60 
W. B. McDOUGALL 
For this reason it is very difficult to find specimens of the early stages. 
The best material for this study was obtained on the seventeenth of 
August from Tilia americana. A section from some of this material 
is shown in fig. lo. The mantle in this case is still very thin, having 
the thickness of from two to four hyphae, and it is incomplete, not 
yet having covered the tip of the root. This shows that infection 
cannot have taken place at the tip, but must have taken place at 
some point farther back. Further, there is not yet any ''lichen 
structure" present. In only a few places have the hyphae begun to 
penetrate between the cells of the root. The formation of the mantle 
and that of the lichen structure are going on more or less simultaneously 
although the mantle is in advance. The outer portion of the epi- 
dermal wall of the root can still be seen on the outside of the fungous 
mantle. The fungus has penetrated the epidermal wall at some point 
back of the root tip, split the wall by dissolving out the middle lamella, 
just as it does when it penetrates between the cells of the root, and 
spreads in all directions. Finally a complete mantle will be formed and 
the outer portion of the epidermal wall will be cast off entirely. Since 
these mycorhizas are never more than a few millimeters in length, 
seven or eight at most, and more often only two or three, it is evident 
that the root does not grow in length after a complete fungous mantle 
has been formed. 
6. Seasonal Relations of the Mycorhizas 
Beginning my collections about the first of July, I found that 
throughout the months of July and August good fresh specimens were 
rather scarce, although the old, black dead ones were very plentiful. 
Throughout October, November and December good, well-developed 
specimens were very abundant on all trees studied. This condition 
persisted during January, February and March, although these were 
very cold months, and the soil in which the specimens were found was 
frozen solid. By the middle of April the mycorhizas of Quercus and 
Carya had begun to die off, and good specimens were becoming rather 
scarce, although they were still very abundant on Tilia and Betula. 
By the middle of May those on the two last named species were also 
getting rather scarce. Throughout June and the fore part of July 
they were very scarce on all species. In August they began to be more 
plentiful again, and, by September, they were as abundant as in Sep- 
tember of the previous year. 
