92 THE RELATION OF PLANTS TO TIDE-LEVELS 
higher levels, and is not wanting from among the Spartina glabra at slightly 
lower levels. Eastward of 430 west to 150 east the algal felts and crusts are 
wanting over most of the loose, gravelly beach. Only where the soil is bound 
together by clumps of Spartina patens do we find small bits of loose alga felts 
on the bases and dead leaves of this grass. From 150 east to 580 east, between 
6.25 and 7 feet, there is a green or blackish felt or crust over all unoccupied mud, 
sand, pebbles, or bits of stubble of Spartina patens and Salicornia europea. 
Not merely are the edges of the pebbles incrusted but the tops of those 4 or 5 
em. across are completely covered. The composition of these felts, which are 
best developed on the Spit, includes the following species: Calothrix (2 spe- 
cies), Lyngbya (38 species), Microcoleus (2 species), Osculatoria (2 species), 
Pleurosigma, Enteromorpha, Rhizoclonium (2 species), Vaucheria, and occa- 
sionally Jlea. Eastward from 580 east to 870 east the coating of alge over the 
soil becomes rapidly sparser and, before reaching the latter point, it disappears 
altogether. Some Calothriz, Lyngbya, and Rhizoclonium may still occur on 
the stalks and dead leaves of Spartina glabra, and rarely on the Salicornia or 
Sueda at higher levels (plate vir). 
The distribution of the felts or tangles of algz on the east and west shores 
and on the Marsh is likewise quite constant and similar in different parts of the 
shore-line, except where affected by fresh water or flood-trash. Only on the 
stone pier at 950 north by 975 east is there any considerable development of 
the dense incrustations found so generally on the south shore of the Spit. On 
the gravelly top of this pier, at 7 to 7.5 feet, the pebbles, especially of the 
northern half, are incrusted by blackish mixtures of alge. On the north or 
shaded side of the little mound of soil about each tuft of Spartina patens, velvety 
layers of Calothriz may cover many square centimeters, and in a few instances 
spread over 1 or 2 dm. 
Green or reddish crusts may occur along other parts of both east and west 
shores, on pebbles over which fresh water is flowing. These are made up of green 
unicellular alge, of brownish Ralfsia ? or the red Hildenbrandia. In the tide- 
pools of the Marsh lying between the 7 and 8 foot levels, we find a still different 
type of association of alge. ‘These alge are chiefly species of Lamprocystis, 
Lyngbya, and Oscillatoria, the two latter of which are woven into very firm 
felts 3 or 4 mm. thick, when moist, and often 2 or 3 sq. meters in extent. These 
felts may be partially floated and partially submerged at high water, and either 
hie on the muddy bottom or be supported by sticks and stubble at low water. 
These felts are very well developed even in tide-pools that are frequently sprayed 
with crude petroleum to kill mosquito larve. New and quite extensive patches 
of algal felts, especially of Vaucheria and Lyngbya, may appear in areas denuded 
of the normal covering of Spartina or Juncus Gerardi. They are often formed 
by masses of flood-trash. The alge may dominate these areas for a season or 
two, but are usually driven out by the reinvasion of the region by the grass or 
rush. 
We may now take up, in systematic sequence, the thirty-odd species of alge 
growing in this belt, indicating briefly the habit and general distribution of 
each species, taking up the genera of each class in alphabetical order. The dis- 
tribution of the more important species is indicated on plates vi1I and Ix. 
