FURTHER STUDIES IN PHOTOPERIODISM, THE RE¬ 
SPONSE OF THE PLANT TO RELATIVE LENGTH OF 
DAY AND NIGHT 1 
By W. W. Garner, Physiologist in Charge , and H. A. Aixard, Physiologist , Tobacco 
and Plant Nutrition Investigations , Bureau of Plant Industry, United States Depart¬ 
ment of Agriculture 
INTRODUCTION 
In an earlier paper (7) 2 considerable data were presented tending to 
show that the length of day exercises a remarkable regulatory action 
in initiating or inhibiting sexual reproduction in plants. In a number 
of species studied it was found that ordinarily the plant can attain the 
flowering and fruiting stages only when the length of day falls within 
certain limits, so that in such cases flowering and fruiting occur only at 
certain seasons of the year. In this respect some species and varieties 
respond to relatively long days while others respond to short days. 
Moreover, some plants are much more sensitive to change in length of day 
than are others. In the absence of the particular day length favorable 
to sexual reproduction vegetative development may continue for a 
more or less indefinite period, thus frequently leading to the phenomenon 
of gigantism. It was discovered, also, that exposure to a daily light 
period intermediate between that favorable only to vegetative develop¬ 
ment, on the one hand, and that favoring only flowering and fruiting on 
the other hand, tends to cause both forms of activity to progress simul¬ 
taneously. This combined form of activity constitutes what is commonly 
known as the “everflowering” or “everbearing” behavior. It was 
suggested that probably the seasonal range in length of day is an impor¬ 
tant factor in the natural distribution of plants and that in agricultural 
practise the correct time for planting many crop plants may be largely 
conditioned by the prevailing length of day. To designate the response 
of the plant to length of day the term “photoperiodism” was suggested.* 
The data on which these conclusions are based were obtained chiefly 
by use of dark chambers into which the test plants could be placed for 
a portion of the day, thus reducing the number of hours of illumina¬ 
tion received during the relatively long days of summer. Brief mention 
also, was made of greenhouse experiments conducted during the winter 
months, in which electric light was used to prolong the daily period of 
illumination. The results of these latter tests confirmed those previ¬ 
ously obtained by use of the dark chambers. Oakley and Westover 
(18) have recently reported an interesting application of these princi¬ 
ples as a means for the quick identification of seed of different alfalfas. 
1 Accepted lor publication May io, 1922. 
* Reference is made by number (italic) to “Literature cited,” p. 919-920. 
* The authors are indebted to Mr. O. F. Cook, of the Bureau of Plant Industry, for suggesting the term 
photoperiodism, which seems to meet satisfactorily all requirements as to both aptness and simplicity. 
Journal of Agricultural Research, 
Washington, D. C. 
adc 
(871) 
Vol. XXIII, No. ti 
Mar. 17, 1923 
Key No. G-289 
