64 



similar trees without grass. Corresponding experiments have been 

 made with trees planted in the open. Though increase of moisture 

 up to a certain point, and increase of food in certain cases, may 

 benefit the trees, the benefit is much too small to do more than very 

 slightly diminish the deleterious effect of the grass. 



The behaviour of a tree in grass is clearly a case of starvation 

 in a land of plenty, and this cannot be explained by supposing 

 (untenable as such a supposition is for other reasons) that the grass 

 roots suck up whatever nourishing solution there is in the soil, 

 leaving none for the tree roots. The pot experiments, just quoted, 

 effectively negative this. Nor can we explain the matter by supposing 

 that the tree was only temporarily affected by the grass, but being in 

 a weak state after transplanting this check resulted in its becoming 

 perma'nently stunted ; for a precisely similar, and even more marked 

 effect has been proved to be produced by grassing over trees which 

 have been established, in one case for four years, and in another case 

 for 12 years ; the effect, indeed, was so great that, in the first instance, 

 many- of the trees have been killed, and, in the second instance, 

 a similar result appears imminent. 



Other explanations which suggested themselves have been 

 investigated, and found equally unacceptable : these were differences 

 in soil temperature, differences in aeration or proportion or carbon 

 dioxide, and difference in the physical condition of the soil. The only 

 other explanation which appears to be possible is that the growth of 

 the grass results in the formation of some substance which is 

 poisonous to the tree. This may be an active poison — a toxin — or the 

 poisonous action may result from an altrration in the proportion 

 of various substance> present in the soil. An active poison may be 

 produced in various ways, such as by the decomposition of the debris 

 of the grass, actual excretion from the grass roots, or as a product of 

 the bacteria present in the soil. As to the origin of the toxin no 

 definite evidence has yet been obtained, but it has been found that 

 toxins may be formed in soils by heat, and other means, producing 

 effects which are analogous in many respects with those produced by 

 grass on trees. Thus, on heating soil, substances are produced which 

 are toxic towards the germination of seeds, and these have been found 

 to be toxic towards plant growth also. That established plants erow 

 better in heated than in unheated soil, is due to the fact that heating 

 causes a considerable increase in the soluble nitrogen present in the 

 soil, and also in the composition of the bacterial flora of the soil. 

 Moreover, the toxin formed as the result of heating the soil soon 

 becomes oxidised and destroyed, allowing the favourable conditions 

 to assert themselves. If, however, the toxin is present in sufficient 

 quantity, it is not all destroyed before the plant grows, and its dele- 

 terious effect becomes apparent. It is noticeable that this effect varies 

 greatly in different cases, and is very much less in the case of grasses 

 than in that of the other plants which have been examined. Earth 

 from grassed ground behaves in the same way as earth which has 

 been slightly heated and which contains only a limited amount of 



