188 THE RELATION OF PLANTS TO TIDE-LEVELS 
to 30° C., while the humidity recorded by a Friez hygrograph in the same place 
varied from 50 to 95 per cent. Atmometer No. 1, in the shelter, showed an 
average corrected rate of loss of 0.43 ¢.¢. per hour. It showed a minimum rate 
of 0.37 c.c. per hour in the period from 12"20™ p. m. August 7, to 7°30" a. m. 
on August 8, when the temperature ranged from 18° to 30° C., averaging 23.5° 
C., and the humidity ranged from 50 to 95 per cent, averaging 79 per cent. The 
maximum rate for this instrument of 0.75 c. c. per hour was shown on August 7 
from 7°20" a. m. to 12°20” p. m., when the temperature ranged from 21° to 27° 
C., averaging 22.2°, and the humidity varied from 53 to 75 per cent, averaging 
69 per cent. 
Atmometer No. 2, placed in a dense stand of Spartina glabra at 2,750 north 
by 290 east, with its cup at the 7.5-foot level and about 1.5 feet above the soil, 
gave an average corrected rate for the whole 25.8 hours of 0.79 c. c. per hour. 
The minimum rate at this station was shown for the period from August 7, 
12°20” p. m. to August 8, 7°30" a. m., the temperature and humidity in the 
shelter for this period being those given above. This corrected minimum rate 
was 0.67 c.c. per hour. The maximum rate for No. 2 of 1.1 c. c. per hour was 
also shown in the same period and under the same conditions as the maximum 
for No. 1,7. e., on August 7, from 7°20” a. m. to 12°20™ p. m. 
Atmometer No. 3 was placed 10 feet south of No. 2 among the stems of 
Spartina, in a reclining position, with its cup 4 inches above the soil and at 
about the 6.3-foot level. The corrected rate of this instrument varied from 0.36 
c.c. per hour to 0.63 c.c. It is interesting to note that the relative rate of this 
instrument varied from 0.5 to 1.7 times that of No. 1. This wide difference in 
rate of two instruments exposed simultaneously is probably due to the differences 
at the station for No. 3 in the direction and strength of the wind, and especially 
to the different amounts of water, from the preceding high tide, left clinging to 
_ the Spartina stalks at the beginning of the exposure. 
The amount of this adhering water would depend on the length of time since 
the tide receded from that level; also on the temperature and on the direction 
and strength of the wind, which might dry it off. It must be borne in mind, 
however, that the variations in evaporating power of the air here mentioned are 
entirely characteristic of locations between tide-marks, and are therefore real © 
factors in the environment of plants, which, like the alge on the mud and on 
stalks of the Spartina, live in these habitats.. 
Another series of records was made in which atmometer No. 1 was placed in 
the shelter, No. 2 was placed on the vertical stone wall, facing east, at 200 north 
by 80 west, near the 6.5-foot level, and thus near the upper edge of the rockweed 
belt, and No. 3 was placed at this same level on the north-facing wall at 10 south 
by 40 east. The three atmometers were exposed simultaneously during a total 
of 32.7 hours of daylight from July 27 to 30, 1909, in clear weather, while 
the temperature ranged from 22.8° to 34.4° C. (averaging 28° C.), and the 
humidity varied from 39 to 87 per cent (averaging 60 per cent). 
The record of No. 1 under these conditions showed an average corrected rate 
of 1.08 c. c. per hour (ranging from 0.89 to 1.56 c.c. per hour) ; No. 2 gave an 
average rate of 0.9 c.c. per hour (ranging from 0.5 to 1.16 ¢.c. per hour) ; 
while in No. 3 the rate averaged 0.72 c.c. per hour (ranging from 0.59 to 1.14 
¢e.c. per hour). 
