The Effect of Weeds upon Cereal Crops. 
59 
capacity for producing fruiting heads or spikes. The wheat, on the 
contrary, gained by the association, being 60—85% heavier, plant 
for plant. This shows that the competition is not confined to that 
between wheat and weed, but that it also exists between the wheat 
plants themselves. When the weeds are so badly handicapped that 
they make little growth, the wheat plants have a comparatively clear 
field and are able to make more growth than when the place of the 
weed is taken by another wheat plant. 
As the spurrey made so very little growth when with wheat it 
might have been expected that the average dry weight of the wheat 
would have increased more than the 65% that it actually did, seeing 
that with poppy, on soil A, the increase reached 85% in the presence 
of a greater amount of weed. This may possibly be accounted for 
in two ways :— 
(1) Soil A is far more favourable to the growth of wheat, so that 
probably greater increase in development was possible in it. 
(2) The spurrey grew fairly well at first, and its habit is to spread 
and cover the surface of the ground, unlike poppies which grow 
erect. It may be that in the early stages of growth the spurry 
offered a real competition to the young wheat plants, checking them 
to a certain degree by robbing them of some amount of light. If 
such a preliminary check did take place, it is conceivable that its 
influence might be felt all through the life of the plant, 
so that the latter could never develop quite so well as if it had 
grown unhindered from the start. 
Brassica and wheat, under the conditions of this experiment, 
seemed to be on equal terms. It is evident that competition exists 
between the Brassica plants as between the wheat plants, and in 
much the same degree, so that when the two were grown together 
the basis of competition was simply shifted without any further 
detriment to the growth of the individual plants. 
From this first experiment there is no evidence that any factor 
is in play other than that of competition, as all the phenomena can 
be explained by the fact of competition without the necessity for 
presupposing any more active factor such as that of toxic root 
excretions. 
Experiment 2, 1913. 
As before described (Experiment 1) the soil was returned to the 
original pots when the roots had been removed, and the pots so 
prepared were utilised for a second crop in the same year. Buck- 
