TIDES AND EVAPORATION 1389 
Other series of atmometer records were started, but, because of accidents or 
lack of time, were not made complete enough to be of significance. 
From what has been said it is evident that the maximum rate of evaporation 
of 1.1 c.c. per hour recorded in the middle of a moderately warm day, by 
an atmometer among the leaves of the Spartina glabra, indicates that these 
plants are at times subjected to a relatively high rate of evaporation. If we 
multiply this hourly rate by 168 we get a weekly rate, 185, that closely ap- 
proaches the highest average weekly evaporation-rate, for the growing-season, in 
the United States east of the Mississippi, given by Livingston (1911, p. 219). 
Kven the average hourly rate for atmometer No. 2 in the Spartina, which 
included one night in the total exposure of 25.8 hours, was 0.79 c.c. per hour, 
or 133 c. c. per week. This considerably exceeds the average weekly evaporation 
for the summer at New York City, as given by Livingston (1911, p. 213), and 
the weekly rate of 95.5 c.c. at 2 meters above the soil found by Fuller (1912, 
p. 426) in a mesophytic beech-maple forest of Indiana. In fact, it very closely 
approaches that given for Salt Lake City (Livingston, p. 210). It nearly 
equals also the average rate of 0.83 c.c. per hour given by Transeau (1908, p. 
219) for his standard instrument, established in an open garden at Cold Spring 
Harbor. The rate of atmometer No. 2 in the Spartina for the period that 
included the afternoon and the night of August 7, was 0.67 c.c. per hour or 
113 c.c. per week. This indicates the correctness of Transeau’s suggestion 
(1908, p. 227) that the low average rate of evaporation from his atmometer, 
placed at the 12-foot level, on top of the Spit, in 1907, was due to the very slight 
evaporation occurring there at night. | 
The rate of instrument No. 2, when placed on the wall of the wharf, amid the 
rockweed, against stones saturated by water, and only 5 feet above the muddy 
bottom, with its trickling rivulets, averaged 1.08 c.c. per hour, or 181.5 c.c. 
per week, while its maximum rate reached 195 c.c. per week. This indicates 
the high rate of evaporation to which the rockweeds, Rhizoclonium, Bostrychia, 
and the Schizophycee of the wharves may be subjected, during low tide. These 
_ rates in fact approach the rate for the area about Lake Erie (200 c. c. per week), 
which is the highest average weekly evaporation-rate given by Livingston (1911, 
p. 219) for any part of the United States east of Texas or the Dakotas. 
It is, of course, realized that the few atmometer records here given can be con- 
sidered adequate to do little more than indicate the importance of this evapora- 
tion-rate as a feature of the environment of shore-plants living between tide- 
marks. To get a really adequate idea of the importance of this factor in the 
environment of plants growing at any level, we must have records by quick-regis- 
tering atmometers exposed at that level from the moment it is bared by the fall- 
ing tide until it is just about to be covered by the rising tide. Moreover, readings 
must be made each hour or half hour of all exposures, night and day, from end 
to end of the season. When this is done, as it is hoped it may be soon at Cold 
Spring Harbor, it is believed that both the average rate, and, in some cases 
especially, the maximum rate of evaporation in their habitats will be found to 
be intimately concerned in determining the upper limit of distribution attained 
by many of the species growing between tide-marks. 
