134 BOTANICAL GAZETTE [AUGUST 
zone. There is a large daily as well as annual range in tempera- 
ture, but the range is considerably less in the soil than in the air 
above. The data obtained are sufficient to strengthen the obser- 
vation made, that in the spring the ice in the central zone melts 
with greater rapidity, and that a higher temperature results from 
the greater insolation and the increased absorption and retention 
of heat rays. On days following a sudden lowering of the air 
temperature, and also on cloudy days, the temperature of the 
surface bog water and bog soil in the sphagnum-covered area 
stands above that of the maple-alder zone. This gain in tempera- 
ture is cumulative and aids in the penetration of heat rays below 
the surface. The heat supply is obviously the most direct factor 
contributing to the substratum temperature, for the variations 
are associated directly with the amount and intensity of sunshine. 
The extreme slowness in the maple-alder zone is explained partly 
by the low conductivity of the partially decayed peat and the lack 
of a free circulation of air above the soil, but largely by the increas- 
ing diffusion of light rays due to the leafing out of trees and shrubs. 
Another point of interest is the fact that notable differences 
are found between the temperatures of the bog island and the 
surrounding lake water. When we compare the effects of gain 
and loss of heat between the free water surface of the lake and 
that of the peat area clothed with vegetation, it will be seen that 
the temperature of the central and the maple-alder zone remains 
higher than that of the lake during the autumn and winter months, 
and that during spring and summer the lake water is warmer at the 
respective (1.5 m.) depths than the peat substratum. Water has a 
specific heat far greater than any soil; it retains its heat longer 
and for this reason is warmer than the peat substratum in spring 
and summer. On the other hand, peat and humus are cooled 
more rapidly at the surface by the evaporation of water during 
the warm days of the seasons. The values of both heat conduc- 
tivity and diffusion are in general lower in peat than in water, and 
hence a rapid loss of temperature in the peat strata below the 
surface vegetation is thus prevented. 
A high temperature phenomenon existing in certain places is 
worthy of special mention. Not infrequently small sheltered 
