1912.] 



A Note on the Protozoa from Stck Soils. 



395 



must, however, be evident that the attempt to solve the problem of how far 

 any protozoon is really doing damage under the normal soil conditions from 

 a study of the forms grown on agar-plates must be a very difficult and 

 dangerous one. It is more than probable that the sample of population 

 cultivated under all the present methods may not by any means be a fair 

 representation of the population of the given soil. On the other hand, it is, 

 I think, important to observe that one finds that each small sample of a given 

 soil for each culture-condition has given rise to a fairly constant specific fauna, 

 whereas samples on other soils under these conditions have also given rise to 

 constant but quite distinct faunas. If these facts are taken into consideration, 

 it seems hard to believe that all these forms are a purely negligible factor in 

 the question of soil-sickness, since it is important to note that many of these 

 forms are new and — a point of difference from the ciliates — have never 

 previously been found in fresh-water pools or infusions. It is, therefore, 

 unlikely that all these organisms are present in the soil as cysts, because they 

 must presumably lead a trophic life somewhere, and it seems probable that 

 their trophic activity is a cause of soil-sickness. On the other hand, it is 

 quite possible that the form or forms which are most dangerous in cases of 

 soil-sickness may not up to the present have survived under the conditions of 

 plate-cultures. I do not think that this view is probable, but the question 

 can, of course, only be decided by parallel cultures of three kinds : (1) Plants 

 grown on the sterilised soil ; (2) plants grown on the sterilised soil to which 

 pure cultures of the various protozoa, with the bacteria on which they feed, 

 have been added ; and (3) plants grown on the sterilised soil to which cultures 

 of the protozoa without the bacteria have been added. 



As regards methods of preparation, wet cover-slip smears were solely used. 

 In this connection a device suggested to me by Mr. Jamieson, of the Natural 

 History Department, Glasgow University, has proved extremely useful. It 

 is obvious that with a large number of cultures on one's hands a number of 

 smears must be fixed which cannot be immediately stained. The smears in 

 these cases after fixation were stacked in tubes in 70-per-cent. alchol. It 

 was found, however, in some cases, after the smears had been some time in 

 the tubes, that in the case of thin smears it was extraordinarily hard to tell 

 the right from the wrong side of the cover-slip. I therefore got some oblong 

 cover-slips of which one corner was cut off, made by Messrs. Frazer, of 

 Edinburgh. It will, I think, be obvious that, if one makes up one's mind to 

 always have the smear on the lower surface of the cover-slip when the top 

 right-hand corner is cut, no mistake of this kind can possibly arise. 



I should like to take this opportunity of thanking Dr. Russel for his 

 kindness in sending me samples of soil, and for much advice, and also 



VOL. LXXXV. — B. 2 E 



