70 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
than the same normality of the other two acids. This difference was even 
more favorable to the sulphuric acid when the root development was com- 
pared. The nitrates and chlorides of aluminum and iron also depressed 
root growth more than the sulphates. 
With solutions ''A" and ''H," an initial H-ion concentration less than 
pH 3.7 had little effect upon the rate of growth. In most cases the plant 
tended to shift the reaction toward neutrality. On the contrary, when 
ferrous sulphate and certain concentrations of the chlorides were added to 
the cultures, plant growth increased the initial H-ion concentration of the 
solution. The reaction was most readily shifted toward neutrality when 
the acidity was due largely to sulphuric acid and least readily when due to 
hydrochloric acid. The amount of shifting of the reaction was approxi- 
matety the inverse of the toxicity of the salt or of the acid; i.e., the change 
in reaction was proportional to the size and the activity of the plant. 
The toxicity of ferrous sulphate showed no relation to the initial H-ion 
concentration. A 0.0002 N concentration depressed growth 27 percent in 
solution "H." The ferrous sulphate was approximately twice as toxic as 
the same normality of the ferric salts. The latter were more readily pre- 
cipitated, and their depressing effect was closely related to the H-ion con- 
centration produced by their hydrolysis or precipitation. 
In solution "A" the aluminum salts were readily precipitated, and their 
toxicity was evidently due largely to the H-ion concentration thus pro- 
duced. In solution ''H" the toxicity was due to the aluminum ion. A 
0.0002 N concentration produced about the same depression as the same 
normality of ferrous sulphate. The development of the secondary roots 
was inhibited by a 0.0006 N concentration. The nitrate seemed to be some- 
what more toxic than the other aluminum salts and showed a greater 
tendency to produce chlorosis. 
Sand cultures greatly increased the concentration of acids necessary 
to produce the same effect when compared to their effect in the solution 
cultures. The same concentration of the salts produced about the same 
depression of the relative weights of the tops in both types of cultures. 
The use of the sand medium lowered the toxicity of the salts to the roots 
proportionally more than it increased the yield of the tops. 
The composition of the nutrient solution had a greater influence in 
determining the toxicity of the salts than the medium in which the plants 
were grown. 
Iron salts when present in injurious concentrations produced a reddish- 
brown discoloration of the lower portion of the nodal area. Aluminum 
salts collected in the same position but produceed no discoloration. 
Botanical Laboratories, 
University of Pennsylvania 
BIBLIOGRAPHY 
Abbott, J. B., Conner, S. B., and Smalley, H. R. ('13). Ind. (Purdue Univ.) Agr. Exp. Sta. 
Bull. 170. 1913. 
Awatsu, H. ('14). Arch. Int. Pharm. Therap. 23: 325-352. 1914. 
