50 THE RELATION OF PLANTS TO TIDE-LEVELS 
Spartina, as we have seen, retreats to the 2.5 or 3 foot level. It is believed that 
the long submergence affects the Spartina unfavorably by preventing the air 
from reaching the submerged rhizomes and roots. In the northwest corner of 
the harbor, out of the main currents, the water moves less, and hence the soil 
about the Spartina roots is less well aerated, and so this plant can not grow 
here at levels where it flourishes at the east end of the Spit, in better aerated 
soil, beneath the swifter tidal currents. It is also to be noted that crab and 
muskrat burrows are less abundant at the lower levels, and hence do not aid in 
aerating the soil. 
The change of water-level, due to the tides, keeps the soil at the upper margin 
of the Spartina belt submerged for 2.5 to 3 hours and exposed for between 9 
and 10 hours each tide. When covered this soil is probably nearly saturated. 
During emergence the water not only runs off the surface, but also settles out 
of at least the upper layers and, by way of the fiddler-crab and muskrat burrows, 
the air penetrates to the soil about the roots and rhizomes of the Spartina. The 
upper layers of the mud are firm enough to allow the fresh water of rains at 
low tide to run off so rapidly that little of it penetrates to the roots of plants. 
The immediate effect of the semidiurnal change of water-level on the leaves 
and shoots of S. glabra is to expose it wholly to the air and the heat of the 
sun during low tide. This occurs for 17 or 18 hours per day for plants at 
the 6-foot level, for about 13 hours at the 4-foot level, and for only about 8 hours 
per day for those at the 2-foot level. The plants thus exposed show a distinctly 
xerophytic structure in their stiff and thick-cuticled leaves, which roll up 
tightly in drying winds. The evident usefulness of these xerophytic structures 
in plants growing at 6.5 feet and lower might suggest that Spartina is kept 
from invading levels above this limit because it can not endure the longer 
exposure of its shoots to desiccating winds and sun. But, on the other hand, 
we must remember that the Spartina growing at 5 or 6 feet is as large and 
vigorous as any plants of this species found, showing that there is no consider- 
able reduction in size and vigor as this plant approaches its usual upper limit. 
Moreover, at high levels, in poorly drained soil, as, e. g., on the west shore and 
on the Marsh south of the harbor, Spartina grows to a good size, although its 
leaves are always exposed to the air. But the surrounding air is in most of 
these cases rendered unusually moist by the presence of abundant water on 
the surface of the soil or by an abundance of neighboring vegetation. 
All of these facts taken together seem to indicate that a low degree of satura- 
tion of the soil with salt or brackish water is a still more effective deterrent to 
the upward spread of the Spartina than the desiccation due to emergence above 
water-level. 
INDIRECT EFFECTS. 
It is clear that the direct effect of submergence or exposure in allowing or 
preventing the competitors of Spartina to grow at certain levels may indirectly 
affect, to a very marked degree, the distribution of Spartina itself. For exam- 
ple, the distribution of such plants as Spartina patens, Distichlhs, Salicornia, — 
and Scirpus americanus, which succeed the Spartina above the 6.5-foot level, 
indicates that these plants can endure neither as long submergence nor, per- 
haps, as high salinity of the soil water as S. glabra. We have already noted that 
S. glabra can grow up to levels as high as 7.5 or 8 feet, often mingled with 
Spartina patens, Scirpus americanus, and other plants. The most important 
