jmnw. of amopial research 
Vol. XXVII Washington, D. C, Jan. 19, 1924 No. 3 
PHOTOPERIODISM IN RELATION TO HYDROGEN-ION 
CONCENTRATION OF THE CELL SAP AND THE CAR¬ 
BOHYDRATE CONTENT OF THE PLANT 1 
By W. W. Garner, Physiologist in Charge , C. W. Bacon, Physiologist , and H. A. 
Allard, Physiologist , Tobacco and Plant Nutrition Investigations , Bureau of Plant 
Industry , United States Department of Agriculture 
INTRODUCTION 
In previous papers (7, 8 ) 2 it has been shown that in many species the 
duration of the daily period of illumination exercises a remarkable forma¬ 
tive action on plant development, and in particular may initiate or in¬ 
hibit flowering and fruiting. The great majority of species, perhaps all 
of them, are more or less responsive to differences in the daily light 
period, though species and varieties differ widely in their sensibility to 
this influence. In many species the action of the light period under or¬ 
dinary circumstances is dominant, so that the response of the plant is 
prompt and certain under a rather wide range of conditions as to tem¬ 
perature, water supply, intensity and composition of the light and the 
supply of plant nutrients. For example, spring plantings of the four 
varieties of soy beans (Soja max (L) Piper) known as Mandarin, Peking, 
Tokyo, and Biloxi, regularly begin flowering in the latitude of Washing¬ 
ton about June 15, July 10, August 1, and September 1, respectively, 
and it has been demonstrated that these differences in time of flowering 
are characteristic varietal responses to the seasonal change in length of 
day. Under normal conditions the later varieties are unable to flower 
until appropriate decrease in length of day has taken place. By artificial 
shortening of the daily light period all varieties are made to flower at 
approximately the same time, while, on the other hand, the later varieties 
may be maintained in the active vegetative condition for a prolonged 
period of time by continued exposure to a relatively long daily period 
of illumination (in excess of 13 hours). Artificial light may be effectively 
used as a supplement to sunlight in prolonging the daily light period. 
Many other species have been found to be equally responsive to change 
in the light period, so that in such cases flowering and fruiting may be 
induced or suppressed at will by proper regulation of the daily illumina¬ 
tion period. 
Plants which behave like the later varieties of soy beans, that is, tend 
toward indeterminate increase in stature without flowering when ex¬ 
posed to a relatively long daily light period but quickly flower in response 
to suitable decrease in the light period, are for convenience designated as 
short-day plants. There is another large group of plants which tend to 
remain in the leaf-rosette stage, without stem elongation, and fre- 
1 Received for publication Sept. 12, 1923. 
8 Reference is made by number (italic) to “literature cited,” p. 155-156. 
Journal of Agricultural Research, 
Washington, D. C. 
(119) 
Vol. XXVII. No. a 
Jan. 19,1924. 
Key No. G-353 
73432—24 - 1 
