SALINITY OF WATER 143 
It is evident that the habitat of the lower plants of Spartina glabra really 
resembles that of shade plants, so far as light supply and moisture conditions 
are concerned. Though they do not show very striking differences in structure 
when compared with plants of the same species growing near its upper limit, 
some differences are discoverable. For example, the plants at the 2-foot level 
have weaker stalks, thinner and narrower leaves, thinner cuticle and _ less- 
developed photosynthetic tissue than plants at the 6-foot level. Of course, many 
plants growing at and below mean low-water level are decidedly like shade plants 
in many respects, as has been suggested by Warming (1909, p. 150). For 
example, Zostera and Ruppia have the attenuated form characteristic of plants 
etiolated by shade. In the case of the alge Ulva, Enteromorpha, Cladophora, 
etc., found near low-water mark the structure is not markedly different, so far 
as was noted, from that of the same species at the highest levels these attain, 
except that the latter are smaller (perhaps we may say more stunted in growth). 
This is probably due to the fact that plants on piles and wharves at higher levels 
are more likely to be torn with the fall of the tide than are plants that lie on 
the bottom. A careful statistical study of the size of plants, of their cells, or the 
thickness of their cell-walls, might show constant differences in plants at 
different levels not hitherto detected. On the other hand, plants at the 7-foot 
level may get the maximum exposure of 6 hours per day to brightest daylight 
on 24 days of the month and lose an hour and a half or more of this light on 
only 4 days per month. Thus plants like Spartina patens, Distichlis, Salicormia, 
and Sueda, growing between 6.5 and 7.5 feet, are probably not much affected 
» by the relatively small proportion of total daylight lost by submergence. 
The general conclusion must be, then, that the shortened light supply of 
plants subject to daily submergence must affect their physiology, their structure, 
and hence their possible vertical range, in a very considerable degree, especially 
if they grow at or below mean sea-level. The exact effect of different exposures 
to light on different species of these plants has yet to be determined experi- 
mentally. 
In summary of the various effects of tidal changes on plants, we find that these 
are of most general importance in affecting, first, the amount of transpiration ; 
second, the time available for gaseous interchange between the shoots and the air ; 
and, finally in limiting the light-supply and hence the effective photospnthetic 
activity of littoral plants. Of secondary and only occasional importance are 
the effects on concentration of the soil-water at high levels, and the exposure of 
plants near mean low water to rains during low tides. 
4. THE SALINITY OF THE WATER OF THE HARBOR. 
The normal specific gravity of the water near the surface of the harbor, at high 
water, was found to be 1.022 (at 15° C.). The salinity, and so the specific 
gravity, varies somewhat with the state of the tide, and may become much lower 
than 1.022 at low water. This lowering of the specific gravity is evidently 
due to the large admixture of fresh water from tributary streams and springs. 
The inflow of this fresh water remains constant, except that from springs 
between tide-marks, while the volume of water in the harbor with which this is 
mixed decreases very rapidly with the falling of the tide. The cubic contents of 
the harbor at high water is about 42,000,000 cubic feet, while at mean low water 
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