. 5 
76 Winifred E. Brenchley. 
of weeds is all to the advantage of the crop—the removal of the 
aerial competition enables the crop to forge ahead far more rapidly 
in spite of what the roots of the weeds may be doing in the soil. 
If the weed roots really excreted a poisonous substance, one 
would have expected, when crop and weed were grown in con¬ 
junction, that the crop would have been less well developed than in 
the absence of weed, or, at least, that it would not have shown any 
increased growth. Instead of this, in nearly every case, the crop 
associated with the weed showed marked individual improvement, 
which points to the fact that the vital factor in Competition is the 
mere presence of other plants, be they what they may, and that 
up to a certain limit, two plants cannot make such good individual 
growth in a given restricted area as can one plant. 
A comparison of the dry weights, pot for pot, for the two years 
1915 and 1916 does not show any definite correlation, in that it is 
not apparent that the crop obtained from any individual pot in 
1915 in any way affected that obtained in 1916, so that the 
differences between the pots in each set of six must be regarded 
as accidental, and not as due to any inherent quality of the soil in 
the various pots. This is a further proof of the absence of toxic 
effects from the roots. If toxins had been present a pot which 
carried a relatively small crop in 1915 might have been expected 
to carry a relatively large one in 1916 and vice versa, as the large 
crop would have left a larger supply of toxin in the soil. As 
no such correlation was proved it seems evident that no toxin 
capable of remaining unchanged from one season to the other was 
present in the soils. 
