Jan. 19, 1924 
Photoperiodism and Hydrogen-Ion Concentration 
125 
this usually requiring 10 to 20 minutes. The temperature of the air was 
noted and the P H value 4 of the sample calculated. The accuracy of the 
outfit used was frequently checked by observing the potential of a stand¬ 
ard acetate solution or of a solution of potassium acid phthalate. 
DAILY PERIODICITY IN ACIDITY OF THE CELL SAP 
Before passing to detailed consideration of the effect of differences in 
the light period on the average level of plant acidity it is necessary to 
discuss briefly the subject of the daily change in acidity during the 
24-hour period, involving both the formation and the decomposition 
of acid. That there is a rhythmic daily change in acidity in succulents 
has been known since [1813], when Heyne ( 11 ) made the observation that 
leaves of Bryophyllum calycinum possess a more acid taste in the early 
morning than in the afternoon. Quantitative observations on the sub¬ 
ject were made by A. E- Mayer (17) in 1875, and he confirmed the fact that 
in succulents there is an increase in acidity at night and a decrease 
during the day. Similar periodicity, however, could not be detected in 
Oxalis species. Mayer found that in the absence of C 0 2 Oxalis gives off 
no oxygen when exposed to sunlight, and therefore concluded that 
organic acids are not intermediate products of photosynthesis. Bryo¬ 
phyllum, on the other hand, does evolve oxygen gas in sunlight in the 
absence of an external supply of C 0 2 . Increase in temperature from 
20 0 to 30° C. causes a decrease in acid content in Oxalis, which was 
ascribed to increased respiratory activity. In later publications ( 18 , 19,) 
Mayer undertakes to show that the evolution of oxygen by succulents 
when exposed to sunlight in an atmosphere free from C0 2 is due to splitting 
of oxygen from the C0 2 which is formed by the plant and again utilized 
in photosynthesis. G. Kraus ( 12 ) studied various phases of acidity 
relations in plants. He observed a decrease in acidity in several species 
of the nonsucculent type when exposed to sunlight and, as a result, made 
the mistake of assuming that the phenomenon of increase in acidity at 
night and decrease during the day applies quite generally to nonsucculents 
as well as to succulents. Kraus found that deacidification is due to 
direct action of light and is not dependent on either respiration or photo¬ 
synthesis. This investigator concluded that the content of mineral salts 
of organic acids in the tissues does not change materially from day to 
night or from day to day, hence daily periodicity in acidity is not due to 
the neutralizing action of bases derived from the soil. 
In an important contribution to the subject of plant acidity de Vries (25) 
brought out several facts of special significance in their bearing on the pres¬ 
ent discussion. The marked increase in acidity in succulents occurring at 
night is due to previous action of sunlight, for the nocturnal rise in acidity 
does not occur in the absence of illumination by day. Moreover, in con¬ 
tinued darkness there is progressive decrease in acidity during both day 
and night after the first 24 hours, and this phenomenon seems to apply to 
thin-leaved species as well as to fleshy plants. The action of sunlight 
in causing subsequent rise in acidity is not due to a heat effect, for 
warming in darkness during the day does not cause increased acidity at 
night. Also, the action of light is not due directly or solely to photo¬ 
synthesis, for exposure to light in an atmosphere free from carbon dioxide 
4 It may be pointed out here that the Ph value represents the potential due to hydrogen ion in a solution, 
and is the negative logarithm of the actual concentration of this ion. Hence, the higher the acidity, the 
smaller the Ph number; and a change of one Ph unit means a tenfold change in hydrogen-ion concentration- 
