plate Iva). Where covered at all it is usually by small patches of 
Spartina patens, Sueda, or Salicornia. From 300 east to 900 east, the similar 
gravelly beach is covered, often completely up to 7.5 feet, by a dense growth of 
Spartina, Distichlis, Sueda, or Salicornia. Any one of these may occur locally 
in a pure stand, or, in other places, all may be more or less mixed together. 
Among these seed plants, on the bases of their stems, or on the gravel and sand 
of the otherwise bare portions of the beach, dense mats of incrusting alge of 
many species occur in patches a yard wide and several yards long, giving the 
beach a dark greenish or black color. These types of vegetation will be described 
_in detail later. 
The extreme eastern end of the Spit, beyond 825 east, is entirely bare between 
the 7-foot and the 8-foot levels, being essentially similar, except for the finer 
sand of which it is made up, to the wave-beaten outer side of the Spit at this 
same level. 
3. TIDE-LEVELS; MODE OF DETERMINING HEIGHT AND RATE OF RISE 
AND FALL OF TIDES. 
One of the primary facts we wished to determine concerning the habitat ot 
each species of plant growing in our area was the position of this habitat with 
reference to the tide-limits. Thus only could we determine how long, in each 
24 hours, the particular habitat is submerged and how long exposed to air, 
sun, and rain. Our aim was to discover just how definitely the distribution of 
beach and marsh plants is dependent upon the relative times of submergence 
and exposure of the plant-shoot and of the substratum upon which it is 
growing. 
The first step in accurately determining the relative times of submergence 
and exposure of any particular zone of beach or bottom is to establish a 
standard datum-level from which to measure the height of tides. This was 
done by placing a graduated tide-stake near the middle of the harbor, with its 
zero-point at mean low water. This stake was planted on a very quiet day, 
when, according to the Tide Tables of-the Coast and Geodetic Survey, the 
