THE LARCH CANKER 47 
and a few drops of water are allowed: to lie between them. 
A cover-slip is then very carefully removed from one of 
the culture-chambers, and placed on the two rings, so that, 
resting above the drops of water, the moisture on its under- 
side does not dry up. The whole can be put under the 
microscope and the spores removed by a sterilized needle 
or glass filament and transferred to culture media in Petri 
dishes or Cogit’s flat glass bottles, the usual precautions 
being taken against the introduction of foreign spores. 
A suitable medium was found to be either gelatine or 
agar-agar containing malt extract (3 per cent.), meat. 
extract (0°3 per cent.), and citric acid (0-03 per cent.). 
There is at first some difference between the cultures 
obtained from the large spores of the parasitic form and 
the smaller spores of the saprophytic form. The germ 
tubes of the latter are smaller, and growth is at first rather. 
slower, but after a day or two very little difference can be 
found, and older streak cultures are indistinguishable. 
The mycelium is characterized by forming a very close 
felt-like mass, and it grows so evenly on all sides that 
a culture from a single spore becomes a circle (fig. 22) and 
a streak culture has the form of an even band. When 
growing on gelatine the mycelium puckers up the sub- 
stratum into folds, but agar-agar remains flat. 
Temperature has a considerable modifying influence on 
the growth of cultures, and the following table is instructive 
in this respect. Twelve streak cultures were grown on agar- 
agar in Petri dishes. The first four, A;—As, were grown at 
the temperature of the room (13°-14°C.), the next four, 
A;—Ag,in an incubator at 22°-23°C., and the last four, Ay—A1pe, 
in an incubator at 26°-27°C. 
It can be seen from the following table that a temperature 
of 12°-13°C. is below the optimum for growth, and 26°-27°C. 
is above the optimum. It is interesting to note that the 
higher temperature encourages germination, which takes 
place very slowly at the lower temperature, but for sub- 
sequent growth the latter is more favourable. Also the 
high temperature encourages excessive branching. 
