jau. it), 1924 Phoioperiodism and Hydrogen-Ion Concentration 153 
definite formative effects of the light period or its action on the growth 
rate on the basis of photosynthesis alone. What these additional pro¬ 
cesses are has not been determined, but there can be no question as to 
their fundamental importance. In the present paper data are presented 
which indicate that die light period in some way profoundly influences 
acidity relations, the form of the carbohydrate present in the plant and 
probably the water content of the tissues. Daily periodicity in content 
of total uncombined acid as affected by light is an outstanding feature of 
acidity relations in fleshy plants, but in the types of thin-leaved species 
here dealt with this daily periodicity is of much smaller magnitude. In 
the present paper the influence of the relative length of day and night 
on the average level of active acidity in the plant, as measured by the 
hydrogen-ion concentration of the sap, is considered in some detail. It 
is shown that growth relations and definite form of expression as con¬ 
trolled by length of day are regularly associated with characteristic 
acidity relations. 
In the case of short-day plants, indeterminate upward elongation of 
the vegetative stem, which is a characteristic response to a relatively 
long daily illumination period, is associated with progressive increase in 
active acidity of the plant, particularly in the region of the growing 
point. This increase continues till the upper portions of the plant 
become more acid than the lower portions. On the other hand, exposure 
to a relatively short daily light period sharply limits increase in stature 
and quickly initiates flowering and fruiting. Under these conditions a 
brief transitory period of decreased acidity is followed by only a moderate 
increase until a level is approached at which flowering is initiated. This 
level of acidity is much below that characteristic of the advanced vege¬ 
tative stage under long-day conditions. Under the short-day exposure 
the upper portions of the plant are less acid than the lower portions. 
After flowering has been initiated there is progressive increase in acidity 
in the vegetative parts of the plant. As to the reproductive structures 
themselves, the embryonic flower bud is relatively low in acidity, while 
growth of the bud is accompanied by increasing acidity which reaches a 
maximum in the unfolded blossom. The developing seed, on the other 
hand, shows a progressive decline in acidity during the period of active 
growth. Abrupt transfer from a long day to a short day causes a sudden 
and sharp decrease in acidity in the region of the growing point, which 
usually occurs about three to five days after the transfer has been made. 
This drop in acidity, which is believed to indicate definite transition 
from the vegetative to the flowering condition, is only temporary and 
is followed by an equally rapid rise to approximately the original level 
of acidity. These changes in acidity are also observed when flowering 
is initiated by natural decrease in length of day, but the extent and the 
sharpness of the changes in acidity are more or less proportional to the 
amount of the change in duration of the light period. The acidity 
relations resulting from exposure to the long days of summer also obtain 
when the short daylight period of winter is prolonged by use of electric 
light of low intensity. 
In the case of long-day plants exposure to a relatively short day tends 
to inhibit stem elongation, resulting in the leaf-rosette type of develop¬ 
ment, with or without tuberization. Under these conditions the acidity 
of the plant remains at a relatively low level. Exposure to a relatively 
long day more commonly results in elongation of the axis, followed by 
