150 THE RELATION OF PLANTS TO TIDE-LEVELS 
and the rockweeds do not grow at levels above 6.5 feet, because they can not 
endure a greater evaporation than that experienced here. Zostera and many 
alge for the same reason are confined to levels below mean low water, except 
where washed by tidal streams during low tide. It seems probable also that the 
too brilliant light or the exposure to rains during low tide prevents certain algee 
from growing above mean low water. On the other hand, certain plants are 
stopped in their spread downward because they can not endure the longer sub- 
mergence, with the lessened aeration and light supply, at lower levels. Spartina 
glabra, for example, as has been shown experimentally, is undble to persist 
more than a few months at levels even slightly below 1.5 feet. The rockweeds 
also seem unable to persist below the level just mentioned in the usually quiet 
and turbid water of the Inner Harbor. In the clearer, rapidly moving water of 
the Inlet, however, and especially in that of the Outer Harbor and Long Island 
Sound, Fucus grows a foot or more below mean low water. 
The degree of salinity clearly determines the horizontal distribution of many 
plants in this harbor. For example, one series of alge, chiefly Chlorophycee, 
occur only in the less saline south end of the harbor, where they are flooded with 
fresh water for from 2 to 10 hours each tide. On the other hand, the majority of 
the Floridee found here grow in the channel to the Outer Harbor, where the 
water is most saline. 
In the list of plants of the harbor (pp. 151 to 161), there are indicated for 
each species the physical characteristics of the habitat which are believed to be 
concerned with its distribution. In but relatively few instances has the connec- 
tion between the distribution and external conditions been experimentally shown. 
Though the attempt has been made in the body of this paper to suggest the 
external factors determining the distribution of each common species, this must 
be regarded as a suggestion of the elements entering into the problem rather 
than as a statement of its definite solution. 
It is believed that the determination of the relative time of submergence and 
exposure of a plant at its upper and its lower limit in this harbor, where the 
range of tide is about 8 feet, will make it possible to predict the vertical range 
of the same species in any other region, if the range of the tide is known. 
That is, the vertical range of a littoral plant is exactly proportional to the range 
of the tide. 
In this study chief attention is devoted to determining and recording as 
accurately as possible the present distribution of the plants of this harbor in 
relation to tide-levels, salinity, and soils. Little has been said of the succession 
in time of the formations occurring at different levels on the shore. It is 
believed that this historical aspect of the problem can be solved more satis- 
factorily by the comparison of the vegetation that will exist here some years 
hence with that which is here recorded. 
