i86 



Journal of Agricultural Research 



Vol. XX, No. 3 



The hydrogen-ion concentration of the sap was also determined by the 

 use of the hydrogen electrode. Mature normal-leaf sap was found to 

 give a P H value of 5.816 and mottled-leaf sap a value of 5.647, which 

 implies hydrogen-ion concentrations of 0.153 X 10 ~ 5 and 0.226 X 10 ~ 5 , 

 respectively. These determinations are probably within the range of 

 variation of different samples of the same leaves. 



After the determination of the hydrogen-ion concentration, total acidity 

 was determined by titration, using the hydrogen electrode to determine 

 the end point. It was found that 10 cc. of the normal sap required 3 cc. 

 N/10 alkali and the mottled-leaf sap 7.05 cc. In other words, the actual 

 acidity (hydrogen-ion concentration) of mottled-leaf sap is approximately 

 the same as that of normal leaves, but the latter sap is more nearly sat- 

 urated with base. It is probable that in each case the ionization of the 

 acids is held at approximately the same level by the buffers present. 



Samples of normal Navel orange leaves approximately one week of 

 age, fully mature leaves, and severely mottled leaves of the previous 

 year's growth were collected in April, 191 9. The sap was expressed, 

 and the hydrogen-ion concentration and total acidity were determined 

 by the hydrogen electrode. Freezing-point depressions were also deter- 

 mined in portions of the unfiltered sap. The acidity is expressed in cubic 

 centimeters of N/10 sodium hydroxid required to neutralize 10 cc. of the 

 sap. 



Table XXI. — Acidity and freezing-point depression of orange- leaf sap 



Condition of leaves. 



Hydrogen-ion con- 

 centration. 



Total acid- 

 ity. 



Freezing- 

 point de- 

 pression. 



Normal, 1 week old 



Normal , mature 



Mottled 



Mottled 



6. 069 

 5.664 

 5- 6 47 

 5- 6 3° 



D. 852 X IO 



. 217 X 10" 



. 226 X 10 



• 235 X 10- 



Cc. 



1.80 

 3.80 

 7. 00 

 8.25 



°c. 



1. 258 

 1.588 



1-734 



These data show that the actual acidity (hydrogen-ion concentration) 

 of mature orange-leaf sap is approximately two and one-half times as 

 great as that of leaves at the age of 1 week; but again it is shown that 

 the acidity of mottled leaves is approximately the same as that of nor- 

 mal leaves. The capacity to neutralize base — that is, total acidity — 

 however, was fully twice as great in mature leaves as in those 1 week of 

 age, while the mottled-leaf sap neutralized about twice as much base 

 as the normal mature leaf sap. 



The freezing-point depressions show that while the normal mature- 

 leaf sap is more concentrated than that of young leaves the sap of mot- 

 tled leaves is more concentrated than either. 



The results of the preceding investigation on the sap of orange leaves 

 are very suggestive. They are in harmony with the preceding ash 



