152 THE RELATION OF PLANTS TO TIDE-LEVELS 
VI. Salinity of the soil water during the growing season: It may be more saline at 
levels above 8 feet in winter, due to storms, or, even in the growing 
season, the salinity may temporarily become somewhat greater 
near the high-water mark from the concentration due to evapo- 
ration. It may again be less saline at these upper levels after rains. 
1. Salt: sa. By this is meant water of the density of that usually found 
at the surface of the harbor at high tide, which has a specific gravity 
of 1.019. 
2. Brackish: br. Distinctly less dense than the above. 
3. Fresh: fr. 
VII. Light demands: That is, the light conditions under which the species usually 
grows. 
1. Sun plants: su. 
2. Shade plants: sd. 
VIII. Upper limit of vertical distribution: The average upper limit of distribution 
of the species is given in feet above mean low water (indicated by 
the plus sign) or below mean low water (indicated by the minus 
sign). Where 12 feet + is used in this column it indicates that the 
plant may grow at this level, which is the highest level reached by 
winter tides, and at any level above this. That is, the plant is not 
confined to sea-coasts. In all other cases the upper limit given is 
the highest level at which the plant has been found about this 
harbor. Where but few individuals have been seen, and so the 
possible vertical range could not be determined with certainty, the 
one level at which it was found is indicated in this column and the 
column for the lower limit is left vacant. 
IX. Lower limit of vertical distribution: The average recorded lower limit of dis- 
tribution is given in feet, above or below mean low-water level for 
the soil upon which the plant grows at this limit. The extremes of 
distribution are given in Section III. The plus and minus signs 
are used as in column VIII to indicate levels above and below. 
X. Emergence or exposure: This is given, in the average number of hours per 
lunar day, for the soil which bears the plants at the upper limit of 
the species. It thus indicates the greatest average exposure endured 
by any plants of the species, aside from the few exceptional indi- 
viduals that may occur at slightly higher levels in habitats where 
the shade or moisture conditions are especially favorable. The data 
for emergence are taken from Table A (p. 135). Additional infor- 
mation concerning the occasional exposure of low levels and the 
duration of continuous exposure of higher levels may be obtained 
from Tables B and C (p. 136). The exposures during the hours of 
daylight may be seen from Table D, on page 142. Of course, the 
exposure of the soil on which a plant is growing indicates merely 
the exposure of the shoot above the soil. The subterranean por- 
tions are still immersed in a soil that may be practically satu- 
rated with salt water, e. g., Spartina glabra, or with fresh water, 
e. g., Sagittaria latifolia. 
XI. Submergence: This is given for the soil on which the plant grows at the usual 
lower limit of distribution of the species, as indicated in column IX 
of Table F. The submergence given thus indicates in hours per 
lunar day of 24.9 hours, the average submergence endured by the 
lowest plants of the species in question during the growing season. 
The duration of submergence here given is taken from Table A 
(p. 185) and is calculated in the manner mentioned in the explana- 
tion of that table. The number of submergences per month or per 
growing season for plants near the high-water level, or the duration 
of continuous submergence of plants near mean low water, can be 
learned from Table B and Table C (p. 136). The occasional sub- 
mergence of levels between 9 feet and 10 feet by storm tides is sug- 
gested in the cases of plants growing at these levels. Only winter — 
storm tides ever cover levels much above 10 feet. 
XII. Ratio of emergence to submergence: This is given for plants at the lower 
limit of vertical distribution of the species and at the upper limit. 
The two figures thus indicate the range in conditions, so far as the 
latter are affected by the tides, under which each species shows 
itself capable of growing here at Cold Spring Harbor. It is probable 
that if one had the tide-curve for a habitat with tides of greater 
magnitude like Nova Scotia or one with tides of lesser magnitude, 
like Virginia, he could predict the vertical range of any species 
common to one of these localities and to Cold Spring Harbor. 
