Feb. •*, 1923 Hydrogen-Ion Concentration and Varietal Resistance 381 
Obviously, geographical differences in the source of the seed have not 
resulted in sufficient differences in the metabolic processes of the seed¬ 
lings to affect the hydrogen-ion concentration of the cell sap in a signifi¬ 
cant degree. It is interesting in this connection to note that Kiessling 
(17), speaking of the resistance of barley to Helminthosporium, states 
that the specific behavior of varieties was independent of the conditions 
under which the seed had been grown. 
As was to be anticipated from the reported results of previous inves¬ 
tigations, it was found that plants cut in the morning almost invariably 
had a higher hydrogen-ion concentration than those cut from the same 
plot in the afternoon. As the result of many researches on the diurnal 
periodicity of the acidity of plant juices, well reviewed and confirmed by 
Richards (24) and by Clevenger (7), it is a well-established fact that there 
is a diminution of acid in many plants during the day and a gradual in¬ 
crease during the night until a maximum is reached just before sunrise. 
Clevenger (7) found that the hydrogen-ion concentration of the cell sap of 
cowpeas was most acid in the morning, decreasing toward night, and in¬ 
creasing after 9.30 p. m.; and Truog and Meacham (30) reported a corres¬ 
ponding change in the hydrogen-ion concentration of alfalfa. Hempel 
(75) proved that plants kept in the dark for not too long a time become 
more acid. In explanation may be mentioned the work of Purjewicz (23) t 
Spoehr (26, 27, 28), and others who have shown that sunlight decom¬ 
poses malic and other organic acids in vitro. The following figures 
taken from some of the data obtained during the present investigation, 
illustrate the magnitude of the difference in reaction between greenhouse 
wheat seedlings cut in the morning and those cut from the same plots 
in the afternoon of the same day, all other conditions being kept as nearly 
identical as possible. 
A . Seedlings 2 weeks old , limed soil 
Cut 9 a. m. 
Cut x p. m. 
Khapli.. 
Ph 
/ (<*) 5 - 94 
l (*) 5 - 92 
f (a) 6.00 
l W 5 - 97 
6. 04 
6. 08 
Ph 
5 - 97 
6. 04 
/ (<*) 6.13 
\ (b) 6. ia 
f (a) 6.16 
l lb) 6. 13 
kittle Club. 
Kanred. 
Turkey. 
Average. 
5 - 99 
6. 09 
B. Seedlings 5 weeks old , unlimed soil 
Cut 9 a. xn. 
Cut x p. m. 
Khapli. 
Ph 
5 - 79 
S- 73 
5 . 8 s 
5 - 79 
5 - 8 * 
Ph 
5.85 
5-80 
5-99 
5 - 8 * 
5-99 
Little Club... 
Turkey. 
D-«:. 
Iumillo. 
Average. 
5-79 
5-89 
