Mar. 8, 1924 
Acidity Changes during the Growth of Wheat 
73i 
same time and handled simultaneously, those external conditions affect¬ 
ing the acidity of one sample also affected that of the other in like degree. 
It is very interesting to find that the hydrogen-ion concentration of such 
* morphologically unlike varieties as Khapli and Little Club and as Pentad 
and Marquis should fluctuate so similarly with small changes in environ¬ 
mental factors. 
Evidence of the diurnal periodicity in the hydrogen-ion concentration 
of these plants appears in the curves of Series C in figure 5 and in the 
corresponding data in Table II from which the curves were plotted. 
The curves for all the varieties in this series fluctuate up and down 
together because at some periods all the acidity determinations were 
made on plants cut in the morning and at others on plants cut in the after¬ 
noon, as indicated on the graph. Reference to figure 3 will show that 
no effect of the difference in the hour at which the plants were cut is 
discernible in the corresponding titratable-acid values of these same 
samples. No such fluctuations appear in the hydrogen-ion curves of 
Series A and B of figure 5 because these were plotted from measurements 
all of which were obtained from plants cut at 1 p. m. in Series A and at 
9 a. m. in Series B. 
significance: of the: data for the: problem of disease resistance 
The variability in the response of all wheat varieties except Khapli 
and Little Club to attack by the many different specialized races of 
stem rust now known (7) would seem to preclude a priori the responsi¬ 
bility of any one character, such as acidity. Moreover, the data of the 
preceding sections, giving evidence of extreme and regular variations in 
acidity during the development of each of the six varieties studied, 
together with what is known regarding the relative susceptibility of sus¬ 
ceptible varieties at different stages of development, afford proof that 
high or low acid content does not influence the plant’s ability to resist 
invasion by the stem-rust fungus. 
A number of investigators have reported observations on the relative 
susceptibility of the wheat plant throughout its growing period. Farrer 
( 1 ) says that the liability of wheat to rust attack begins as soon as the 
plant has flowered and ends when its foliage begins to change color. 
Freeman and Johnson (2) found wheat most susceptible to Puccinia 
graminis from the time the heads emerged from the boot until they were 
in full bloom. Melchers and Parker (5) used both heading plants and 
seedlings for the determination of varietal resistance of wheat to stem 
rust. Stakman and Piemeisel (8) state that cereals, including wheat, are 
usually susceptible at any age up to ripening time. Peltier (6) obtained 
infection of wheat by P. graminis tritici in the seedling, stooling, joint¬ 
ing, and heading stages, and his data indicate that the lowest degree of 
infection occurred in the seedling stage. So it seems obvious that the 
development of the fungus is not favored by the low acid concentration 
characterizing the post-seedling stage nor hindered by the relatively high 
concentration at flowering time. 
Gassner (3) also, as the result of an extended experimental study of 
the relation between stage of development and degree of susceptibility to 
attack by Puccinia graminis , concludes that the susceptibility of wheat 
and other cereals increases with age, the most susceptible period occurring 
between the flowering stage and maturity. Equally interesting, in view 
of the fact that very young wheat seedlings are more acid than older 
plants, though usually not so acid as those which have passed the flower- 
