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W. B. McDOUGALL 
Form 5. — A form collected from Larix laricina is brown in color 
and the thick pseudoparenchymatous mantle is smooth on the out- 
side (fig. 6). There are no radially elongated root cells, but the outer- 
most cells of the root have been crowded apart by the fungus until 
some of them are isolated as islands far out in the fungous mantle. 
The fungus penetrates nearly to the central cylinder so that nearly 
all of the cortical cells are entirely separated from each other. 
Form 6. — A form found on Tilia americana may be called a hetero- 
trophic mycorhiza. In external appearance, and in internal appear- 
ance, too, for that matter, it looks like an ectotrophic mycorhiza. It 
has a semi-pseudoparenchymatous fungous mantle which is smooth on 
the outside. It also has the radially elongated external layer of root 
cells, characteristic of so many ectotrophic mycorhizas, entirely 
surrounded and separated from each other by the fungous tissue. The 
novel thing about this mycorhiza is that, occasionally, the filaments, 
which extend in between the root cells, enter the next cells beneath, 
and there appear very much like endotrophic filaments (fig. 7). 
All forms of mycorhizas described above agree in containing no second- 
ary growth. Also no fungous hyphae are ever found in those parts of 
a root that are not covered by the fungous mantle, so that, if two or 
more branches of the same root are transformed into mycorhizas, 
there can be no connection of the fungus of one of these mycorhizas 
with that of another except through the soil. 
2. Mycorhizal Fungi 
On August 6, 1912, I found the ground around the Tilia, from which 
I had been collecting regularly, fairly covered with the sporophores of 
a Russula. On digging up these mushrooms, twenty-two in number, 
a cluster of fresh young mycorhizas was found immediately below 
each one, while in three separate cases an actual connection between 
the mycelium of the mushroom and that of the mycorhizas was easily 
demonstrated (Text-fig. i). These mushrooms were found only on 
the area which might be occupied by the roots of the Tilia in question. 
In searching over the greater part of the woods, only one other speci- 
men was discovered. That one was near another Tilia, and when 
it was dug up a cluster of mycorhizas was found immediately beneath it. 
I have not been able yet to determine the species of this Russula, 
and it may be an undescribed species. It is a small red mushroom, 
and it is very acrid. Evidently it belongs to the same group as do 
