6 4 
similar trees without grass. Corresponding experiments have been 
made vrith 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 
permanently 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 alteration 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 stow 
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 
