376 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
similar way. One or two of the results obtained are illustrated in 
rig. 56. The extent to which surface growth affects a plant varies 
between wide limits, according to the nature and vigour of that growth, 
and the nature and vigour of the plant affected : the smallest effect 
observed has been a reduction of 15 per cent., the greatest, ore of over 
99 per cent. ; the average reduction in these pot experiments is over 
50 per cent., and is approximately equal to that observed in the case 
of the action of grass on trees. 
Special mention may be made of one experiment which appears to 
be a crucial one in establishing the fact that the damage done to the 
plant in the pot is caused solely by the action of something produced 
in the soil by the surface crop. In this there were two sets of pots 
with the perforated trays, as in other cases, one carrying a surface crop, 
the other without such a crop, but there were also two similar sets 
wherein the perforations of the trays had been blocked, so that no 
leachings from the surface crop could reach the plants in the pots 
below. Where the passage of the leachings was thus prevented, 
the presence of the surface crop had no effect on the plant in the pot, 
instead of reducing its vigour by 63 per cent., as was the case where 
the leachings were allowed to reach the plant (fig. 57).* 
Two points of importance must be noticed : it is highly probable, 
both from general considerations, and also from a consideration of 
the results of our experiments, that different plants differ both in their 
susceptibility to the action of surface growth, and to the effect pro- 
duced by them when grown as a surface crop (cf . the different extent 
of the action of grass and clover as illustrated in fig. 56) : but posi- 
tive proof of this is very difficult to obtain, since for any such com- 
parison we must have both plants and surface growth of a like degree 
of vigour in the cases to be compared together. The second point is 
that the action of surface growth is equally apparent when the plant 
affected is of the same nature as that constituting the surface growth, 
as it is when the plants are not the same ; indeed, so far as our results 
can tell, the action of a plant on one of its own nature is even greater 
than its action on one of a different nature ; but, for the same reasons 
as those given above, positive proof of this can hardly be obtained. 
If a plant can affect others of its own nature, it is clear that it must 
affect itself, and it is, therefore, impossible to grow any plant without 
it suffering from its own poison. The plants grown in the pots when 
there is no surface growth in the trays must suffer in this way, and the 
reason why they suffer more when there is growth in the trays, is that 
they are not only subjected to the influence of the toxin produced by 
* In a similar experiment quoted in the Annals of Botany, xxxi. 182, a 
reduction of over 99 per cent, was observed. This is explained by the fact that 
the experiment in question was carried out in the winter, when growth was so slow 
that the toxin had an abnormally long time in which to act upon and stunt the 
young plants. 
It will be seen from the illustration here given that the growth of the plants 
in the pots is not so satisfactory when the trays are not perforated as when 
they are : this is due to the difficulty in properly adjusting the water supply 
in such a case. The plants in the unperfo rated trays also suffer, owing to the 
toxin formed by them not being washed away. 
