1906] TRANSEAU—BOGS OF THE HURON RIVER VALLEY 39 
dry sand. Palisade tissue was developed in the leaves of these plants 
in diffuse light, and it is shown that palisade tissue is to be correlated 
with physiological drought. An analogy between the bog habitat 
and the dry sand habitat is established. 
Experiments with Larix indicate that mycorhizas develop only in 
poorly aerated substrata; their growth is exceedingly slow; the 
acidity of the substratum is not a factor in their development; a 
naturally or artificially aerated substratum favors the development 
of normal roots, and these roots when not surrounded by water 
develop root hairs abundantly. Larix seedlings can withstand 
prolonged submergence. When exposed to low substratum tem- 
peratures and poorly aerated soil conditions, Larix produces more 
xerophilous leaves. 
Further field work on the bog plant societies needs to be carried 
on in the region extending from Winnipeg to New Brunswick. Data 
on the soil and air temperatures, the acidity, the chemical composition 
of the soil solution, and the plants associated in bog areas throughout 
this region will go far toward solving the problems of the distribution 
of bog plants. Experimentation on the production of xerophilous 
structures by bog conditions should be continued on a larger scale 
than is possible in the ordinary university plant-house. 
To Professor V. M. SpaLpineG and Professor F. C. NEWCOMBE, 
of the University of Michigan, under whose direction this work was 
planned and carried out, I desire to express my sincere thanks both 
for helpful suggestions and the facilities of the institution which were 
freely placed at my disposal. Many thanks are also due Professor 
I. C. Russexz for criticism of the physiographic part of this paper. 
I wish to acknowledge the kindness of Mr. FRANK LEVERETT, of 
the U. S. Geological Survey, whose intimate knowledge of the glacial 
geology of this region has been most helpful to me in the prosecution 
of my own field work. To Mrs. N. L. Brirron I am indebted for 
the determination of the mosses. Finally I take this opportunity 
to express my appreciation of my friend and former instructor, Dr. 
H. C. Cowxes, to whose writings and lectures I owe my interest 
in ecological botany. 
UNIVERSITY OF MICHIGAN. 
