173 PHILLIPS— GEOTROPISM [1920 



were run on the sand and water, and were used to correct the 

 results. There was not enough color in the material to interfere 

 seriously with the phenolphthalein endpoint, but the endpoint is 

 somewhat slow, and, especially with material containing so little 

 acid, the unavoidable errors are apt to cause differences which 

 represent a large percentage of the total titration. The results for 

 corn nodes, calculated as cubic centimeters 0.05 N NaOH per 

 gram of fresh material, are given in tables I and II. The differ- 

 ences found between the two flanks are small. The convex side 

 seems quite uniformly to be the more acid. 



A few measurements of the hydrogen ion concentration of the 

 press juice of corn nodes which had bent from 5 to 15 were 

 obtained. The measurements were made electrometrically, using 

 a modified form of the Barendrecht electrode. The following P H 

 values were obtained, that for the upper flank being given first in 

 each case: 4.919, 5012; 5. 136, 5.246; 5.104, 5. 198. In these 

 three cases, therefore, the hydrogen ion concentration of the juice 

 of the concave flank was the greater, although, as has been noted, 

 the titration acidity varied quite uniformly in the other direction. 



The titration results with Vicia Faba are given in table III. 

 The differences are slight and irregular, and do not correspond at 

 all closely with those reported by Miss Schley. 



Determinations of hydrogen ion concentration, and electrometric 

 titrations, were made on the press juice of the upper and lower 

 flanks of Vicia Faba seedlings that had been exposed to gravity. 

 The material was frozen immediately after collection. A special 

 hand press was used which would remove the juice very completely 

 from samples containing not more than 10 gm. of the fresh material. 

 Five cc. of the juice was taken for the determination. The hydro- 

 gen ion concentration was determined immediately, after adding 

 1 cc. of 0.10 N NaOH free from carbonates. This is practically 

 the method used by Emslander (4) in his work with beer. Pre- 

 liminary experiments showed that the part of the titration curve 

 including these two points is always, for this material, the straight 

 line part of the curve which crosses the neutral line. Usually the 

 two points obtained were on opposite sides of neutrality, so that the 

 cubic centimeters of 0.10 N NaOH required to titrate to P H = 7-o 



