Pickering . — The Effect of one Plant on Another. 185 
may suffer from trees above it, even in cases where there certainly has 
been an increase in fertility, and where, also, the shading effect is inoperative, 
so that the damage to the crop can only be attributed to the toxic action 
of the trees. Thus, a quarter of an acre of land over which some fifteen 
apple-trees, twenty years of age, were distributed, was planted uniformly 
with Brussels sprouts : those under the trees suffered to the extent of 
48 per cent, in their growth ; but there were patches in the ground where 
trees had been growing until the preceding winter, when they had been 
cut down, leaving the roots undisturbed in the soil, and in these patches 
the sprouts did better than elsewhere to the extent of 12 per cent. In 
other parts of the ground canvas screens had been erected, at a height of 
6 feet above the surface, to simulate, and even exaggerate, the shading of 
the trees, and under these the sprouts gave exactly the same values as on 
the unshaded ground. Thus, the trees themselves materially injured the 
crop, though the soil under the trees was more fertile than elsewhere, and 
though the shading was inoperative. 
Though differences in the toxicity and in the susceptibility of different 
plants may be overshadowed by differences due to other causes, it is 
highly probable, as has already been mentioned, that such differences do 
exist. The only case of differences of a positive character noticed at present 
in our experiments, is that the effect of a plant on plants of its own kind 
is generally greater than that on plants of another kind. This may be 
fallacious ; but, certainly, a plant affects its own kind just as much as any 
other kind ; and hence we must conclude that the toxin formed by any 
individual plant will affect that individual itself. This has been proved 
by growing plants in pots divided into compartments, so that there was 
no root interference, and comparing these with other plants grown in similar 
pots not so divided : in the former case each plant will be affected only by 
the toxin produced by itself, in the latter it is affected partially by its own 
toxin, and partially by that of its neighbour, but the amount per plant 
must be the same in both cases, and, as a matter of fact, the plants all 
gave the same results, except for a slight advantage in favour of those 
in undivided pots, due to conditions which can be easily specified. 
When a stronger and weaker plant, or an older and younger one, 
are growing side by side, we find that the latter rarely picks up, and 
generally gets more and more behind its stronger brother. This cannot 
be due to the stronger one monopolizing the food-supply ; for if it exhausted 
this supply, both plants would suffer at the same time, and, till that supply 
is exhausted, both would flourish equally. The inadequacy of any such 
explanation is demonstrated by taking a pot of soil capable of growing, 
say, six plants, sowing the seed for three of them first, and that for the 
other three a certain number of days later. In the case of mustard, when 
the difference of date is only four days, it is found that, at the end of 
