SUMMARY AND CONCLUSIONS 149 
and well-drained, Spartina glabra grows but little above this and is succeeded 
by Salicornia and Sueda, while on the peaty soil of flatter parts of the shore 
S. glabra may extend up to the 7.5-foot level and there be succeeded by Spartina 
patens or Distichlis. If fresh water is present in the fine soil of the upper levels, 
Spartina glabra is succeeded by Scirpus americanus and sometimes mingled 
with it up to the 8-foot level. On the Marsh the character of the vegetation of 
the surface is correlated with the local thickness of the peat above the underlying 
gravel. 
The effects of water-currents on the distribution of plants are exercised by the 
dissemination of spores and seeds and by the breaking off and transportation of 
the shoots of Zostera, and of alge like Ulva, Enteromorpha, Fucus, etc., which 
persist and grow in their new lodging-places. In other cases water-currents, by 
mechanically injuring the plants or by determining the character of the sub- 
stratum, may favor or retard the further extension of a species. On the other 
hand, the rapid movement of the water is an evident advantage to some species, 
perhaps by increasing the interchange of material between the plant and the 
surrounding water, and possibly also by injuring competitors. Thus Zostera, 
Cladophora, Pylaella, Chondrus, Polysiphonia, Porphyra, etc., are most abun- 
dant in or beside the rapid tidal current of the Inlet. Jlea, Monostroma, and 
Pylatella likewise are abundant only in the rapidly flowing Creek entering the 
south end of the harbor. 
A careful study of the vertical distribution of the littoral plants about this 
harbor shows that this depends primarily and very definitely on the relative time 
of their submergence and emergence with the rise and fall of the tide. Moreover, 
the vertical range of littoral species is strictly, sometimes very narrowly, limited. 
There are no species here, except two or three alge, that are distributed 
“between tide-marks” (1. e., from low water up to high water), as is so often 
reported. The nearest approach to this range found for any seed plant is that 
of Spartina glabra, whose vertical range of 5 feet (from 1.5 to 6.5 feet above 
mean low water) lies midway between the limits of the average (8-foot) 
tide. The range of this salt reed-grass covers but five-eighths of the mean tide- 
range and only half the range of many spring tides of the growing season (10 
to 11 feet). This Spartina never gets below 1.5 feet and only in exceptionally 
moist and shaded areas does it grow at any appreciable distance above 6.5 feet. 
The alga Fucus ranges from just below mean low water up to 7.25 feet and 
Enteromorpha intestinalis has a similar range on shores where fresh-water 
rivulets flow in over the upper beach. The other seed plants of the shore range 
down to but 1.5 or 2 feet below mean high-water level (7.75 feet), with the 
exception of Lilgopsis, which is found between 5 and 6.5 feet. Zostera and 
Ruppia range upward for only 1 or 1.5 feet above mean low water. 
The essential feature, for our purpose, of the tidal oscillation of water-level 
is the relative times of submergence and exposure experienced by the various 
shore-plants. For the sake of comparison this relation is most significantly 
expressed in a fraction for the upper limit and one for the lower limit of distri- 
bution of each species (p. 135). The influence of this change in water-level 
on the plant is effected through differences, at different levels, in evaporation 
rate ; in aeration; in salinity of soil-water at higher levels; in exposure to rain 
and in light supply. It seems evident, for example, that Spartina glabra 
