tg11] BURNS—HURON RIVER VALLEY 113 
temperatures for the last two were lower than the author obtained 
for these zones. As has been pointed out, there is great variation 
in the poplar zone, and one should have a better knowledge of 
the exact location before attempting to explain this difference. 
The temperature under the bog sedge also varies with the width 
of the floating bog and the proximity to the zone of bog shrubs. 
That is, the temperature remains more constant when one gets 
away from the open water. 
Average records made by a class with ordinary thermometers 
for six weeks in the summer of 1906, 3 inches (7.6 cm.) below the 
surface, were as follows: open water, 18° C.; bog sedge, 17° C.; 
tamarack, 15° C.; maple-poplar, 17° C.; marginal willows, 18° C.; 
outer edge of marginal zone with no shade at 1:15 P.M., 22°C. 
These readings show a wide divergence of soil temperatures in 
the zones under consideration, and also at different depths in the 
same zone. This calls for a study of the depth of the root systems. 
For example, the high temperature of the soil at 25 mm. repre- 
sents the condition for germination of the many seeds of “drained 
swamp” plants which yearly lodge in the bog. This taken with 
the weak light and humid air, which will be considered later, makes 
an excellent place for germination of these seeds. On the other 
hand, the study of the root system shows that temperatures found 
at a depth of 45.8 cm. in the bog sedge and tamarack zones, 
especially, could not have a direct influence on the plants growing 
above, as they did not root in that layer but only in the surface 
layers. This is true even of the largest trees. Added to this 
fact is the additional consideration that peat is a very poor con- 
ductor of heat. The soil readings in these zones which are of the 
greatest importance are those taken near the surface in a study of 
reproduction, and those at moderate depths in the study of the 
present flora. 
These data become ecologically important in the light of the 
work of TRANSEAU (18, p. 22), who has shown by experiment 
that a temperature of 10°8 C. causes a diminution in the develop- 
ment of both roots and leaves. Although the trees began to open 
their buds the last of May, it was not until July that the tempera- 
ture about the roots of the tamaracks reached a higher temperature. 
