444 Mr. T. Goodey. Investigations on Protozoa in 



The V. microstoma curve is also very interesting and shows the influence 

 of some factor which had become operative by, the end of 63 days and which 

 subsequently kept the numbers of bacteria in check, though only at about the 

 same level as the untreated soil. 



The untreated curve also shows that the bacteria have gradually decreased 

 in numbers after reaching and maintaining a high level for 181 days. These 

 results are very interesting when considered in relation to the number of 

 protozoa in the soils. 



In the untreated 1846 soil no protozoa are present, so that the gradual 

 decrease [in the number of bacteria cannot be due to the activity of the 

 protozoa. The V. microstoma soil, however, contained, a few weeks after 

 inociilation, about 300 vorticellae per gramme. By December, 1913, however, 

 all the vorticellae had died out, for they failed to appear in cultures of the 

 soil made at that time and on all occasions since.* Flagellates and amoebae 

 are present in this soil, probably due to infection of the mass culture or 

 during the initial air-drying of the inoculated soils, to the extent of 1000 

 flagellates and 100-200 amoebae per gramme. 



It is probable that these lead an active trophic existence in the soil and so 

 might be considered responsible for the limiting action on the bacteria. This 

 can scarcely be the case, however, when we consider this soil in relation 

 to the bacteria-inoculated soil. In the latter there are flagellates present 

 to the extent of about 100 per gramme. If now the limiting factor in 

 this soil is considered as due to the action of these flagellates, we should 

 expect to find not so great a decrease in the bacterial numbers as in the 

 vorticella soil, where the flagellates are more than ten times as numerous, 

 and where there are, in addition, 100-200 amoebae per gramme. The 

 reverse of this is the ascertained result, and clearly negatives the idea 

 that the flagellates and amoebae are responsible for the limiting action on 

 the bacteria. 



Another point of interest is that the two curves for the untreated and 



* This dying out of the Vorticella microstoma is very interesting. At first I thought 

 its failure to appear might be due to an unsuitable culture medium. I, therefore, tried 

 to obtain it again, taking care of the reaction of the hay-infusion, but with no better 

 success. It also failed to appear on a nutrient bouillon agar, favourable to the growth of 

 all the other protozoa under consideration. I afterwards remembered that in some 

 earlier experiments I had failed to obtain Vorticella from a soil which had been kept in 

 the laboratory for some months and from which I had obtained a very fine culture of the 

 organism when the soil was fresh. At another time too I failed to get the excystation of 

 V. microstoma from cysts which I had obtained in a hay-infusion culture and which had 

 been stored for a few months. From this evidence it would appear that V. microstoma in 

 its encysted condition does not retain its vitality for more than a few months, and the 

 dying out in my soil is easily accounted for on this supposition. 



