Feb.3.19*3 Hydrogen-Ion Concentration and Varietal Resistance 379 
tions in the environmental factors affecting growth rather than to errors 
in operation. In agreement with this conclusion is the statement of 
Richards ( 24 ) that— 
Experimentally considered this close connection of acidity with weather conditions 
gives an exceedingly small expectation of precisely similar results, even with material 
from the same plant. . . . Even under apparently like conditions the plants are 
not constant in acidity. 
Therefore, for varietal comparisons, no importance can be attached to 
differences of o. 1 P H between single observations when the plants are not 
grown at the same time and under identical conditions. Since so few 
determinations can be made in a single day, the necessity for averaging 
a considerable number of observations, if conclusions are to be drawn 
from small differences, is evident. 
It was found that the hydrogen-ion concentration of expressed wheat 
juice increases on standing, which is in agreement with results reported 
by Clevenger (7) and Haas (11) for alfalfa and red-clover juice. Some 
figures showing the rate at which this gradual increase in hydrogen ions 
takes place in expressed wheat juice, kept in closed weighing bottles at a 
temperature of 25 0 C., may be of interest. Three separate determina¬ 
tions were made on the same sample at intervals during the day as 
indicated below. 
A. From plants (Iumillo) 31 days old, grown in limed soil in greenhouse: 
a. m. 
io-33. 
10.45... 
11.00. 
p. m. 
1 - 5°. 
2.05... 
2 - lS. 
4.10 .. 
4-25 . 
4-35.. 
B. From plants (Kanred) 8 days old, grown in germinating chamber: 
Ph 
6.122 
6. 129 
6.132 
6. 089 
6. 081 
6. 071 
6. 015 
5-998 
5-996 
a. m. 
9-50- 
10.05. 
10.20, 
11 - 35 - 
ii.45' 
12 . 00 , 
2 . 00 , 
2 . 15 . 
4 . 00 , 
4 . 10 . 
Ph 
5-941 
5- 944 
5- 939 
5- 923 
5- 9i3 
5-913 
5- 859 
5- 849 
5- 813 
5- 805 
These data show that among other precautions, uniform time intervals 
between the expression of the juice and the determination of its reaction 
must be observed to avoid errors due to oxidations or other changes in the 
juice. 
Another possible source of error to be investigated was that of dilution. 
The surface moisture on both greenhouse and germinator plants, es¬ 
pecially the latter, varied at different times, and elimination of the 
resulting slight differences in concentration seemed impossible. Hempel 
(13) states that a tenfold dilution of the sap of succulent plants produces 
no essential change in the acidity. Boas (3) found that the hydrogen- 
