39 



XV. D. W. Cutler. " A Method for Estimating the 

 i\' umber of Active Protozoa in the Soil." Journal of 

 Agricultural Science, 1920. Vol. X. pp. 135-143. 



This method constitutes a great advance on those previously 

 in use, since it discriminates between active and encysted forms ; 

 it has, therefore, been adopted in all the succeeding work. The 

 soil is passed through a 3mm. sieve and two samples of 10 grams 

 each are taken. In one the total number of protozoa (active forms 

 plus cysts) is determined as follows : 10 grams of the sieved soil are 

 added to 100 cc. of sterile tap water or physiological salt solution. 

 This gives a 1 /TO dilution. From it further dilutions are made as 

 shown below. 



Nutrient agar is poured into sterile Petri dishes. When the 

 medium has solidified, the dishes are inoculated in pairs with 1 cc. 

 of each dilution. Incubation at 20"^ is continued for 28 days, and 

 the plates examined at intervals of 7 days, 14 days, 21 days and 

 28 days. This long period of incubation is necessary in order to 

 ensure accurate results. 



In the other 10 gram sample the cysts only are determined, 

 advantage being taken of the fact that they survive treatment with 

 2% hydrochloric acid while active forms do not. The soil is there- 

 fore treated with sufficient 2% HCl to neutralise the carbonate 

 present and still leave an excess of unchanged 2% acid. The acid 

 is allowed to act overnight. After treatment, the number of 

 j)rotozoa in the sample is ascertained by the dilution method ; this 

 gives the number of cysts since the acid has killed all the active 

 forms, leaving most of the cysts unharmed. The number of cysts 

 subtracted from the total number of organisms given by the first 

 count gives the number of active protozoa per gram of the soil 

 sample. 



XVI. D. W. Cutler and L. M. Crump. " Daily Period- 

 icity in the Numbers of Active Soil Flagellates , with 

 a brief note on the Relation of Trophic Amoebce and 

 Bacterial Numbers." Annals of Applied Biologv, 

 1920. Vol. VII. pp. 11-24. 



Using the preceding method, it was found that the numbers of 

 protozoa varied so rapidly that weekly counts did not fairly repre- 

 sent the changes taking place. A series of daily counts was there- 



