48 THE RELATION OF PLANTS TO TIDE-LEVELS 
%-foot level, all in a horizontal distance of 0.5 or 1 meter. The Spartina at 
the foot and part way up this slope is quite dense. On the upper half of the 
slope it thins out, and when the top of the slope is reached at 6.75 or 7 feet it is 
replaced by its commonest successor, Spartina patens, except for a bare sprink- 
ling of small shoots of the S. glabra in areas with wetter soil. Where, however, 
there are tide-pools farther up on the Marsh, the 8. glabra extends back along 
the edges of the draining ditches and often occupies not only the sides but 
sometimes the bottom of the pool itself, though this may lie above the 7.5-foot 
level (e. g., the tide-pool at 100 south by 900 east). Other areas, with a firm 
bottom, that lie but a few inches below the surface of the surrounding Marsh, 
and have no outlet on the surface, may bear small plants of S. glabra in rather 
dense stand—100 or 200 plants per square meter. Such an area is found at 
0 north by 1,025 east (plate xr) and another very interesting one just above 
the 8-foot level at 240 south by 1,135 east, as is indicated in plate x1, area 25. 
(See C. A. Davis, 1910.) 
Along the portion of the shore from 100 south by 400 east to 250 south by 
600 east, the upper margin of the Spartina belt much resembles that of the 
Spit. The soil here is sandy, and at the 6 to 6.5 foot levels S. glabra meets and 
mingles with 8. patens and more rarely with Distichlis. 1t also comes in con- 
tact occasionally with scattered plants, but never with dense stands, of Solidago 
sempervirens, Salicornia europea, and Spergularia. Near the fresh-water rivu- 
let at 320 south by 640 east the Spartima encounters and mingles with Scirpus 
americanus, which also occurs on both sides of the main stream beyond 400 
south. The related species, S. robustus, is mingled with the Spartina between 
the 6.5 and 7 foot levels at several points where the soil is considerably saturated 
with fresh water (460 south by 820 east, 100 south by 1,180 east). 
Some details of the distribution of these competitors of Spartina and also of 
certain rarer ones are indicated in plate x1. 
Throughout this Spartina belt about the south end of the harbor, aside from 
the competitors very near the upper margin, no other seed plants are found, 
except the small patches of Lileopsis (60 north by 1,050 east and 150 north by 
1,140 east), which grow on bottom between the 5 and 6.5 foot levels. Dr. Shreve 
records the occurrence of Limosella aquatica var. tenuifolva on the edge of the 
shoal near 199 north by 600 east, but this has not been seen in recent years. 
Most of the alge found scattered among the Spartina on the other shores are 
equally abundant here, and will be referred to again in a later section. 
THE FACTORS DETERMINING THE DISTRIBUTION OF SPARTINA GLABRA. 
Having sketched in some detail the distribution of this salt reed-grass about 
the whole harbor, and described the habitats in which it grows, we may now 
attempt to discover the enviromental factors that condition the distribution 
which we actually find. What factors have favored the entrance and persistence 
of the grass in the areas now occupied, and what factors limit its extension 
upward, downward, and horizontally along the shore? We should keep in mind 
that many of these factors probably act in a similar way on other plants, and 
that for this reason Spartina is a type whose relation to external factors is in 
many respects resembled by that of many other plants about the harbor. 
Temperature and light of the range of intensities found, so far as has been 
made out, play no decisive part in limiting the distribution of S. glabra. It 
