Jan. 19, 1934 
Photo per iodism and Hydrogen-Ion Concentration 
123 
tigator, however, failed to recognize the effect of alternative forms of 
activity on the distribution of acidity in the different parts of the plant, 
so that his results as a whole can not be readily interpreted. In brief, 
he found that in the leaf the acidity decreases with increasing age of the 
organ, in the stem the acidity decreases from the apex downward, while 
in the floral organs acidity decreases from the button stage up to unfold¬ 
ing of the blossom, and the developing fruit also shows decreased acidity. 
Various functions in the economy of the plant have been ascribed to 
organic acids, but detailed discussion of the different theories which 
have been advanced on the subject will not be here undertaken. It was 
long ago suggested that the free acids and their soluble salts play an 
important r 61 e in promoting osmotic pressure, turgor, and growth, and 
this theory in its relation to change in the acidity of plants when exposed 
alternately to light and to darkness has been widely discussed (cf. Kraus 
(72)). This is the most important of the older theories regarding the 
effect of acid content of the plant fluids on metabolism, and the general 
procedure which has been followed in studying the subject involves 
measurement of total acidity by titration. In the application of more 
recent conceptions regarding the effect of acidity on the properties of 
plant colloids, however, it becomes necessary to give special considera¬ 
tion to relations of active acidity 3 of the tissue fluids, as measured by 
concentration of hydrogen ions. The latter phase of plant acidity has 
not thus far been fully developed. According to Atkins ( 2) plant cells 
rarely show an alkaline reaction and the P H value of the sap does not 
exceed the number 8 . On the other hand, in certain fruits the P H value 
of the sap may fall as low as 1.4. According to this author “the P H 
value met with in a tissue is usually near, but slightly less than, the 
optimum for the activity of the characteristic enzyme at ordinary tem¬ 
perature.’' Clevenger (6 ) studied the range in hydrogen-ion concentra¬ 
tion in leaf, stem and root of cowpeas during the 24-hour period, and 
found the highest concentration in leaf and stem to occur during the 
morning hours, while the lowest concentration was observed during the 
night. The root showed a narrower daily range in acidity and the 
maximum occurred during the day. Of the three plant parts the stem 
averaged highest and the root lowest in active acidity. From observa¬ 
tions on several crop plants Haas (10) finds that the active acidity of 
the sap is affected by changes in illumination, the soil solution, the age 
of the plant, and other conditions. Several publications have appeared 
in recent years which deal with the hydrogen-ion concentration of the 
cell sap as affected by the reaction of the soil solution or the culture 
medium, but these problems are not directly involved in the present dis¬ 
cussion and hence may be passed over. 
The significance of measuring hydrogen-ion concentration in studying 
the relation of acidity to the properties of colloidal proteins has been 
especially emphasized by Loeb (16). In view of the effects of acidity, 
as thus measured, on the swelling of gels and on the viscosity and osmotic 
pressure of protein solutions, it is desirable to obtain much more complete 
information than has hitherto been available as to the influence of 
external environment on the hydrogen-ion concentration of the cell 
fluids. It is hoped that the present contribution will add materially to 
The term “active acidity” refers to the portion of any acid substance which has become ionized and 
thereby rendered active in producing the effects commonly ascribed to acids; it is measured in terms of 
hydrogen-ion concentration. 
