ENHALID FORMATION 23 
mud, each of these branches consisting of from 12 to 20 short internodes, and 
bearing from 4 to 8 functional leaves. The width of these leaves varies from 
5 to 8 mm. and the longer of them are from 1 to 2 meters in length. The 
fertile, or floral, shoots of Zostera, at this season, vary in length from 50 cm. 
to 1.5 meters. Each consists of about 15 or 20 internodes, which are from 1.5 to 
2 mm. in diameter and from 5 to 20 cm long. Each node bears a leaf which 
is about 4 mm. wide and 15 cm. long. On older parts of the shoot nothing is 
left of the leaves but the sheath and perhaps a small bit of the blade. Inflores- 
cences in all stages of development are present on each fertile shoot, from 
floral rudiments just initiated at the top to spathes at the base from which the 
fruits have already been discharged. 
The densest stands of Zostera seen in the harbor are that east of the channel 
to the Outer Harbor, northeastward from 1,200 north by 800 east, those along 
the two banks of the tide-stream from 2,000 north by 400 west to the depression 
at 1,500 north by 300 east, and that on an area of several hundred square yards 
extent southwest of the deep hole about 900 north by 200 east. On these areas 
there may be from 500 to 2,000 leaf-clusters of Zostera to each square yard “ 
bottom. In other parts of ‘the Zostera region indicated on the map, e. g., 
much of the area near the main north-and-south axis from 1,000 north to 2, 000 
north, the stand of Zostera is far less dense, with an average ‘of 200 or 300 leaf- 
clusters per square yard. For some distance outside the indicated area, espe- 
cially to the north and west, plants of Zostera are very infrequent, perhaps 20 to 
100 small tufts to each square of the map, 7. ¢., to each 3,333 square yards. 
These tufts are mostly scattered and show but 2 or 3 to 10 or 12 leaf-clusters 
each. The extreme limit of distribution of Zostera is shown on plate x111 by the 
use of the letter Z, which is used as a symbol for indicating the position of 
outlying plants. These plants are usually small, being 1.5 to 6 dm. from the 
rhizome to tips of leaves, and their distribution varies somewhat from year to 
year. For example, numerous scattered plants of Zostera were found at 2,000 
north by 0 to 600 east in 1905 and 1906. From 1907 on, following the 
occupation of much of this area by beds of mussels (Mytilus edulis), Zostera 
has been nearly or quite wanting here. The presence of the mussels has 
evidently led to a gradual silting over of the bottom of this area, raising it to 
or above mean low water, which is probably a direct cause of the disappear- 
ance of Zostera. It may well be that the mussels also cause other changes in 
the soil, making it injurious to the Zostera, e. g., in the content of such gases 
as air, CO,, or H,S. Such changes may explain the disappearance of Zostera 
from bottom not continuously occupied by mussels that has not yet been raised 
above the usual upper limit of this plant. 
Though the horizontal distribution may seem decidedly irregular (plate x111), 
especially the scattered marginal tufts, the distribution in depth is pretty con- 
stantly limited. It is found at levels extending from mean low water down to 
3.5 feet below this. The parts of the harbor where Zostera occurs above the 
upper limit mentioned, are certain areas where bottom at 6 to 12 inches above 
mean low water is overflowed more or less at low tide by water from inflowing 
streams. For example, the southern prolongation of the Zostera area near 
600 north by 400 east overlaps the mean low-water line very considerably, and 
scattered outlying plants of Zostera are found as far south as 300 north by 
200 to 400 east or 275 north by 725 east, and even a few tufts near 210 north 
