374 
Journal of Agricultural Research voi. xxm, no. $ 
MATERIAL, 5 APPARATUS, AND METHODS 
Most of the plants were grown in the greenhouse; but when very young 
material was desired the seedlings were grown to a height of 3 or 4 inches, 
without light, in the germinating chambers of the Seed-Testing Labora¬ 
tory of the Bureau of Plant Industry. By this means, the possibility 
of error from soil adhering to the plants was avoided. The greenhouse 
plants were grown either on the benches or in pots; the germinator seed¬ 
lings on clean cloths on trays in the practically saturated atmosphere of 
the incubators at a temperature of approximately 20° C. The varieties 
were always grown and studied in pairs, consisting of a resistant and a 
susceptible one, as similar morphologically as possible, or in groups of 
three or four, in order that varying environmental conditions might 
affect in like degree the resistant and susceptible types in any given set of 
determinations and thus not obscure inherited differences. 
For analysis, germinator seedlings were cut off just above the seed 
and soil-grown seedlings 1 or 2 inches from the ground in order to avoid 
contamination by soil that might be clinging to the lower part of the 
culms. They were ground to a fine pulp in a food chopper which, being 
without any sharp cutting edges, macerated the material thoroughly. 
The juice was then squeezed from this pulp by hand through small 
muslin bags which had been washed and boiled thoroughly in distilled 
water and used but once. The juice was kept covered in glass-stoppered 
weighing bottles, and the determinations were made as quickly as possible 
in order to avoid excessive change in reaction due to oxidations. Centri¬ 
fuging was not necessary. 
The plants were not frozen, as recommended by Dixon and Atkins (<?), 
nor otherwise treated to make the cells permeable before expressing the 
sap. Harvey ( 12 ) has shown that freezing changes the reaction of cab¬ 
bage juice. It is true that Andre (r, 2) found that the concentration of 
the juice of untreated tissue varies inversely with the pressure applied, 
and that Marie and Gatin ( 18 ), Dixon and Atkins (8), and others have 
shown that the first sample pressed out may be more concentrated than 
subsequent ones. But Harvey (13) points out that in a buffered plant 
juice minor differences in concentration do not change the hydrogen-ion 
measurements appreciably. 
The hydrogen-ion concentrations were determined electrometrically. 
Potential differences were measured with a high-resistance Wolff poten¬ 
tiometer which could be read to 0.00001 volt. The accessory equipment 
consisted of a Siemens and Halske galvanometer, a Weston standard 
cell standardized by the Bureau of Standards, and a 2-volt storage 
battery in series with a Wolff resistance box as the external regulating 
resistance. The hydrogen electrode usually attained equilibrium in the 
wheat juice in 10 or 15 minutes. The final reading was taken when the 
potential difference had not changed by more than 0.0002 volt in five 
minutes. The saturated calomel electrodes 6 * 8 used were made with the 
usual precautions and frequently checked by the potential difference 
obtained with the hydrogen electrode immersed in a M/20 potassium a cid 
phthalate solution, P H 3.97 (6). 
6 The seed for these experiments was obtained from the agricultural experiment stations at Manhattan, 
Kans., Akron, Colo., Moccasin, Mont., Hays, Kans., Highmore, S. Dak., St. Paul, Minn., Pullman, Wash., 
Moro, Oreg., Dickinson, N. Dak., and from the agronomic division of the Office of Cereal Investigations, 
Bureau of Plant Industry. 
• The form of these cells was designed by Dr. R. B. Harvey. 
