5? THE RELATION OF PLANTS TO TIDE-LEVELS 
The past history of this Spartina belt is indicated by stumps and other plant 
remains found in the peaty subsoil of the Marsh at the head of the harbor. It 
seems evident that the history of these Spartina marshes has been that outlined 
by C. A. Davis for the salt, tidal marshes near Boston, which have similar fresh- 
water deposits, stumps of trees, and other features like the marshes we are 
describing (see Davis, 1910, p. 635, ff.). The minimum amount of subsidence, 
as indicated by the plant remains found in the section of the Marsh on the east 
bank of the stream at 100 to 200 south, is about 6 feet. It is difficult to see how 
these remains can have attained their present relation to tide-levels by change in 
magnitude of the tides or by the settling of a floating bog, as held by D. W. 
Johnson (1913). We know of no evidence for the former change, and the fact 
that the salt-marsh peat does not overlie the parts of the gravelly shore (e. g., 
near 100 to 300 south by 1,200 east) above the present marsh-level, seems to 
exclude the latter explanation. 
2. THe ALG& oF THE Mip-LiTToRAL MarsuH (1.5 To 6.5 FEET). 
The number of species of alge growing among the Spartina, on the natural 
shore lying between the 1.5 and 6.5 foot levels, is quite large. At least 36 species 
have been collected, and the distribution of most of these has been studied with 
some thoroughness. The classes and genera represented here, many of them by 
small patches or scattered individuals, are the following: 
Schizomycetes: Beggiatoa. 
Schizophycee: Anabena, Chroococcus, Lyngbya (2 species), Microcoleus (2 
species), Oscillatoria (3 species), Polycystis, Rwularia, and Spirulina. 
Bacillariales: Melosira, Plewrosigma, and many less abundant species of 
other genera. 
Chlorophycee: Chetomorpha, Cladophora, Enteromorpha (3 species), Llea, 
Monostroma, Rhizoclonium (2 species), Ulva, and Vaucheria. 
Pheophyceer: Ascophyllum, Ectocarpus, Fucus (2 species), Pylaella, and 
Ralfsia. 
Rhodophycew: Bostrychia, Delesseria, Hildenbrandia, and Petrocelis (?). 
Of all these species growing on the Spartina Marsh, the most prominent, 
because of their size, are Ascophyllum and Fucus. The most universally 
present are two species of Rhizoclonium (R. ripartum and R. tortuosum). In 
some places these Rhizoclonwums develop nearly pure growths of considerable 
extent, while in other habitats one or both species may form an important 
constituent, often the major one, of tangled mats, or, near the 6.5-foot level, 
of dense felts of alge. These mats or tangles are mixtures of varying propor- 
tions of Cladophora expansa, Enteromorpha clathrata, and Vaucheria thu- 
retwi?), and often include also several or many species belonging to the genera 
Calothriz, Lyngbya, Microcoleus, and Oscilatoria. The closer felts or incrus- 
tations are more characteristic of the next higher belt and will be discussed in 
that connection. Below 6.5 feet it is only on the occasional spots bare of 
Spartina that these felts of alge invade the present belt. They occur on the 
mud at the edge of the rivulets and between the stems of Spartina, and some- 
times grow upon the stalks and stubble of this grass. 
Keeping in mind the general constitution and distribution of these com- 
posite mixtures, we will now discuss the species of alge occurring on this mid- 
littoral marsh in systematic sequence. 
