562 
Journal of Agricultural Research 
Vol. XXXI, No. 6 
materially delayed. In any event, the localized effect of the long 
and short daily illumination period is plainly evident. 
Extensive data presented m the earlier papers, already referred 
to, make it clear that, when compared with the action of a longer daily 
illumination period, the effects of a shorter light period may fail to 
show any tendency to approach those produced by total darkness. It 
seems quite clear that, in general, the formative action of light on 
the aerial portions of the plant may readily extend to parts which are 
under ground and therefore excluded from light, influencing their de¬ 
velopment in various ways. That distinctive differences in the forma¬ 
tive effects of long and short daily illumination periods may be 
thus transferred to subterranean organs in the case of the potato 
has been shown in a previous paper. 5 On the other hand, the above 
tests indicate that the local action of a relatively short daily il¬ 
lumination period in initiating flowering in yellow Cosmos is not 
extended to an adjoining portion of the plant which is exposed to a 
light period unfavorable to sexual reproduction. It seemed worth 
while, therefore, to determine the comparative effects of a short 
and a long daylight period when acting on the lower portion of the 
plant, in influencing the activities of the upper part exposed to con¬ 
tinuous darkness. The first tests were carried out during the summer 
of 1923. 
Using a combination of two light-proof chambers, one arranged 
immediately above the other as shown in Plate 3 c, the upper protion 
of a Cosmos plant was maintained in continuous darkness, while 
the lower portion was exposed to 10 hours of daylight. These 
treatments were begun on July 6, and on July 26 the upper portion 
was restored to the natural length of daylight by opening the dark 
chamber. During the period of darkening, the portion of the stem 
excluded from light increased in length from 3 inches to 21 inches. 
When returned to the light this part of the stem was badly etiolated, 
had lost the power to stand erect, and no flower buds could be seen 
(pi. 4, B). During the time the upper portion was darkened, the lower 
portion of the stem which was exposed to the 10 hours of daylight 
wilted badly, but this condition soon disappeared after the upper 
! >ortion was restored to the light. Flower buds had appeared on the 
ower portion by July 18, and the first blossom opened August 11. 
On the upper part of the stem, which had been darkened, flower buds 
appeared by August 20, and the first blossom opened September 7. 
It will be observed that the upper portion of the plant, restored to 
the full natural length of daylight alter the exposure to continuous 
darkness for 20 days, flowered as soon as it had recovered sufficiently 
to do so. It appears, therefore, that the action of the short day on 
the lower portion of the stem was in some way transmitted to the 
upper portion which was darkened. These results are in-line with 
the fact just mentioned that the formative action of light may be 
transmitted to parts of the plant which receive no light. Tins is 
in contrast, however, with the relations which obtain when one 
f )art of the plant is exposed to a short light period and the other to a 
ong period. The experiment was repeated, beginning August 20 
and maintaining the upper portion of the plant in continuous darkness 
for 28 days. The results were similar to those of the first test. 
* 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: 891. 1923. 
