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at 22° C. and for seven days at 30° ('., determinations of total numberi- 

 of protozoa and cysts being made before each change of temperature. 

 Table 6 shows the results. 



It will be noted that after nine days at 5-7° i'. practically no change 

 from the original numbers is observed. This is as was to be expected, 

 for the temperature was about the same as that to which the soil had 

 been exposed in the garden and the only change in the conditions was 

 that the soil in the flowerpot had received about 3 % more water 

 than was present in the plot from which it was taken. But after a 

 period of seven days at 22° C. quite a considerable increase in the total 

 number has taken place while the cysts have remained practically 

 stationary. Exposure to a temperature of 30° C. for seven days has 

 caused a fall in the total numbers but a distinct rise in the number of 

 cysts. The fall in the total numbers is readily explained when one 

 bears in mind that certain of the soil protozoa in active form are killed 

 by a temperature of 25° C. Doflein^ refers to the work of Grosse- 

 Allermann who showed that Amoeba lerricola (Greet) is killed after a 

 few hours at 25° C. But apart from this 30° C. is evidently too high 

 a temperature to allow of the activity of quite a number of the protozoa 

 in soils as is shown by the increase in the number of cysts. As the 

 result of these experiments, therefore, a temperature in the neighbour- 

 hood of 22° C. seems to be the most suitable for the activity of the 

 majority of the soil protozoa. 



But although 22° C. is the optimum for the majority of the protozoa 

 in soils, it does not exclude the possibility of the presence of other 

 organisms adapted to higher temperatures. In order to try to throw 

 some light on this point a further experiment was undertaken. The 

 protozoal content of a sample of soil which had been saturated with 

 water and kept for eight days at 22° C. was determined. The soil 

 was then placed in the 30° C. incubator for 38 days, during which time 

 it was kept saturated with water. Determinations of the numbers of 

 protozoa in the soil after eight and 38 days respectively were made. 

 For all three determinations quantities of the soil corresponding to the 

 same dry weight were employed so that the figures in Table 7 are 

 comparable. 



A fall in the total numbers of protozoa is observed after eight days 

 at 30° C. as was to be expected from the results given above. But 

 later the organisms which are adapted to the higher temperature show 

 a marked increase in numbers. It is evident, therefore, that soil 



' Lehrb. d. Protonoenkumle, p. 319. 



