PRESIDENTIAL ADDRESS. 67 



The claims that have been made regarding the functions 

 of the protozoa, have led investigators to endeavour to count 

 their number in the soil. With one exception, Remy's 

 method has been adopted of shaking the soil with water, and 

 diluting the suspensions until there is reached a limit 

 beyond which no protozoa develop. The method is useful 

 ior the enumeration of bacteria or vegetating protozoa, but 

 the principle is useless for counting the encysted protozoa. 

 It does not appear to have been observed that when certain 

 protozoa, such as the ciliates and amoebae, encyst, they 

 extrude a quantity of slime by which they attach them- 

 selves to solid particles or surfaces. They occasionally 

 appear to aggregate together, but this may be due to some 

 inequality of the surface. However, I am convinced from 

 my observations, that any method of enumerating protozoa 

 by means of aqueous suspensions is doomed to failure. This 

 has been the experience of an investigator, the reference 

 to whose paper has been mislaid. He found that fairly 

 concordant results could be obtained in no other way than 

 by adding definite weights of soil, even to fractions of a 

 milligram, directly to nutritive fluids. 



The function of the Protozoa. 



Since the evidence shows that the protozoa do not destroy 

 the actual numbers of bacteria in the soil, they must have 

 another function to perform. This is probably the destruc- 

 tion of certain forms of organic matter. In experimenting 

 with irrigated soils, Wolff found that as soon as the air- 

 spaces of the soil were filled up with water, the protozoa 

 grew with a new intensity. He suggested that their func- 

 tion was to take up and destroy moulds, algae and bacteria, 

 to withdraw important substances from the soil water, and 

 to preserve them by assimilation before they sank into the 

 subsoil, and to exert a solvent action through the acid 

 products of metabolism. We have no evidence regarding 



