The Effect of Weeds upon Cereal Crops. 
75 
The weight of the Alopecurus grown with wheat must have 
been very small, so that the wheat plant itself gained considerably, 
though less than when spurrey was the competing plant. The 
Alopecurus developed strongly at first, so that it was probably a 
real competitive factor at the outset. As it died off later without 
making any considerable growth, the active competition was largely 
withdrawn, but even so the wheat failed to make any such great 
improvement as it did in the case of spurrey, in which the competing 
plant made even less growth and died off sooner. This seems to 
indicate either:— 
(1) that the Alopecurus was a very real competitive factor 
at the beginning, so that the wheat suffered such an initial check 
that even when the Alopecurus died off it was unable to make up 
the lee-way so well as it did when grown with spurrey, or— 
(2) that the Alopecurus, as well as or instead of being 
merely an initial competitive factor, so working its mischief, was in 
some way actively inimical to the wheat plant, so that when the 
competition was removed and the wheat had apparently full 
opportunity to push ahead, the deleterious factor still remained in 
play and prevented the development otherwise possible. 
CONCLUSIONS. 
All things considered, in the light of the water culture and soil 
culture experiments, it seems probable that the first of the above 
explanations is the true one. There is no evidence, and indeed no 
indication, that any direct toxic action comes into play. It is 
evident that the mere competition of plant with plant, irrespective 
of species, has much to do with development and that the time 
and duration of competitive check are the chief factors involved. 
Negative results are necessarily inconclusive, so that it is still 
impossible to say definitely whether or not such plants as Alope¬ 
curus are harmful to crops in any other way than that of direct 
“vegetative” competition. It is clear that such competition is 
more potent than is generally realised. In the experiments all the 
crop plants were at least 4 or 5 inches apart, and the effects of 
overcrowding were most obvious. In a field where weeds are at all 
prevalent, the plants are still more closely placed, and the struggle 
must be still keener. Even when the weeds are suppressed by 
cultivating and hoeing the roots remain in the soil to a large extent, 
and those of the perennials at least continue functioning in a 
normal way, though to a less degree ; nevertheless, such suppression 
