378 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
Two other experiments in this series are quoted, though they do not 
bear directly on the particular point now being dealt with : in one the 
grass was allowed to re-establish itself gradually, instead, of the turf 
being replaced at once, the result being that the deleterious effect 
also became established only gradually ; in the other the turf was 
replaced, but was fed off first by sheep, and then by poultry ; yet this 
feeding off effected no reduction in the toxic action except in the last 
year, when the reduction was explicable by the grass above two of 
the trees having been almost entirely killed by the poultry. 
The action of one plant on another being a general one, it follows 
that trees should be prejudicial to grass, just as grass is prejudicial 
to trees. We have been able to establish this by pot experiments, in 
which grass was grown in the pots, and apple seedlings grown as a 
surface crop in the trays. That in ordinary practice crops under trees 
suffer is well known, and the following experiment shows that this 
cannot be fully accounted for, either by the shading due to the trees, 
or by the exhaustion of the ground by their roots. A plot of ground 
which was partially and irregularly occupied with fruit trees of differ- 
ent sizes, mostly standard apple trees, was selected, and, after due pre- 
paration, was planted with Brussels sprouts. The areas of the patches 
of ground under the trees varied from four to eighteen feet square, 
and similar patches were selected in parts of the ground where there 
were no trees, and a shading effect, certainly greater than that 
produced by the trees themselves, effected by erecting over them 
canvas screens. At the same time other patches of the ground were 
marked off where trees had been growing, but had been cut down just 
before the sprouts were planted, the roots being left undisturbed in 
the soil. The results of two years' observations showed that the 
sprouts under the trees were only 70 and 50 per cent, as vigorous as 
those in the unoccupied ground, but that this reduction in vigour 
was not in any way attributable to the shading, for those under the 
screens showed no reduction at all ; nor could it be attributed to the 
exhaustion of the soil by the growth of the trees in it during the 
previous twenty years, for the sprouts planted on the sites from which 
the trees had been removed gave, on the average, exactly the same 
values as those planted in ground where no trees had been growing ; 
indeed, during the first season, they actually flourished better. 
Relative Weights of Brussels Sprouts. 
Situation. 
Open ground 
Under trees 
Under screens . 
On former tree sites 
1916. 
100 
72 
100 
112 
100 
52 
103 
89 
Important evidence bearing on the question of toxin production 
by plant-growth is afforded by the results obtained on investigating 
the character and behaviour of soil which has been heated. 
