GROWTH 217 



quently occurs that a first doubling period is completed at even 35°C. 

 or 36°C. in the minimum time, — i.e., thirty minutes or so — but the 

 second doubling of the same filament will require a longer time; 

 and the third may occupy nearly twice as long, and so on." 



A very good confirmation of the existence of the 

 Tammann principle in the growth at high temperatures 

 is given further by Fawcett (1921). He grew four 

 plant-pathogenic molds on agar at different temperatures, 

 using as inoculum a disk of 1.25 mm. radius cut out of a 

 five day old agar culture of the mold kept at 20°C. 

 This disk was inverted and placed in the center of the 

 surface of a new agar plate containing 15 c.c. of solidified 

 cornmeal agar. The rate of growth was measured by 

 the increase of the radius of the colony. Most of the 

 data are averages of 8-12 separate experiments. 



Table 55 represents the experiments with Phyti- 

 acystis citrophthora. The data show the daily increment, 

 i.e. the rate of growth. The maximum growth rate, 

 indicated by heavier type, shifts from 27.5°C. on the 

 first day to 23.5°C. on the fourth day and remains there. 

 This is probably the optimum temperature for continuous 

 growth, while the initial growth was faster at 27.5°C. 

 The growth rate of molds is nothing very definite, and 

 we observe an increase of the ^^ growth rate" at all 

 temperatures below 30°C. Whether this is a question 

 of lag, or of adjustment, or of recovery from injury by 

 tearing the mycelium in transplanting does not interest 

 here especially. It is of great interest, however, that 

 above 30°C., the growth rates of the second day are 

 much lower than on the first day. This is even more 

 characteristic of the Tammann principle than the 

 shifting of the optimum temperature with time. 



Phytophthora terrestria shows a shift of the optimum temperature 

 from 34.5°C. on the first day to 31°C. on the fifth day. Above 34.5°C. 



