Jan. I®, i9*4 
Photoperiodism and Hydrogen-Ion Concentration 
127 
shown in this office in the case of tobacco leaves during the process of 
curing (9). During the progress of the curing there is a decided decrease 
in total content of malic acid, while there is an equally marked* increase 
in content of citric acid. 
From the above-mentioned investigations it appears that clearly 
defined daily periodicity in acid content, with marked decrease in acidity 
during the daylight period and a corresponding increase in acidity at 
night, is characteristic of succulents having structural features which 
are unfavorable for rapid gas exchange. This periodicity is much less 
evident in thin-leaved species and in some of these has not been observed 
to occur at all. Formation of acids in plant tissues results from incom¬ 
plete oxidation of carbohydrate and is favored by a limited oxygen 
supply. While, in the case of succulents, the acids accumulate chiefly 
during the night, nevertheless the rate of acid formation is in some way 
dependent primarily on the conditions of illumination during the day. 
Decomposition of acids in the tissues of the plant is facilitated by in¬ 
creased oxygen supply, by exposure to light, and by relatively high 
temperatures. Exposure to light, however, is not a necessary con¬ 
dition for acid decomposition. There seems to be considerable doubt, 
moreover, as to whether deacidification is to be regarded as a part of 
the respiratory process. 
The present investigations, relating primarily to the average level 
of acidity of the plant sap as affected by the duration of the light period, 
have incidentally emphasized the relatively narrow daily range in acidity 
of the thin-leaved species which have been studied. Direct observations 
also have been made on this point, in the case of Biloxi soy beans. A 
series of readings were obtained on the sap of the young, topmost leaves 
of the plants collected early in the morning, at noon, and late in the 
afternoon. In plantings made in the field the average P H values of 
the saps during the midsummer, when the plants were most active 
vegetatively, were 6.22, 6.19 and 6.14, respectively, for the samples 
collected in the morning, at midday and in the afternoon. Similar 
readings taken later in die season, when the plants were approaching 
or had actually reached the flowering stage, showed practically no change 
in active acidity during the day. These results were confirmed by a 
second series of observations on plantings made in boxes and exposed 
to the natural length of day of early and late summer. Finally, in 
plantings in boxes exposed to only 10 hours of light daily, a light period 
which, of course, quickly initiates flowering, there was at most only a 
slight increase in acidity during the day. Thus, in contrast with the 
behavior of succulents, Biloxi soy beans show an appreciable increase 
in hydrogen-ion concentration of the sap from morning to afternoon 
as long as the plants continue in the active vegetative stage as a result 
of exposure to relatively long days. When the flowering stage is reached 
as a result of exposure to relatively short days there is little or no daily 
change in active acidity. 
In plants which show the behavior of succulents it might be expected, 
perhaps, that an increase in the number of hours of daily illumination 
would tend to reduce the average acidity for the 24-hour period. In any 
event, with respect to thin-leaved species it may be stated that in the 
limited number observed in the present investigation the general ten¬ 
dency has been toward an increase in average active acidity with increase 
in duration of the daily light period. This holds true in general both 
