BY R. GREIG-SMITH. 639 



The growth of the bacteria in the extract was compared with the 

 growth in the water used in preparing the extracts, both extract 

 and water being primarily freed from bacteria l>y filtration 

 through porcelain. 



At the end of the experiment, the soil was tested for protozoa, 

 and representatives of the three classes, ciliates, monads, and 

 amcebse, were found. 



The experiment is similar to others in which the development 

 of toxicity is not so marked as the lessened nutritive action. It 

 is possible that the preliminary heating played some part in pre- 

 venting the development of toxins in sufficient quantity to show 

 a direct toxic action, such as was obtained in the moisture- 

 experiment (p. 636, antea). 



Soils seem to develop toxicity much more readily under natural 

 conditions than when bottled up in the laboratory. In casting 

 about for a reason for a difference in the behaviour, one noted 

 that the garden-soil had a covering of grass, which was absent in 

 the experimental soil. This may be a reason for the difference, 

 for the Duke of Bedford and Pickering have recently shown that 

 the leachings of grassed soils are toxic to fruit-trees, to various 

 plants, and to grass itself. They consider that these toxins are 

 not secreted by the plant-roots directly, but result from the 

 decay of the debris of the growing roots. Thus the bacteria are 

 brought into the matter, and an increase of root-debris should 

 mean a greater bacterial fermentation and increased amount of 

 bacterial products, toxic to the bacteria themselves and to plants. 

 Whether these two are the same or not, has yet to be determined, 

 but, at present, these authors have indicated a relation between 

 plant-growth and bacteriotoxins. This relation was to be ex- 

 pected, as any substance which increases bacterial growth will 

 necessarily increase the bacterial by-products. 



In commenting upon these experiments of the Duke of Bed- 

 ford and Pickering, Miller* says, "It does not seem very clear 

 why the leachings from the trays are injurious to the plants in 

 the pots .... whilst it is without action, as soil-solution, before 

 it drains out of its own pot." The leachings are really unfiltered 

 * Annual Reports Progress of Chemistry, Chem. See, 1914, 232. 



