ENpoTHta CANKER OF CHESTNUT 565 
were girdled and killed by the growth of the fungus. It grew and pro- 
duced spore horns also, on the wounded tissue at the point of inoculation, 
on all the others except the maple and the tulip. On Quercus alba and 
Q. prinus the cankers continued to spread for several weeks, and fan- 
shaped mats of mycelium were found under the bark. From the fact that 
such mats are never found, even on the chestnut, except where the parasite 
is invading living tissue, it was decided that in these cases, at least, 
it had established parasitic relations. None of these trees were killed. 
The fungus was reisolated from all these hosts, and in culture proved to be 
the same as that isolated from chestnuts. 
It may safely be said that at present the canker fungus is not a serious 
menace to any other forest tree except the chestnut. 
Life history 
Germination of pycnospores 
The pycnospores cannot be made to germinate in pure water. The 
writers have found the most satisfactory medium for this purpose to be 
a decoct’on made by boiling chestnut bark. The spores will germinate 
also on sterilized twigs of a large number of trees, on various nutrient 
media, and even on humus alone; but these media are not favorable for 
observation of the process under the microscope. 
The time required for germination varies with the temperature. . Fulton 
(1912:52) states that he found conidia germinated best at a temperature 
of 60° F., and distinctly less rapidly at temperatures 10° below or above 
that point. The writers obtained the most rapid germination at 80° 
F., the shortest time for the appearance of germ tubes being twelve hours. 
At temperatures ranging from 60° to-75° F., germination occurs in eighteen 
to thirty-six hours; at lower temperatures the process often requires four 
or five days. From this it appears that the warm periods of summer are 
the most favorable for infection by pycnospores. All attempts to produce 
the disease by inoculating with pycnospores in winter have failed. 
The process of germination begins with an enormous swelling of the 
spores. Spores averaging. 1.28 by 3.56 u before germination, were found, 
at the end of eighteen hours in chestnut-bark decoction, to average 6.86 
by 10.53 uw, the largest observed being 9.05 by 14.48 u — representing an 
increase in volume of over one hundred times that of the original spore. 
A germ tube grows out from one end, and usually this is followed later by 
a second one from the opposite end. The rate of growth and the manner 
of septation and branching of the germ tube are best understood by 
reference to the series of camera lucida drawings of a single spore at short 
intervals, reproduced in Fig. 90. The swelling of the spores is due, not 
merely to a mechanical imbibition of water, but also to a process of growth. 
