38 



surface film of water on the ag'ar, and (ii.) the rate of multiplica- 

 tion previous to the drying- of this film. In the present medium 

 this rate of multiplication has been much reduced so that spreading- 

 colonies are greatly restricted. The medium has the following 

 composition:— K.HPO^, 1.0 gram; MgSO^, 7 H.O, 0.2 grs. ; 

 CaCU, 0.1 gr. ; NaCl, 0.1 gr. ; FeClg, .002 grs. ; KNO3, 0.5 grs. 

 Asparagine, 0.5 grs. ; mannitol, 1.0 gram; agar, 15.0 grs. ; water 

 to 1000 cc. Reaction brought to Ph 7.4 before sterilisation. 

 (For the rigid test of this medium, see Paper XIV., p. 35.) 



PROTOZOA. 



XX. D. W. Cutler, Lettice M. Crump, and H. Sandon. 



''A Quantitative Investigation of the Bacterial and 

 Protozoan Population of the Soil," Phil. Trans. 

 Roy. Soc, London, B., 1922. Vol. CCXI. pp. 

 317-350. 



The results of 365 consecutive daily counts of the numbers of 

 bacteria and of six species of protozoa in a normal field soil are 

 given, and the methods of counting bacteria and protozoa are 

 described. 



The numbers of both bacteria and protozoa rarely remain the 

 same from one day to the next. The fluctuations are very great, 

 but it has not been found possible to connect them with meteoro- 

 logical or general soil conditions. 



Fourteen-day averages of the daily numbers demonstrate that 

 well-marked seasonal changes in the soil population are super- 

 imposed on the daily variations in numbers. In general, both 

 bacteria and protozoa are most numerous at the end of November 

 and fewest in February. These changes are not directly influenced 

 by temperature or rainfall, but show a similarity to the seasonal 

 fluctuations recorded for many acquatic organisms. 



There is a slight tendency for the various species of flagellates 

 to fluctuate together from day to day, but this is not shown by 

 the two species of amoebae. 



An inverse relationship is found between the numbers of 

 bacteria and active amoebai in 86% of the total observations. 



A two-day periodicity obtains for the active numbers of one 

 species of flagellate {Oiconionas termo). 



XXI. D. W. Cutler. ''The Action of Protozoa on Bacteria 



when Inoculated into Sterile Soil." Annals of 

 Applied Biology, 1923. Vol. X. pp. 137-141. 



Soil sterilized by heat was inoculated with : — 



(a) Bacteria alone ; 



(h) ,, + one species of amoeba; 



(c) ,, + one species of flagellate. 



Daily bacterial counts made on each portion of soil showed that 

 the one containing no protozoa sustained a greater number of 

 bacteria than those containing protozoa. Also the bacteria in the 

 |)rotozoa free soil did not exhibit the fluctuations in numbers 

 characteristic of soil in which protozoa were living. 



