24 THE RELATION OF PLANTS TO TIDE-LEVELS 
by 500 east and at the 1.5-foot level. It seems evident from what has just been 
said that Zostera is excluded from bottom much above mean low water by its 
liability to death from exposure at low tide. In those places where it exceeds 
this usual upper limit it is protected from desiccation by water from streams, 
which runs over it at low tide. 
The environmental factors determining the lower limit of the Zostera have 
not been distinguished with absolute certainty. So far as could be discovered, 
from a study of the relatively short lower margin of the Zostera area about the 
deep hole, this plant does not encroach on bottom lower than that indicated, 
because this lower bottom is sandy or shelly. Such soils, in this harbor, are 
nearly always bare, no matter what the depth. In the few places where Zos- 
tera seems to be growing on such bottom, the use of a sounding-rod usually shows 
that there is a softer subsoil of mud, an inch or two below the surface. It seems 
likely that the Zostera in these spots originally became established on a mud 
bottom which later, by some change in water-currents, was covered by a layer 
of sand and shell fragments. Only a few plants, which had very short, narrow 
leaves, were found growing on an apparently pure sandy bottom. Possibly the 
turbid water of this harbor may make the light supply inadequate at greater 
depths. In the clearer water of Casco Bay, Maine, Zostera grows on soft 
bottom 10 feet below low-water mark, also in Great South Bay, New York. A 
similar distribution of Zostera is found in Carmel Bay, California. 
C. H. Ostenfeld (1909) has found a similar relative abundance and size of 
the Zostera (growing, however, in much deeper water), on sandy and on muddy 
bottoms of the coast of Denmark. He states (p. 33) that on the bare, firm, 
sandy bottom there is only a sparse growth of Zostera with short, narrow- 
leaved shoots, which are free from growths of epiphytic plants and of animals. 
On soft, muddy bottom, on the contrary, he finds a dense pure growth of 
Zostera with larger and broader leaves, which latter are occupied by many 
- epiphytic diatoms, brown and red alge, and various animals, e. g., hydroids, 
mollusca, bryozoans, and ascidians. On pebbly bottom, where the interspaces 
between pebbles and boulders are occupied by softer soil, Ostenfeld finds a 
“mixed Zostera vegetation” in which, on certain areas, Zostera grows in the 
mud, while the pebbles give fixing-points for Fucus, Laminaria, and other 
coarse brown and red alge. Another type of mixed Zostera vegetation is that 
mentioned by Ostenfeld as occurring in brackish waters with bottom of sandy 
mud, and is characterized by the abundance of the green alge Ulva, E'ntero- 
morpha, Cladophora rupestris, Chetomorpha linum along with Chara, Toly- 
pella, Lamprothamnus, and also the seed plants Ruppia, Zannichellia, and 
Potamogeton pectinatus. This last type is the one that approaches most 
nearly, on the whole, to the Zostera vegetation of the harbor we are studying. 
Perhaps the most striking difference between the two is the absence, from our 
area, of the Characeew and Potamogeton. The similarity will be clear when we 
consider the distribution of the alge of the harbor bottom. 
In our harbor the differences in the character of the bottom, and so the 
presence or absence of Zostera, seem primarily due to the differences in the 
swiftness of the water-currents. The bottom of the deep hole, and of the 
channels north of it, is of sand or gravel and particles of shell, with a few 
bits of organic material, all of which may be seen shifting with the current 
when the tide is swiftest. The only fixed plants seen here were Ceramiums, 
