SPARTINA GLABRA ASSOCIATION 51 
cause of the usual absence of S. glabra from the upper littoral region is, it 
seems clear, the competition of the plants above mentioned. Wherever the 
physical conditions allow these competitors to grow they prevent S. glabra 
from occupying levels higher than 6.5 feet. It is difficult in this, as in all 
cases of direct competition, to discover exactly how these plants of higher 
levels (e. 9., S. patens) can prevent the advance of S. glabra into the 
areas occupied by them. If it is really these competitors that prevent the 
growth of S. glabra in certain areas, it must be by some effect exercised beneath 
the soil, since we can not believe that the shoots of the larger plant can be 
crowded or shaded out. It is possible that the dense impervious turf of 
S. patens cuts off the air from the deeper-growing rhizomes and roots of 
S. glabra. This would be in accord with the suggestion offered above, concern- 
ing the absence of 8. glabra from poorly aerated mud of bottom below the 1.5- 
foot level. We are now attempting to solve this question experimentally. 
Currents created by the rise and fall of the tides apparently have little effect 
on the distribution of Spartina, except that already noted of undermining the 
turf beside the two chief streams of the Marsh, and a similar, though relatively 
slight, effect along the tide-channel, cutting through the Spartina belt at the 
end of the Spit (near 800 east). 
FRESH WATER IN THE SOIL. 
In places where the fresh water runs over the soil the S. glabra is wanting. 
Often the peat is cut away down to the gravel by the flow of the rivulet. In other 
places the fresh water merely seeps out or trickles over the peaty mud. On this 
kind of area Spartina is sparse or wanting, and its competitor, Scirpus ameri- 
canus, becomes more and more abundant as the soil becomes more nearly 
saturated with fresh water. Cases of this sort, of which there are many about 
the harbor, might seem to indicate that S. glabra can not grow in a soil saturated 
or nearly saturated with fresh water. But this Spartina does grow in the pool 
below the wheel of the old mill. The soil here at the time these observations 
were made was submerged in fresh water for 6 hours each tide, and could hardly 
become very strongly saline even at high water. It therefore seems quite 
possible that the competition of the Scirpus americanus plays an important 
part in crowding Spartina out of areas wet by fresh water. 
The usual intermingling of Spartina and Scirpus americanus in wet soils 
above the 6-foot level may quite possibly indicate that some individuals or 
strains of Spartina possess much greater ability to withstand fresh water than 
others and so push farther up the beach. Likewise, it may well be that the 
lowest plants of Scirpus americanus are really the individuals most able to 
endure salt water about both stems and roots. Only after experimental study 
of the problem can it be determined whether the position of the line of contact 
of these two species is dependent upon physical conditions directly or upon 
some kind of competition in which one plant disturbs the other physiologically. 
This study must include a determination of the power of different plants of 
the two species to endure considerable, and rapid, changes in the osmotic 
pressure of the water about roots and shoots. 
* A thin sprinkling of S. glabra grows south of the Causeway about 450 south and 
800 east at the 7.5-foot level in soil water with a specific gravity of 1.000. Turfs of 
this grass planted in the pond 200 yards south of the Causeway survived but a single 
season. 
