414 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
Arrhenius (i) calls attention to the double-maximum curves secured by 
Hixon and Salter and Mcllvaine and states that the same type of curve is 
also secured when the growth of older plants is plotted against the pH. He 
cites data for 2j-months-old plants in soil cultures showing that the relative 
leaf area of bersim ( Trifolium alexandrium) , barley, corn, and cotton showed 
curves with double maxima. The minimum between the two maxima for 
bersim was found at pH 6.0, for barley at pH 8, for corn at pH 5 and pH 7, 
and for cotton at pH 8. In solution cultures 2§ months old, wheat plants 
and radish plants showed a double-maximum curve for the weight of roots 
plotted against the pH, the minimum between the two maxima falling at 
pH 6.0. The weight of stems of wheat plotted against the pH of the 
solution also showed a double-maximum curve with a minimum at pH 6.0. 
The cause of this double maximum and minimum is suggested by 
Arrhenius as being due to changed intensity of permeability for the different 
salt nutrients, or to the fact that the solubility of salts differs at different 
hydrogen-ion concentrations. 
Although attention was directed by Webb, Hixon, Hopkins, and 
Arrhenius to the breaks or minimum points in the curve of the germination 
of spores and seeds, of the growth of plants or of infection of host plants by 
fungi when the intensity of the process is plotted against the pH of the 
medium in which the plant material is placed, similar observations can be 
found in the older literature. In these cases the hydrogen-ion concentra¬ 
tion was not measured, and the results are therefore less certain in their 
interpretation. 
Fischer (8) found that treatment with dilute acids or with solutions of 
acid salts markedly increased the germination of seeds of Sagittaria sagittaria 
or Sagittaria platyphylla , which showed almost no germination in distilled 
water or in solutions of neutral salts such as NaCl or CaCL. Dilutions of 
KOH or NaOH had the same favorable effect as the acids. Copper sulfate 
also increased the percentage of germination. 
N 
Dachnowski (6) found that in dilute acids, -, and in dilute alkalies, 
800 
, bean and corn seeds showed a greater maximum water absorption 
800 
than in distilled water. 
F. E. Lloyd (12) studied the growth rates and accompanying phenomena 
in pollen tubes of Phaseolus odoratus. Increasing the concentration of 
acetic, malic, or citric acid in 40 percent cane sugar increased the rate of 
N N N 
growth of the pollen tubes to a maximum at 
or 
re- 
3200 12800 12800 
spectively. Further increase in the concentration of the acid decreased the 
growth rate. Increasing the amount of NaOH in 20 percent cane sugar 
N 
also increased the growth rate to a maximum at- . Further increase in 
3200 
