THE TOXICITY TO FUNGI OF VARIOUS OILS AND SALTS. 9 
more involved aspects of the question and merely bring forward a 
few of the points which serve to illustrate certain phases. 
In the previous discussion it has been seen that the degree of 
toxicity manifested is relative and closely associated with the envi- 
ronmental conditions and the particular physiological constitution 
of the individual organisms under consideration, as well as with the 
concentration of the different toxic substances employed and the 
chemical and physical relations which these bear to the media upon 
which the organisms are grown. Why certain concentrations of 
substances are toxic to one plant and not to another, or why the same 
species varies in its tolerance to a certain toxic agent, is more or less 
obscure. According to Heald (12, p. 126) it may be a case of " adapta- 
tion and adjustment," and this is at least suggested by the work of 
Pulst (23) in increasing the resistance of Penicillium glaucum to cop- 
per sulphate. In support of his view Heald further states : 
Those substances which are poisonous to plants are generally such substances as are 
not accessible to plants in their normal habitats, at least to any extent, while those 
substances which are generally present in the soil have no injurious effect, or at least 
not in the same degree of concentration at which we find them in the soil. 
However, for the purposes of the present paper the question of 
how the toxic substances exert their effect is not so near to the point 
as is the question of what particular components of the substances 
are the effective ones. On the basis of the separation of compounds 
into their constituent ions (elements or radicals) when brought into 
solution, many efforts have been made by comparison of different 
substances which have certain ions present in varying proportions to 
determine the most active part of the molecule. As many substances, 
particularly the more complex, do not become completely dissociated 
in solution, experimental work largely draws its inferences from the 
simpler compounds, mainly the inorganic. 
As a result of work on such ionized molecular solutions, investiga- 
tors quite generally agree that in case of the salts of heavy metals, 
like copper and mercury, it is the metallic ion that is largely effective. 
In the case of strong acids, such as hydrochloric and sulphuric, the 
hydrogen ion is said to be the principal toxic element. The work of 
Kahlenberg and True (14) proves the great activity of hydrogen and 
shows that in mixtures of such acids the toxicity is proportional to 
the number of free hydrogen ions present. 
In 1900 True (28) published an account of the investigation of 20 
acids, both inorganic and organic, together with their sodium salts, 
in an effort to extend our knowledge of the effective toxic elements, 
the toxicity tests being conducted on the roots of Lupinus albus. 
With the simple inorganic acids, which readily dissociate in solution, 
he corroborated earlier views that the H ion gives the greater part of 
the toxicity to the solution, the corresponding sodium salts of the 
88340°— Bull. 227—15 2 
