52 W. B. McDOUGALL 
with the cytological papers, which dealt entirely with endotrophic 
mycorhizas, numerous theories were advanced as to the relation 
between the host plant and the fungus. 
Since the publication of Gallaud's (6) extensive work on endo- 
trophic mycorhizas, several important papers of varying nature have 
appeared. Bernard (i) in 1909 published his excellent work on the 
endotrophic mycorhizas of the orchids. This is the most complete 
and satisfying work yet done on the mycorhizas of any group of plants 
and is to a large extent verified by the work of Burgeff (2) which ap- 
peared a few months later. Other papefs will be mentioned farther on. 
It is a notable fact that all of the more extensive papers have dealt 
principally with endotrophic mycorhizas, and that no very intensive 
work has been done on ectotrophic forms. Mangin (13) in 1910 
published a paper dealing with the ectotrophic mycorhizas of forest 
trees, but it is of an introductory nature, and the figures which accom- 
pany it are very diagrammatic. 
The work on which the present paper is based was carried on at 
the University of Michigan during 191 1 and 1912. The primary 
object from the beginning was to work out the seasonal relations 
of the mycorhizas of our forest trees. During the course of the work, 
however, a number of other interesting facts were brought to 
light and they will be presented here. I am indebted especially to 
Professor F. C. Newcombe, under whose supervision the problem was 
worked out, for invaluable advice and suggestive criticisms throughout 
the course of the work. I am also indebted to Professor C. H. Kauff- 
man for his constant interest in the work and for assistance in dealing 
with the mycological side of the subject. 
II. MATERIAL AND METHODS 
I. Species of Trees Studied 
The species of trees of whose mycorhizas the most extensive study 
was made are Carya ovata (Mill.) K. Koch, Quercus alba L., Tilia 
americana L., and Betula alba var. papyrifera (Marsh.) Spach., which 
were found to have ectotropic mycorhizas, and Acer saccharinum L., 
and Acer rubrum L., which have endotrophic mycorhizas. Some work 
was also done, for purposes of comparison, on Larix laricina (Du Roi.) 
K. Koch., Quercus rubra L., Quercus velutina Lam., Populus grandi- 
dentata Michx., Fagus grandifolia Ehrh., Ostrya virginiana (Mill.) K. 
Koch., and Carpinus caroliniana Walt., all of which produce ecto- 
