72 
W. B. McDOUGALL 
SUMMARY. 
1. Six forms of ectotrophic mycorhizas, including a heterotrophic 
form, are described. 
2. Four species are added to the known list of ectotrophic myco- 
rhiza forming fungi: Russula sp. on Tilia americana, Boletus scaher 
var. fuscus on Betula alba var. papyrifera, Cortinarius sp. on Betula 
alba var. papyrifera, and Scleroderma vulgare on Quercus alba. 
3. At least four, and probably more, different species of mushrooms 
may form mycorhizas on the same tree. 
4. The infection of a young root and its transformation into a 
mycorhiza depend on the chance presence of a fungus which is capable 
of forming mycorhizas with that particular species of root, or on some 
chance condition of the root or the fungus. 
5. Infection for the formation of ectotrophic mycorhizas takes 
place by a fungous filament penetrating the outer portion of the epi- 
dermal wall of the root and then branching and spreading in all 
directions, by dissolving the middle lamellae, until a complete mantle, 
covering the rootlet, is formed. While this mantle is being formed, 
other branches of the mycelium are penetrating between the epidermal 
cells of the root by dissolving the middle lamellae and splitting the 
cells apart. 
6. As soon as a complete mantle of mycelium is formed over the 
root any further growth of the root in length is inhibited. Because 
of this fact the root is stimulated to produce branches. These branches 
are in turn infected, and the result is a coral like cluster of mycorhizas. 
7. The fruiting bodies of an ectotrophic fungus are usually produced 
soon after the mycorhiza is formed. 
8. Both ectotrophic and endotrophic mycorhizas are normally 
annual. They are formed during summer, reach their fullest develop- 
ment in late autumn, persist unchanged throughout the winter, and 
die in spring. 
9. The roots of Acer are infected through root hairs in the produc- 
tion of endotrophic mycorhizas. The modified bead-like character 
of the mycorhizas is due to the stimulus of infection. 
10. The matter presented in this paper indicates that the endo- 
trophic mycorhizas of the maples are sometimes symbiotic associations, 
and sometimes associations in which the fungus can only be considered 
as an internal parasite of the roots. The ectotrophic mycorhizas of 
