56 



R. GREIG-SMITH, 



and, at the same time, the safeguarding of the conclusion, 

 in case the hypothesis should prove to be faulty. I was also 

 impressed with the fact that the hypothesis was based 

 upon the supposed absence of toxins in the soil. I could not 

 believe that toxins really were absent, for it is a well- 

 known fact in bacteriology, that bacteria growing in a 

 nutritive medium, and the soil moisture must be considered 

 as such, give rise to waste- or by-products, and unless these 

 are removed or destroyed, they increase to an extent which 

 ultimately poisons, checks or inhibits further growth. 

 Some bacteria may produce lysins, which dissolve the 

 formed cells, others may give rise to acids, others to poisons 

 and so on. It is impossible to obtain a culture of bacteria 

 in a medium in which all the nutriment has been converted 

 into bacterial protoplasm, that is to say, the growth of 

 the bacteria increases to a maximum, when it is slowed 

 down by the action of the accumulated by-products, and 

 these in time weaken and eventually kill off the cells, and 

 the culture becomes dead. The word "toxin" is a general 

 name for an injurious product, but Russell and his col- 

 leagues, while denying the presence of "toxins" in soils, 

 still admit that there may be "inhibiting substances." I 

 cannot see why there should be any discrimination until 

 the nature of the toxins or inhibiting substances is 

 determined. 



To argue that because the delicate nitrate bacteria can 

 grow in soil, there can be no toxins present or they would 

 be destroyed, appeared to be wrong, and I set about to 

 prove, that the general bacteriological knowledge regarding 

 the presence of injurious bacterial by-products in a nutritive 

 medium, such as the soil moisture, was not at fault. This 

 I succeeded in doing by the use of porcelain-filtered extracts 

 of the soil. The extract, after being filtered through por- 

 celain, was seeded with known numbers of bacteria which 

 after a time were counted. A loss showed a toxic effect. 



