194 Temperature and Growth of F ‘ungr [392 
that the results obtained may be of value, not only in inter- 
preting the geographical distribution and seasonal occurrence 
of these diseases, but in suggesting further means for their 
control. 
A suitable solid medium in petri dishes is employed, a 
transfer (of spores or a small piece of mycelium) being 
made to the center of each culture dish, and the resulting 
growth is measured in terms of the 24-hourly increase in 
the mean diameter or radius of the nearly circular area 
occupied by the fungus. Various precautions are taken to 
have all conditions, excepting that of temperature, as nearly 
alike as possible throughout the entire investigation. 
The preliminary work so far carried out has been con- 
fined largely to Pythiacystis citrophthora, which attacks both 
the trunk and fruit of the lemon tree. To illustrate the 
kind of results obtained, at the temperatures 10°, 20°, 28° 
and 38°C. the radial, 24-hourly growth-rates of this fungus 
were 2.5, 6, 7.5 and 2.6mm., respectively. For a rise of 
temperature from 10° to 20° the growth rate was thus 
somewhat more than doubled, from 20° to 28° it increased 
25 per cent., and at 33° the rate was nearly the same as 
at 10°. This kind of a relation between the growth-rate and 
temperature was of course to be expected, and interest in 
this research will lie largely in the differences between the 
temperature-growth curves of the different fungi, especially 
in the differences between their optimum temperatures for 
growth. 
Although bacteria and fungi, as studied by other work- 
ers, appear to exhibit gradually diminished growth-rates 
when temperature and the other environmental conditions 
are maintained unchanged for a long time, yet no such slow- 
ing down of growth has been encountered with this Pythia- 
cystis; for example, the same growth-rate has been observed 
to continue unchanged for a period of eight days or more. 
