146 THE RELATION OF PLANTS TO TIDE-LEVELS 
this is reduced to about 2,250,000 cubic feet, and at a low water of —1-foot, 
during spring tides, the volume may be only 700,000 cubic feet, or one-sixtieth 
of that at high water. 
There is no adequate evidence that these variations in salinity are of con- 
sequence to the plants found established in the harbor. The low salinity at low 
water, however, may well be an important factor in preventing other red alge, 
now occurring in the Outer Harbor, from getting a foothold in the Inner Harbor. 
Of far more importance are the relatively rapid changes from water of a 
specific gravity of 1.019 to absolutely fresh water, and the reverse, to which 
plants growing between tide-lines in the fresh-water tributaries are subjected 
twice each day. We have already spoken of the algew Hnteromorpha intestinahs, 
Ilea, and Monostroma as occurring in or near fresh-water tributaries, where at 
high water they are surrounded by salt water, but, with the fall of the tide, are 
left with fresh water running over them, often for 8 or 10 hours continuously. 
The transition from one extreme to the other may, in the case of the small 
rivulets, occur very suddenly, probably in the course of a very few minutes. 
This is true because where there is but a small flow from a rivulet it may cause 
little mingling of the fresh and salt water about the alge until the water has 
fallen almost to their level. Up to this time the fresh water, being lighter, simply 
spreads out on top of the salt water and leaves the alge, which may be but an 
inch or two below, surrounded by salt water. Even in the case of larger streams 
such as that entering the harbor from the ponds of the New York State Fish 
Hatchery at 600 south by 720 east, Miss Streeter found that the water at the 
bottom, surrounding the alge, may change from a specific gravity of 1.014 to 
one of 1.000 in an hour’s time, with a fall of but 1 foot in the tide. 
From a careful study of the floras of the between-tides portions of fresh-water 
tributaries, all about the harbor, it is evident that the rapid changes in salinity 
of the water must prevent many plants found elsewhere from growing in these 
‘areas. On the other hand, the ability of the alge mentioned to withstand these 
conditions make these areas places of refuge for these alge, where they are free 
from the competition of other species; Ilea, e. g., for example, covers many 
square meters of the pebbly bottom of the Creek, between 200 and 500 south, 
practically to the exclusion of other species, except the inconspicuous diatoms. 
It must also be recalled that these rivulets have another advantage, perhaps the 
principal one, in that they form habitats where these more delicate alge are not 
subject to desiccation during low tide, as they would be elsewhere at the same 
levels. 
The salinity of the soil-water on various portions of beach and marsh, both at 
the same and at different levels, is undoubtedly a factor of very great importance 
in determining the distribution of plants. Reference has been made in the body 
of the paper to the fact that Lilea subulata, Scirpus americanus, and S. robustus 
are found chiefly in soils more or less saturated with fresh water, and to the 
fact that Iris versicolor, and probably Hibiscus moscheutos, push down to their 
lowest levels in spots where the soil, though below high-water mark, is saturated 
by running or seeping fresh water. A series of quantitative determinations of 
the salinity of the soil-water in various of these habitats has been initiated, but 
determinations are not yet numerous or complete enough to allow of detailed 
discussion. The method being used is like that used by Harshberger (1909, 
