ON THE MYCORHIZAS OF FOREST TREES 
6i 
7. Time of Fruiting 
As far as my observations go, it would seem that the fruiting bodies 
may be looked for any time after the first of August. In all cases of 
mycorhiza producing fungi reported in this paper, the fruiting bodies 
were produced relatively soon after the rootlets were infected. 
B. Endotrophic Mycorhizas 
I. Descriptive 
The endotrophic mycorhizas of Acer saccharinum and Acer ruhrum 
have a very characteristic appearance and are easily distinguished 
from the normal roots. They are constricted at intervals so that they 
have a beaded appearance (fig. 12). The beadlets are seldom more 
than one millimeter in diameter, and usually less than that. A 
mycorhiza may consist of only one such beadlet or there may be as 
many as six or more. The cortical cells in these swellings are enlarged 
much beyond the normal size, so that, in section, the cortical cells 
in the constriction appear to be greatly compressed (fig. 14). In 
some cases, only occasional cells contain the fungous filaments, while in 
other cases nearly every cell of the cortex is infected. I have never 
seen any of the filaments within the cells of the central cylinder. The 
filaments are often seen curled about the nucleus (fig. 16), but in 
other cases they do not appear to bear any relation whatever to the 
nucleus. They are often seen to pass through the cell wall from one 
cell to another (figs. 16 and 19), but they never split the cell walls and 
follow them as do the ectrotophic filaments. Peculiar swellings and 
various modes of branching are observed (figs. 15 and 18). The 
vesicles described by several authors are occasionally seen (fig. 17). 
These are large swellings, usually at the ends of the hyphae, and have 
thickened walls. The significance of these modifications is not known. 
Usually when only a few cells of the rootlet are occupied by the endo- 
phyte, root hairs are present in variable numbers; but root hairs are 
entirely lacking in those mycorhizas in which nearly all cells of the 
cortex are infected. 
2. The Chance of Mycorhiza Formation 
As in the case of ectotrophic mycorhizas, so here the infection of a 
rootlet depends on chance conditions. The glass plate method of 
observation did not prove as satisfactory in this case as with ecto- 
trophic mycorhizas, but one of the plates did give some results. Under 
