PRESIDENTIAL ADDRESS. 75 



A probable indirect production of toxicity has been shown 

 to occur in the case of the leachings of soil which has been 

 growing plants. The Duke of Norfolk and Pickering, x after 

 noting that grass under growing fruit-trees caused the trees 

 to grow badly, found that the leachings from soil growing 

 grass were injurious to growing trees and other plants. 

 Similarly the leachings from clover soils were injurious to 

 tobacco, tomatoes and mustard, and to clover itself. 

 Indeed it appears that the leachings from soil growing 

 any kind of plant is toxic to the plant itself and to all other 

 plants. They did not believe that the toxic substances 

 were secreted directly by the growing plants, but that they 

 were derived from the decay of the plant debris. Thus 

 the soil bacteria, or at any rate the soil micro-organisms, 

 which produce the decay of the vegetable matter, are 

 inferentially considered to be the active agents in the 

 formation of the plant toxins. 



There can be little doubt that the life in the soil is inter- 

 related. The plant has an influence on the bacterial flora 

 and upon other plants ; the bacteria influence the growth 

 of plants, of other bacteria, of moulds, and of protozoa, and 

 this is entirely caused by the secretion or excretion of 

 various bodies of an acid, a nutritive, a toxic or an enzymic 

 nature. The amount of each of these relative to the others 

 and the alterability of one to the other is of importance. 

 The nutritive quality of the soil is naturally of paramount 

 importance, for the food of the bacteria, protozoa or of 

 plants, depends upon this factor, and doubtless it is also 

 responsible for the nature of the bacterial flora and protozoa 

 fauna. But behind all this there is the action of the grow- 

 ing plant, for we see from the occurrence of soil sickness 

 that the crop influences the balance of the bacterial flora. 

 The bacteria again are influenced in their activities by the 



1 Joum. Agri. Sci., 6, 136. 



