F. FLETCHER. 5 
It is to be noted that pending further and more precise 
experiment the figures in Table II are to be considered as indi- 
cating only the order of the influence of various plants on one 
another and that too only under the particular conditions of soil 
and climate under which the experiment was conducted. On 
lighter soils and under a more evenly distributed rainfall the 
percentage reductions are apparently less. With this reservation 
and since the decreased yields are not restored by either irriga- 
tion, manure, aeration or light,* it appears legitimate to draw the 
following conclusions :— 
(1) All plants excrete substances which are toxic both to 
themselves and to other species. 
(2) The quantity of material excreted by the different crops 
varies when reckoned per unit area of a field sown in the ordinary 
way. 
(3) The sensitiveness of crops to the same quantity of the 
excreted substance varies with the variety of the crop. 
(4) The substance excreted by ali crops is apparently 
identical. 
The last statement is made under a further reservation 
pending the isolation in a pure state and analysis of the excreted 
substance or substances. There is, however, nothing in the facts 
so far observed either in the field or in water culture inconsistent 
with the identity of the substance excreted by all plants. On the 
other hand, if the substance excreted varied with the species, 
we should not expect the regularity found in Table II. Thus, 
reading that table vertically, the order of sensitiveness to a given 
amount of the substance excreted by sorghum appears to be 
(beginning with the least sensitive) :—sorghum, cajanus, cotton, 
sesamum. The same order holds good for the same plants 
towards the substance excreted by cajanus, by cotton and by 
sesamum. If now the excrement from sorghum differed not 
only in quantity but also in kind from that of cajanus, we should 
* All these factors have been proved by experiment to be incapable of correcting except 
very partially the poorer growth except in the case of certain manurial substances (see later). 
