128 
Journal of Agricultural Research 
Vol. XXVII, No. 3 
for plants which flower when exposed to relatively short days and for 
those which flower when exposed to long days. In detail, however, 
there are important differences in the actual course of the acidity level 
during growth and development in the two classes of plants, for changes 
in form of development resulting from differences in the light period 
are associated with definite changes in average acidity of the cell sap. 
Moreover, decrease in the duration of the light period to which the plant 
is exposed may not result in immediate decrease in average active 
acidity. Apparently, the full effect of such change in illumination is 
not seen, as a rule, till about the third to fifth day, thus suggesting 
similarity to the behavior of succulents in continued darkness. 
EFFECT OF THE LIGHT PERIOD ON ACIDITY RELATIONS IN 
SHORT-DAY PLANTS 
In the study of acidity of the cell sap as affected by the duration of the 
illumination period four general forms or features of plant development 
have been included, namely, (i) the sterile vegetative stem; (2) stem 
FEB. MB. PPF. Af4V JI/ME JPLY 4t/G. SEPT OC7. MOV. PEC.J4N. 
Fig. i.—A pproximate length of day, from sunrise to sunset, for the latitude of Washington, D. C. 
elongation accompanied by flowering; (3) the leaf-rosette type of growth, 
with clearly defined tuberization; (4) the leaf-rosette type of growth, 
without pronounced tuberization. For comparative purposes the plants 
may be grown throughout their development under the respective light 
periods favorable to these different types of activity or at the proper 
time the plants may be shifted from one light period to another so as to 
change the type of development. Both plans have been followed. To 
facilitate comparison of the artificially regulated daily light periods em¬ 
ployed in the experiments with the prevailing natural length of day (sun¬ 
rise to sunset), the approximate annual range in day length in the lati¬ 
tude of Washington is shown in figure 1. 
As already explained, the term “short-day plants” is applied to those 
species and varieties which tend to produce indeterminate vegetative 
