414 BOTANICAL GAZETTE [MAY 
solubility in water are not toxic. Substances easily soluble in water act either 
as poisons or as nutrients according to their relatively higher or lower solubility 
in oil. Substances easily soluble in water and slightly soluble in oil act as 
nutrients, but not as poisons. In other cases, where this theory fails to explain 
the toxicity of substances, as with some dibasic acids easily soluble in water 
and scarcely soluble in oil, which nevertheless are toxic, the toxicity is attrib- 
cannot be explained on the basis of any one characteristic of the substances. 
The hydrogen ion is toxic, but acetic acid possesses a toxicity far in excess of 
that attributable to its hydrogen ion. Cane sugar, regardless of its solubility 
in oil or in water, can be utilized only by those organisms which contain 
invertase. 
The inhibiting and sometimes fatal effects which the accumulated products 
of metabolism exert on organisms producing them are matters of general 
observation. Different organisms vary much, however, in their behavior 
toward their own products. An interesting illustration of this difference of 
behavior is brought out by WEHMER’ in his studies of the effect on Penicillium 
variabile and Aspergillus niger of acids accumulating in culture solutions upon 
which these fungi are growing. The behavior of. these two organisms differs 
widely. In cultures of Penicillium on solutions containing ammonium sul- 
phate, WEHMER observed inhibition of growth and ultimately death of the 
fungus as a result of the accumulation of free acid in the solution. This result 
does not come about in cultures in which potassium nitrate, ammonium 
nitrate, ammonium chloride, or ammonium salts of organic acids are the 
source of nitrogen. In the case of nitrates, both ions are consumed, although 
“ait also nitric acid accumulates in the cultures at first. Hydrochloric acid 
s to be comparatively harmless to this fungus. In cultures of Asper- 
pias niger, also, acid accumulates in the solution when ammonium salts of 
inorganic acids are offered as sources of nitrogen, but in the course of a few 
weeks there is in all cases a diminution of the acidity of the solution. The 
diminution is most marked with sulphuric acid and least with hydrochloric 
acid. Growth is not injured by the temporary accumulation of acid, but 
spore production is inhibited. The acidity is due to the accumulation of 
inorganic acid and not to the production of organic acids. The author attrib- 
utes the lowering of the acidity of the cultures to a neutralization of the aci 
by the products of the protein decomposition in the older parts of the mycelium. 
Thus the ammonia consumed during the early growth of the culture is finally 
7 WEHMER, C., neat ACE. in Penicillium-Kulturen als Folge der Stickstoff. 
Ernahrung. Ber. Deutsch. Bot. Gesells. 31:210-225. 1913. 
Gang der Aciditaét in Kulturen von Wiles, case niger bei wechseln- 
der iin Biochem. Zeitschr. 59:63-76. 1914. 
