NOTES AND ABSTRACTS. 



141 



planted trees (p. 27). The physical alteration of the soil as a 

 possible cause of the action of grass has been examined, but with 

 negative results (pp. 106-122). 



The question of soil bacteria was also partially examined (p. 123), 

 and the conclusion arrived at that neither the mere number of soil 

 bacteria, nor the absence of those of a beneficial character, could 

 explain the grass-effect; while the absence of any ill effect from grass 

 leachings, or from soil taken from grassed ground, militates against the 

 theory that it may be due to the presence of maleficent bacteria. 



Trees were grown in soil partially sterilized by heating, and they 

 were found to behave in the same way as other plants, being later in 

 starting into growth but growing better afterwards (p. 124). The 

 destruction of the greater part of the bacteria and the whole of certain 

 protozoa which feed upon them results in the unchecked multiplication 

 of the bacteria after a time, so that the soil becomes richer in bacteria 

 and in the nitrates formed by them. This alone does not explain the 

 delayed germination of seeds in soil which has been heated (pp. 

 127-30). The heating also results in the production of some substance 

 which is actively toxic to plant growth, and plants will not flourish, 

 or seeds even germinate, in a soil while it is present in sufficient 

 quantity. The toxin is rapidly oxidized by the action of air and 

 moisture and is destroyed under cultivation in a few weeks, after which 

 plants grow better in it than in soil which has not been heated. Thus 

 plants may behave in diametrically opposite ways in soils which have 

 been heated according to the conditions under which they are grown 

 (p. 126). It was observed that the soil also from grassed ground was 

 more favourable to the germination of seeds than that from tilled 

 ground. Another point of resemblance was that soils which had been 

 heated were less readily wetted with water than unheated soils, the 

 water standing on them as if they were oiled, and the same difficulty 

 in wetting was conspicuous in several cases of the soil from grassed 

 land as compared with that from tilled (p. 141). It was found that 

 pot-grown trees, when watered with the leachings obtained from trays 

 containing grass growing in sand, flourished more than when water 

 alone was supplied (p. 87); but when the trays, which were made 

 specially to fit the pots, were placed on the surface of the soil in such 

 a Way that the separation of the medium in which the grass was grow- 

 ing from that in which the trees were growing was almost, if not quite, 

 complete, and the leachings from the grass reached the tree roots with 

 practically no exposure to the air (p. 150), they had a very deleterious 

 effect (except in the case of the trees growing in sand), nearly as great 

 as when the grass was growing above the roots in the ordinary way 

 (p. 94). 



All these observations point to the conclusion that a toxic substance 

 IS formed by the grass which is readily oxidizable into some substance 

 which favours plant growth, the injurious effect upon trees being due 

 to the fact that the toxic substance is brought into contact with the 

 roots before it can be oxidized (p. 151). The most weighty evidence on 



