Pickering . — The Effect of one Plant on Another. 183 
elusion that any particular kinds of plants are more susceptible than others, 
or that any particular surface crop is more toxic than another ; that such 
differences exist is highly probable, but all the variations observed so far 
may be explained by the greater or lesser vigour of the plants in the 
particular experiments in question. Similarly as regards the effect of grass 
on fruit-trees, though the extent of it varies very greatly, and in many 
soils is certainly small, we must hesitate to attribute this to any specific 
properties of the soils in question ; for when soils from different localities 
(including those from places where the grass effect is small) have been 
examined in pot experiments, they have all given very similar results ; and 
this applies equally to cases where pure sand, with the addition of artificial 
nutrients, has been taken as the medium of growth. 
In searching for an explanation of the effect of grass on trees, various 
possibilities suggested themselves, and these were excluded, one by one, till 
the only possibility left was that of the formation of some deleterious 
substance by the growing grass. It would be impossible in this short com- 
munication to give any account of all the suggestions which were negatived, 
but these included the robbing of the tree of necessary moisture and food by 
the grass, alterations in the temperature, alkalinity or physical condition of 
the soil, and alterations in its carbon dioxide and bacterial contents, the 
exclusion of all which suggestions is embraced in experiments such as those 
mentioned above, where the grass, or other crop, is grown in a separate 
vessel, merely resting on the surface of the ground, without any possibility 
of it extracting anything from the soil in which the plant affected is 
growing. 
From the outset the behaviour of trees in grassed land suggested the 
action of some toxin : not only is the growth arrested, but a peculiar 
alteration in the colouring of the bark, leaves and fruit occurs, unlike that 
attending other forms of ill-treatment : indeed, the high colour developed by 
fruit under grass is, in some cases, so great, that expert fruit-growers have 
been unable to correctly name the varieties after being affected, and if this 
action of grass could be limited, and suitably adjusted to every tree, it would 
prove beneficial from the point of view of the fruit-grower, if not from that of 
the tree itself, especially as a limited check to the growth of a tree generally 
results in heavy cropping. The extent of the grass action which brings 
about these notable colour changes is very small, for they are apparent in 
cases of trees weighing about two hundredweight when only three to six 
ounces of their roots extend into grassed ground. Such an effect is in itself 
strongly suggestive of toxic action. 
To some agricultural chemists the mention of a toxin as being formed 
in the soil by a growing plant is as a red rag to a bull, chiefly, perhaps, 
because it conjures up the picture of the plant ejecting some virulent poison ; 
but, though the excretion of toxin from the roots is possible, there is no 
