SPARTINA GLABRA ASSOCIATION 49 
seems to grow equally well on warm tidal flats, or where flooded by the cold 
water of tidal inlets or fresh-water streams. It is possible that it is the low 
temperature of the spring-water of the rivulets along the east and west shores 
that prevents the Spartina from gradually invading the beds of these streams. 
In certain areas along the west shore it endures considerable shade, though it 
commonly grows in full sunlight. These shaded plants, however, are at high 
levels and hence they enjoy, from the neighboring trees, about the same degree 
of protection from light and transpiration that plants at lower levels do from 
submergence. 
The character of the soil is evidently a factor of much importance near the 
upper and lower limits of the Spartina’s distribution. Between the 2-foot and 
6-foot levels Spartina may grow on mud, peat, sand, or gravel, as may be seen 
along the Spit. It has been pointed out, however, that the belt of this grass 
stops most abruptly at its upper margin on a gravel beach, just east of the 
middle of the Spit. At its lower margin, in like manner, the Spartina usually 
terminates abruptly at a point where the very soft mud succeeds the rather firm 
peat. This is equally true whether the change in character of the soil occurs 
at the 1.5-foot, the 2-foot, the 3-foot, or even at a higher level, as it does in the 
northwest corner of the harbor and about certain tide-pools. 
From these facts we might conclude that this plant is gradually advancing 
over the soft bottom and binding it together with its roots and rhizomes. But 
on this assumption it is rather surprising to find such a very regular lower limit 
of distribution at the 1.5-foot level in a harbor where this grass has been growing 
for hundreds of years. It will also be difficult to explain the greater width of 
the Spartina belt at the east end of the Spit. On the whole it seems clear that 
the firm soil bearing Spartina is in some way the product of the growth of this 
grass and that it is the conditions limiting the spread of Spartina that second- 
arily determine the extent of this firmer bottom. 
All observations thus far made on this harbor indicate that the decisive 
factors preventing the Spartina from spreading further upward or downward 
_ are those connected, directly or indirectly, with the water-level about the leaves 
or in the soil about the roots. In short, the primary factors may be considered 
under two heads—tide-levels and fresh-water streams or springs. 
TIDE-LEVELS. 
Differences in the elevation of soil above mean low water may, in the first 
place, influence the distribution of Spartina directly by determining the time 
of submergence. Upon this depends the level maintained by the salt water in 
the soil about the roots of the grass and the time of exposure of the leaves to 
the evaporating influence of the air. These differences of level, and so of dura- 
tion of submergence, may affect the distribution of Spartina indirectly, since the 
longer submergence evidently keeps the competitors of Spartina off the lower 
parts-of the beach. The duration of submergence also determines the types of 
plants that grow just above the Spartina and thus the conditions in the soil 
into which the latter must push if it is to advance upwards. 
At the lower margin of the Spartina belt at the east end of the Spit the 
soil at the 1.5-foot level is submerged for about 8.5 hours each tide, or for 17 
hours daily, and exposed for only 7.5 or 8 hours daily. In certain areas remote 
from the main tidal currents, as in the northwest corner of the harbor, the 
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