Andreav Ounninghaji 67 



no traces of protozoa, active or encysted, could be found. The protozoa 

 had probably died off without encysting and then been attacked by the 

 bacteria. This view receives support from the frequent observation 

 in ammonifying solutions of protozoa, showing absolutely no signs of 

 life but yet without any traces of a cyst membrane surrounding them. 

 It is quite probable, therefore, that some species of protozoa die off 

 without being able to encyst when the concentration of ammonia or 

 other products of the activity of bacteria reaches a particular level. 

 Their bodies would then be a ready prey to the attacks of bacteria 

 and the latter might increase in numbers as a consequence. 



The reductions in the numbers of bacteria as obtained in these 

 experiments are on the average smaller than those given in Table 14. 

 But it must be remembered that the bacterial content of the protozoa 

 cultures at the beginning was in all cases larger, probably often much 

 larger, than that of the protozoa-free culture. The only satisfactory 

 method for securing comparable results, therefore, is the inoculation 

 of equal numbers of bacteria from a protozoa culture in the one instance 

 and from a protozoa-free culture (prepared from the protozoa culture) 

 in the other, on to fresh media and the determination of bacterial 

 numbers in both solutions at intervals. 



The results given in this section j^rove conclusively that the soil protozoa , 

 in solutions at all events, exercise a very decided limiting effect on the numbers 

 of bacteria. The question of the relative activity in this direction of the 

 three main groups of protozoa — flagellates, ciliates and amoebae — remains 

 to be investigated. 



V. The Influence of Protozoa on Ammonification 

 IN Solution Tests. 



As a preliminary experiment in this direction, the quantities of 

 ammonia produced in some of the cultures used in the last section 

 were determined. The conditions in these cultures may be briefly 

 recapitulated. Each culture contained -4 grm. bloodmeal + -05 grm. 

 K2HPO4 in 100 c.c. water. After sterilisation in the autoclave at two 

 atmospheres pressure, each was inoculated with one loopful of a pro- 

 tozoa-free bloodmeal culture and incubated for two days at 22° C. 

 Some of the cultures then received each one loopful of a bloodmeal 

 protozoa culture from soil. At the end of the incubation period (20 

 days at 22° C.) all were distilled with magnesia and the ammonia 



