146 BOTANICAL GAZETTE [AUGUST 
external factor which exerts a direct influence on the evaporation 
at the bog island is the water area surrounding the island. Evapo- 
ration from the water surface and from the vegetation produces 
a vapor blanket, the action of which influences to a great extent 
the normal range of evaporation under the varying temperature 
conditions and consequently the rate of transpiration. The 
evaporation blanket is readily transported over the open central 
zone, its rate of movement and consequently the rate of evapo- 
ration varying particularly with the action of the wind. In the 
relatively forested maple-alder zone, however, the vapor blanket 
is more stationary and hence more uniform in its influence. This 
vapor blanket covers the locality to a definite height vertically. 
Studies on the phenomena of evaporation of water over lakes and 
reservoirs (3) have shown that the vapor pressure of the vertical 
gradient varies, beginning nearest the evaporating surface with 
a maximum, and rapidly diminishing within several feet above 
the evaporating surface, until it approximates to that in the 
free air. A few isolated readings confirmed this for the stations 
in question, as table VIII will show. At first the readings were 
taken every hour from 6 A.M. to 6 p.m.; later at intervals of six 
hours. For convenience, the larger time values covering the 
period from July 30 to August 3 are given here (fig. 8). The 
amounts are in the ratios too, 71, and 50 for positions at 5 feet 
(1.5 m.), 1 foot (0.3 m.), and 3 inches (7.5 cm.), respectively. 
Hence in general, the lower stratum of a vegetation has a smaller 
range in humidity variations and possesses an atmosphere usually 
much more humid than the upper vegetation stratum or the free 
air above the vegetation level. The data confirm the noteworthy 
results of the evaporation experiments by Yapp (34), and show 
that the bog vegetation at the lower levels is exposed to trans- 
piration conditions much less severe than those existing at posi- 
tions above the substratum and those quite clear of vegetation. 
But the growth of bog plants and their successful occupation 
of the habitat do not depend so much upon the total amount 
of evaporation or the time factor of this exposure, that is, the 
amount of moisture which the air contains during critical periods 
of the growing season. The functional activity of the plants is 
