ON THE MYCORHIZAS OF FOREST TREES 
65 
amounts, fewer mycorhizas will be produced and more roots will de- 
velop normally. The same explanation may be applied to the fact 
that the same tree may have mycorhizas one year and the following 
year may have none at all. This fact was brought out by Kauffman 
(9), and has been verified by me in several instances. 
3. Mycorhiza Development 
Frank (4) stated that an ectotrophic mycorhiza is produced by a 
fungous filament applying itself to the side of a rootlet, and then 
branching and spreading until it covers the whole rootlet. He evi- 
dently believed that the mantle of fungous tissue is put on first, and 
that the formation of the lichen structure within the rootlet is a 
secondary process. These statements, however, were probably mere 
guesses, with no direct evidence to back them. Mangin (13) does not 
make himself very clear on this question. Judging from his figures, 
one gets the idea that the fungous mantle is put on over the tip of the 
rootlet first and progresses from that point until it covers the rootlet. 
Moller (14), on the other hand, reports, for the mycorhizas of Picea, 
that infection takes place back of the root tip, and that the lichen 
structure within the root is formed first, and the mantle put on later. 
Moller does not give any figures to enforce his statements, and it is 
of course impossible to judge of the accuracy of his observations. In 
the course of my work I have never seen any evidence of either of the 
methods of development indicated by Moller and Mangin, and the 
method given by me in this paper is certainly correct for the mycorhizas 
which I have studied. 
Moller also reported that the growth of a mycorhiza is resumed 
in spring after a winter rest, basing his conclusions on the fact that 
specimens are found in spring with bright, fresh appearing tips. He 
therefore agreed with Frank that the mycorhiza as a whole grows in 
length. Such specimens with fresh tips were common on all ecto- 
trophic trees studied by me, in the spring of the year. A number of 
the mycorhizas, which were already present when my glass plates were 
first placed, showed this characteristic. These were carefully meas- 
ured from time to time during a month, at the end of which time they 
were all dead. In none of them was I able to discover any appreciable 
increase in length. It appears, then, that the fresh tips are due merely 
to a freshening up of the mycelium. 
The fact that the root is inhibited from further growth by the 
