ON THE MYCORHIZAS OF FOREST TREES 53 
trophic mycorhizas, and Acer saccharum var. nigrum (Michx. f.) 
Britton, Juglans nigra L., Crataegus sp., and Aesculus Hippocastanum 
L., which bear endotrophic mycorhizas. No mycorhizas were found 
on Cornus florida L., Ulmus americana L., Sassafras variifolium 
(SaUsb.) Ktze., and several species of Salix. These trees all occur in 
the woodlots in the vicinity of Ann Arbor, Michigan, or at the Forestry 
Farm 3 miles west of Ann Arbor. 
2. Collection of Material 
The collection of material was begun July i, 1911, and continued 
till December, 191 2. Throughout the warmer parts of the year some 
collecting was done every week, while during the winter collections 
were made less often, but specimens were obtained from each species 
every month, the object being to get series of specimens, extending 
throughout the whole twelve months, from individual trees of each 
species. Altogether one hundred and twenty-five collections were 
made. Great care was taken always to get reliable specimens. As is 
well known the mycorhizas occur only on the smallest rootlets of the 
tree and in the more superficial layers of the soil. In a mixed forest, 
where there are dozens of kinds of roots within a small area, it is often- 
times by no means easy to know to what tree the roots one is working 
with belong. In the case of a tree with the habits of Acer the task of 
collecting reliable specimens is not a very difficult one, because large 
roots of the maple often extend for considerable distances along the 
surface, or just beneath the surface, of the soil, where they can be 
easily followed. These always give off numerous short branches which 
can be easily traced to their ends, where mycorhizas are likely to be 
found. In the case of such trees as Quercus or Carya, however, it is 
much more difficult. On these trees the roots are usually interwoven 
and tangled together, so that it is very difficult to follow one for any 
great distance. Moreover, the large roots of these trees usually grow 
out only a few feet from the base of the tree when they turn downward 
into deeper soil, and it is more distant branches from these whose ends 
bear mycorhizas, often fifteen or twenty or more feet from the tree. 
These same trees, however, often give off short roots directly 
from the base of the trunk, or from the large roots near the trunk, and 
these very often yield an abundance of mycorhizas. When these can 
be found they are, of course, easily obtained. When they cannot be 
found, the only way of obtaining reliable specimens is to go some dis- 
