48 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
concluded that the toxicity is due to the hydrolysis of the aluminum salts 
with the accompanying liberation of nitric acid. Miyake ('i6) has com- 
pared the effect on rice seedlings of the same normality of aluminum chloride 
and hydrochloric acid. The toxicity of these two solutions was not greatly 
different. The H-ion concentration produced by the acid was three times 
as great. This indicated that the toxicity must be due to something other 
than the H-ion concentration. A similar conclusion was reached by Hart- 
well and Pember ('i8) in their study of the effect of aluminum sulphate on 
barley and rye. The sulphuric acid produced a H-ion concentration four 
times as great as the same normality of aluminum sulphate. The two 
substances were, however, alike toxic to barley. The acid depressed the 
growth of rye similarly to that of barley, but the salt had little effect on 
the rate of growth of rye. This indicated that plants may vary greatly 
in their tolerance of aluminum salts. Duggar ('ii) states that the various 
soluble inorganic salts of the same metal are of about equal toxicity. 
Rothert ('06) has shown that this may not be strictly true for aluminum 
salts. Aluminum chloride was found to be much more toxic to corn than 
the sulphate. The toxicity of the salts was also dependent upon the method 
of application. They were most toxic when used alone in distilled water; 
less toxic in Knop's solution; and least toxic in soil cultures. 
The toxicity of iron salts, particularly of the ferrous salts, when present 
in excess, is well known. Hartwell and Pember ('08) determined the 
effect of ferrous sulphate upon the growth of rye and barley. Katayama 
('06) found that a concentration of ferrous sulphate of less than o.oi percent 
stimulated the growth of barley. Higher concentrations were toxic. 
Clover, according to Rupprecht ('15), is seriously injured under certain 
conditions by ferrous salts in a concentration above 4 p. p.m. Iron hydrate 
when added to sand cultures is toxic to spinach. Analyses by Czadek 
('04) have shown that iron in this form is easily absorbed by spinach. 
Iron salts, as aluminum salts, are readily hydrolyzed. Thus, they 
produce an increased acidity of the sand or solution cultures. Boiret and 
Paturel ('92) suggested that the toxicity of ferrous sulphate is due to the 
acid radicle. Ferric salts are even more easily hydrolyzed and precipitated 
in nutrient and soil solutions. The generally recognized greater toxicity 
of the ferrous salts, as compared to the ferric salts, indicates that at least 
in the case of the ferrous salts the toxicity is possibly due to something 
other than the acidity. This may be due in part, as has been suggested 
by Aw^atsu ('14), to an abnormal stimulation of the physiological activities. 
The ferrous salts produce the greater effect on these activities. Maquenne / 
and Demousy ('20) found that the addition of small amounts of copper 
sulphate to solutions containing a toxic concentration of ferrous sulphate 
reduced the toxicity. The addition of the copper to like solutions of ferric 
salts did not reduce the toxicity. They believe that the unlike effect is 
due to the catalytic action of the copper which hastens the oxidation of 
