194 Temperature and Growth of Fungi [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 33 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. 



