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this plate, which was placed on Acer saccharinum, four new rootlets 
were produced. One of these became infected and developed into a 
typical mycorhiza consisting of two beadlets, while the other three 
developed normally and attained lengths of 2.8 cm., 3.1 cm., and 3.4 
cm. respectively. 
3. The Infection of Rootlets 
A few sections were obtained in which the fungous hyphae were 
found within root hairs, or extending from root-hair cells into adjacent 
cells (fig. 19). This indicates that infection takes place through root 
hairs just as has been shown by several authors for other endotrophic 
mycorhizas. 
The fungus is found only in the cortex of the beads. I have never 
seen it in unmodified portions of the root, nor in the constrictions 
between the beads. Each bead, therefore, must be infected directly 
from mycelium in the soil. Since the beaded appearance is only 
found in the case of infected rootlets, and roots which are not infected 
develop normally, the modified character of the mycorhiza must be 
due to the stimulus of infection. 
4. Seasonal Relations 
In general the seasonal relations of the endotrophic mycorhizas of 
Acer are the same as those of the ectotrophic mycorhizas of other 
forest trees. They are formed during the summer, reach their fullest 
development in late autumn, and die during the spring. They are, 
therefore, annual. They do not die off so rapidly in spring, however, 
as do the ectotrophic forms, and new ones are beginning to be pro- 
duced long before the old ones are all dead, so that there is never a 
time when they are as scarce as the ectotrophic forms become. 
IV. DISCUSSION 
The work presented in this paper may be discussed under five 
heads: (i) the heterotrophic form of mycorhiza; (2) mycorhizal fungi 
and their relation to the chance formation of mycorhizas; (3) mycor- 
hiza development; (4) seasonal relations; (5) physiological relations. 
I, The Heterotrophic Form of Mycorhiza 
A mycorhiza of this sort has, I believe, never been reported before. 
Stahl (24) reported that the mycorhiza of Juniperus nana is sometimes 
ectotrophic and sometimes endotrophic, and the one on Picea studied by 
