LOCALIZATION OF THE RESPONSE IN PLANTS TO RELA¬ 
TIVE LENGTH OF DAY AND NIGHT 1 
By W. W. Garner, Senior Physiologist in Charge , and H. A. Allard, Physiologist , 
Tobacco and Plant Nutrition Investigations, Bureau of Plant Industry , United 
States Department of Agriculture 
INTRODUCTION 
In earlier papers 2 it has been seen, with many species of plants, 
that modification of the duration of the daily illumination period 
seems to have definite formative effects on growth and of bringing 
about certain well-defined chemical changes within the plant. 
As an aid in attempting to determine the manner in which these 
formative effects are brought about, it is of considerable interest to 
know the results when different parts of the plant are exposed to 
different periods of illumination. In other words, when two parts 
of the same plant are exposed to different illumination periods, will 
each part show the characteristic response to its particular light 
period, more or less independently of the action of a different light 
period on the other plant part? This question has already been 
answered in part in an earlier paper, 3 but in recent experiments the 
results have been considerably extended. 4 
In the earlier work, specimens of Cosmos bipinnatus Cav. were cut 
back to the first node above the cotyledons, from which two branches 
were allowed to grow under a daily illumination period of 16 hours. 
At the end of a month a vertical cardboard screen was arranged 
between the two branches of each plant, one branch being exposed 
only to the natural daylight period of winter, while the other branch 
was exposed to electric illumination from sunset till midnight, in 
addition to the natural illumination. Each branch "showed the 
characteristic response to its illumination period—that is, the branch 
exposed to the short illumination period promptly flowered and 
developed seed, while the branch exposed to the longer light period 
remained vegetative to the end of the test. 
EXPERIMENTAL DATA 
The tests previously reported had to do with two coordinate 
branches of a plant exposed to different day lengths. The present 
experiments deal with different portions of the primary stem exposed 
1 Received for publication Oct. 16, 1924; issued October, 1926. 
2 Garner, W. W., and Allard, H. A. effect of the relative length of day and night and other 
FACTORS OF THE ENVIRONMENT ON GROWTH AND REPRODUCTION IN PLANTS. Jour. Agr. Research 18: 553- 
606, illus. 1920. 
-and Allard, H. A. further studies in photoperiodsm, the response of the plant to 
relative length of day and night. Jour. Agr. Research 23 * 871-920, illus. 1923. 
- , Bacon, C. W., and Allard, H. A. photoperiodism in relation to hydrogen-ion concen¬ 
tration OF THE CELL SAP AND THE CARBOHYDRATE CONTENT OF THE PLANT. Jour. Agr. Research 271 119- 
156, illus. 1924. 
* Garner, W. W., and Allard, H. A. further studies in photoperiodism, the response of the 
plant to relative length of day and night. Jour. Agr. Research 23: 871-920, illus. 1923. 
4 In this paper and in earlier experiments dealing with plants, different parts of which have been 
exposed to different illumination periods, only short-day plants, flowering in response to relatively 
short daylight periods, have been used. Since the present paper was submitted for publication the 
work has been extended to a series of typical long-day plants, in which flowering is induced by exposure 
to relatively long daily illumination periods. Results secured to date (September 1, 1925) indicate that 
these plants, like the short-day plants, show localized responses, flowering only on those stems receiving 
a long daily illumination period. Publication of the details of these tests is deferred to a later date. 
Journal of Agricultural Research, 
Washington, D. C. 
( 555 ) 
Vol. XXXI, No. 6 
Sept. 15, 1925 
Key No. G-498 
