14 THE RELATION OF PLANTS TO TIDE-LEVELS 
clearly, between 7 and 7.5 feet, in both rise and fall. This record was taken on 
a night when the water was so quiet as to insure accuracy, and the irregularities 
referred to are thus not at all due to waves or other local irregularities in level. 
An examination of the map shows that the 1.5-foot contour is widely 
separated~horizontally from the zero or mean low-water contour. That is, 
the area of the harbor bottom to be covered increases with great rapidity as the 
water rises from 0 to 1.5 or 2 feet. This seems evidently part of the explanation 
of the slow rise shown by the start of the curve in plate vi. On the other hand, 
the horizontal distance between the 2 and 6-foot contours is relatively small, 
which is probably related to the steepness of the curve between 2 and 6 feet. 
While the 6 and 8-foot contours are close together about much of the harbor, 
they are widely separated on the Marsh at the head of the harbor. This 
probably is a partial explanation of the flattening of the top of the tide-curve. 
It must be kept in mind, however, that the form of the tide-curve in the Outer 
Harbor, or the varying cross-section of the Inlet, may be concerned in deter- 
mining the form of the curve for the Inner Harbor. In fact, published curves 
for the tides of more open water show that there is often a flat crest and trough, 
even at the open shore of the ocean. (See Darwin 1910.) 
The matter of primary importance for us is the actual form of the curve for 
this particular harbor, without regard to the ultimate causes of this form. It 
will become evident, however, that those factors that determine the time at 
which the water reaches and leaves a given level on the beach give the time of 
submergence and exposure and determine, in part at least, which plants shall 
grow at that level. 
One can easily determine the time of submergence or exposure of any given 
level along the beach or wharf by consulting the tide-curves. This can also 
be calculated by subtracting the time of submergence from the time between 
one low water and the next following low water. The latter time varies from 
less than 12 hours, in the case of spring tides, to 13 hours, in the case of neap 
tides, the average interval being not far from 12.5 hours. Table A (p. 135) 
gives the average time of submergence and exposure for various levels from 
0 to 8 feet. This will be useful for reference when we come to describe the 
distribution of the plants of the harbor. 
A comparison of curves or records for neap and spring tides shows that 
the time of daily submergence of levels below 2 feet and that of emergence of 
levels above 6 feet is decidedly greater during neap tides than during spring 
tides. On the other hand, for levels between 2 feet and 6 feet the time of sub- 
mergence, like the emergence, is practically the same for both kinds of tide. 
This latter fact is clearly indicated by the straightness of both sides of the tide- 
curve between the levels mentioned. This means, of course, that plants growing 
in this zone of about 4 feet in vertical width have a practically constant propor- 
tion of daily submergence and exposure, from end to end of the growing season. 
On the beaches it is just this zone that is dominated by Spartina glabra alterni- 
flora (plate vit A). | 
The plants of the harbor bottom below the 2-foot level and those of the 
beach above the 6-foot level are subject to much greater variation in the 
proportion of daily submergence and exposure. It might be assumed that 
this variation within each fortnightly period, ¢. e., within each set of tides, is 
