Mar. 17,1923 
887 
Further Studies in Photoperiodism 
This relationship seems to hold whether or not flowering is involved and 
applies also to branches and leaf stems as well as to the primary axis. 
The seasonal length of day may be either too long or too short for maximal 
apogeotropic response. 
It is probable that the first important result of departure from the 
optimal day length for apogeotropism is a foreshortening of the stem 
which results in, or at least is accompanied by, transformation of the 
terminal portion into an inflorescence. With a more marked departure 
from this optimum, such as would completely eliminate apogeotropic 
stem elongation, flowering gives way to leaf development. In other 
words, the foreshortening is pushed to the point where the terminal 
flovrer bud is replaced by the leaf rosette. This, at least, expresses the 
result morphologically even if it should have no physiological basis. 
As has been previously pointed out, many short-day plants of the type 
of Cosmos and Bidens continue for a more or less indefinite period verti¬ 
cal elongation of the axis under the influence of long days. As the day 
length is reduced to suboptimum, elongation of the axis is checked and 
flowering quickly follows. In these plants stem elongation can scarcely 
be completely eliminated by reducing the day length, and likewise 
flowering can not be readily inhibited by this means. In many long-dav 
plants, on the other hand, apogeotropism may not be pushed so far as 
to inhibit the foreshortening and flowering, but by reducing the day 
length initial elongation of the axis is readily prevented. It is probable, 
moreover, that some of the woody perennials will be found to occupy an 
intermediate position in which more or less indefinite apogeotropic 
growth is induced by the optimal light period, on the one hand, and such 
growth completely inhibited, on the other hand, by suitable change of 
the light period to suboptimum. 
A few examples illustrative of the above-mentioned responses will be of 
interest. In Comos, as stated on page 875, the axis reached a height of 
15 feet under the influence of a long daily light exposure, and then flower¬ 
ing occurred only because of a decrease in the 'light period, whereas the 
final height attained under short-day conditions throughout was only 30 
inches. Bidens, which behaves like Cosmos under long-day conditions, 
may attain a height of only 2 inches when exposed to a short-day length, 
the foreshortening of the axis being followed by flowering. It was pointed 
out, also, that while buckwheat flowers under all day lengths employed, 
under the longer light periods flowering is accompanied by marked in¬ 
crease in stature, the final height in this case being more or less inde¬ 
terminate. Similarly, under an increase in duration of the light period 
from the normal day length of winter to about 18 hours, which was with¬ 
out effect on the time of flowering in Connecticut Broadleaf tobacco 
(Nicotiana tabacum L-), the height was increased by 50 per cent and the 
number of leaves was doubled. In the violet the leaf stalk is greatly 
lengthened by increase in the length of day. Goldenrod, as described on 
page 877 and illustrated in Plate 5, B, furnishes a striking example of 
inhibition of stem elongation by a reduced day length. 
Summer radish (.Raphanus sativus T.) when exposed to a daily light 
period of 7 hours is unable to develop a flowering stem, but under these 
conditions the leaves reach an unusually large size and the root continues 
to enlarge. Two plants which had received 7 hours of light daily during 
the preceding summer and fall were exposed to the natural day length of 
winter in the greenhouse till February 1. At that time it was seen that 
