126 
Journal of Agricultural Research 
Voi. xxyii, No. 3 
causes rise in acidity at night. Very weak light which could not cause 
marked photosynthetic activity is able to promote nightly rise in acidity. 
A very short period of illumination, on the other hand, causes no rise in 
acidity at night. The increase in acidity at night is due to a greater rate 
of formation than of the destruction of acid, while under high tempera¬ 
tures the rate of destruction may keep pace with the rate of formation. 
De Vries concludes that accumulation of acids at night is largely limited 
to fleshy plants. Finally, light action is not the cause of deacidification, 
but merely promotes the process. Warburg (26) in his paper on the 
subject gives a thorough review of previous work, and records observa¬ 
tions on a large number of species. This author finds that decrease in 
acidity in the presence of sunlight does not take place in all thin-leaved 
species. In general, the daily decrease in acidity is proportional to the de¬ 
gree of protection against transpiration afforded by the structure of the 
plant organ. Plant parts which are free from chlorophyll show little or no 
decrease of acidity in sunlight. In the green parts of succulents deacidifi¬ 
cation both in the presence or the absence of light is coupled with the 
presence of oxygen. Increase in acidity at night is probably dependent 
on increase in sugar content which results from illumination during the 
day. The presence of oxygen, though in only small quantity, is required 
for acid formation. The acids of succulents are to be regarded simply 
as products of incomplete oxidation. The formation of acids in plant 
tissues is proportional to (1) the intensity of metabolism, (2) the degree 
of protection against entrance of oxygen; the decomposition of acids is 
proportional to (1) the intensity of metabolism, (2) the accessibility of 
atmospheric oxygen, (3) the temperature. 
Purievich (22) finds that decrease in acidity of leaves in prolonged dark¬ 
ness is not due to physiological translocation or to neutralization of the acid 
by bases derived from the soil. The formation of acid in the dark is propor¬ 
tional to the intensity and duration of the preceding illumination and leaves 
placed in sugar solutions showed increased formation of acid. Transfer 
from the light to darkness causes again in acidity for a period ranging from 
8 hours in some species to more than 24 hours in other species, and these 
differences are believed to be correlated with differences in the relative 
stability of malic, oxalic, tartaric, and citric acids, since different species 
do not contain the same acids. In the spontaneous decomposition of 
solutions of these acids in vitro when exposed to sunlight malic acid is 
most easily broken down, followed by tartaric and oxalic, respectively, 
while citric acid is not affected. Oxalic and malic acids, however, are 
most affected by increase in temperature. In a comparatively recent 
review and study of acidity in succulents, with special reference to cacti, 
Richards (23) finds maximum acidity of the plant juice at about 7 o’clock 
a. m. and the minimum at about 5 p. m., in the case of Opuntia versicolor . 
The greatest daily range in acidity occurs in summer. The daily range 
was greater in younger joints of the plant than in older ones. The diurnal 
change in acidity is due mainly to sunlight, while temperature also is a 
factor. The formation of acid in the plant is due to an inadequate supply 
of oxygen in the tissues. Deacidification is not believed to be a part of 
the respiratory process. As bearing on the work of Purievich relating 
to the relative rate of decomposition of different acids in sunlight and 
the more recent study of the subject by Spoehr (24), attention is called 
to the fact that the same difference in ease of decomposition between 
malic and citric acids applies to deacidification in darkness, as has been 
