The Effect of one Plant on Another. 
BY 
SPENCER PICKERING. 
With three Figures in the Text. 
F IG. 3 shows a pot with two mustard plants growing in earth, on the 
surface of which rested a perforated tray with five inches of earth in it; 
through this all the water given to the plants percolated. The tray has 
been removed so as to be more clearly visible in the photograph. The 
presence of such a tray makes practically no difference in the behaviour of 
the plants in the pot below. Figs, i and 2 show a like arrangement, 1 but 
with a crop of mustard growing in the trays : in one case (Fig. i) the effect 
has been to reduce the plants in the pot to one-hundredth of their 
normal size ; in the other (Fig. 2) there has been no effect. The only 
difference in the two cases is that in Fig. 1 the washings from the surface 
growth were allowed to reach the plants in the pot (though the penetration 
of any roots through the perforations of the trays was prevented by a layer 
of very fine metal gauze at the bottoms of the trays), whilst, in the other 
(Fig. 2), no washings passed, the holes in the trays having been blocked, and 
such water as the plants required was given to them direct. The conclusion 
is obvious : the teachings from the plants growing in the trays must contain 
something which is toxic to other plant-growth. 
Such a very simple experiment must definitely settle the question of 
toxin production, and should have been made long ago, 2 but as a matter of 
fact it comes only as the (at present) final step in a series which originated 
in 1895 in observations on the effect of grass on fruit-trees. As so often 
happens, we start with the more complex problems, and only gradually 
work down to the simpler ones. 
It has now been established with a reasonable amount of certainty that 
the deleterious effect of one growing plant on another is a general pheno- 
menon. By means chiefly of pot experiments such as those indicated above, 
the following plants have been found susceptible to such influence : apples, 
1 In Fig. 2 there appears to be only one plant : this is due to a fault in the photograph. 
2 The details of this, and many of the experiments alluded to below, have not yet been pub- 
lished : a description of the other work on the subject will be found in the Third, Thirteenth and 
Fourteenth Reports of the Woburn Experimental Fruit Farm, 1903, 1911 and 1914. 
[Annuls of Botany, VoJ. XXXI. No. CXXII. April, 1917.] 
O 
