Mar. 17,1923 
Further Studies in Photoperiodism 
909 
As reported in the former paper, this species flowers precociously under 
the short days of winter but flowering is readily inhibited by lengthening 
the light period. The two phases of activity are represented in a single 
plant in Plate 16, A. Under the natural day length of winter this plant 
flowered early in February at a height of 5 inches. At the time of 
flowering the plant was transferred to the greenhouse electrically lighted 
from sunset till midnight. The original stem died to the ground, but 
under the lengthened light period new shoots appeared and these reached 
a height of more than 4 feet. It seems clear that in typical annuals 
not only may general senescence be deferred more or less indefinitely by 
preventing exposure to the particular light period which favors reproduc¬ 
tive activities but even after this stage has been reached rejuvenescence 
may be effected through regenerative processes. The final stages in an 
interesting type of rejuvenescence in the inflorescence of poinsettia, the 
earlier stages of which are referred to on page 903, are illustrated in 
Plate 19, B. 
LOCALIZATION OF THE PHOTOPERIODIC RESPONSE 
An experiment was made with Cosmos to determine whether the 
stimulus imparted to a particular portion of the plant by change in the 
light period extends its influence to other parts of the individual. The 
results were clean cut, and some interesting facts were brought out. 
Young seedlings, propagated under long-day conditions, were cut back 
to the first node above the cotyledons so as to cause two opposite branches 
to develop. A month after the plants were cut back a cardboard screen 
was interposed vertically between the branches of each individual, the 
plane of the screen extending in north-and-south direction. On one 
side of the screen a 100-watt, electric light, with reflector, was turned 
on each day from sunset till midnight. On the other side of the screen 
only the natural daylight period of the winter season prevailed. The 
plants were so adjusted with reference to the screen that the portion of 
the plant below the point of origin of the two branches was on the long- 
day side in some instances and on the short-day side in others. In all 
cases the branches exposed to the short day promptly flowered while 
those exposed to the long day continued the sterile, vegetative type of 
development, without regard to the exposure of the basal portions of 
the plant. Moreover, in all cases the sterile branch assumed a dominant 
influence over the basal portion; that is, the basal portion of the plant 
continued to enlarge in harmony with the growth of the vegetative branch 
while the flowering branch soon ceased growth and passed into a state of 
decline. These relations are clearly shown in Plate 16, B. In this 
connection it is to be recalled that the determinative influence of the 
light period on an aerial part may be transmitted to a subterranean 
part of the plant, as shown in the case of the potato. In this instance 
the apogeo tropic influence of the long light period inhibited tuber forma¬ 
tion and, instead, buds at the nodes of the stolons were caused to germinate 
without a rest period, thus forming offsets. The experiment with 
Cosmos shows that such influence on an aerial plant part may not be 
transmitted to another coordinate member, and to this extent perhaps 
the stimulus may be regarded as localized. It is not entirely clear as 
to how or why the branch exposed to the longer light period is dominant 
over the branch exposed to the shorter light period in its influence on 
other portions of the plant. 
27975—23-6 
