Mar. 17,1933 
Further Studies in Photoperiodism 
907 
began to appear on the plants which had been exposed to a io-hour day 
during the preceding summer. The renewed growth was rapid and vigo¬ 
rous. The new foliage showed the abnormally dark green color of the 
original leaves which had developed under the io-hour day. The control 
plants did not renew their growth till February 16 (PI. 15, A and B). 
Beginning August 16, a third series of plants were placed in the greenhouse 
artificially lighted from sunset till midnight. In this case dormancy was 
inhibited and rapid growth continued during the winter months. More¬ 
over, no woody stems were formed, even the tissues of the basal portions 
of the stem remaining soft, in contrast with the usual behavior of Kudzu. 
As has been repeatedly pointed out, there is a well-defined quantitative 
aspect of the influence which the length of day exercises on the formation 
of seeds and vegetative resting and reproductive structures so that, other 
conditions equal, it is to be expected that emergence from the rest 
period will be materially influenced by the prevailing length of day under 
which the state of dormancy was developed. There can be no doubt 
that both the quantity and the character of the plastic “reserve 0 ma¬ 
terial available for subsequent needs of the resting embryo are conditioned 
in large measure by this factor. 
As regards the direct action of length of day on emergence from the rest 
period, it would seem that successful emergence without the aid of a 
favorable day length will depend on whether suitable temperature condi¬ 
tions and other environmental conditions can set in action the necessary 
internal processes for renewing active growth. Since ultimately both the 
character and the extent of growth in the plant are dependent largely 
on the prevailing day length, the point at which this factor must inter¬ 
vene in the renewal of activity by the resting structure will be governed 
by the amount of potential growtii energy that becomes available from 
stored materials under the existing environment. In the case of the vege¬ 
tative bud the action of a favorable light period may come into play 
almost at the outset. What action, if any, the light period may exercise 
directly on nonchlorophyllous structures can not be stated at this time. 
It has been shown by MacDougall {13) that in certain cases resting buds 
attached to large storage organs may maintain renewed growth for a year 
or more without receiving any light. By way of contrast, however, the 
behavior of Hibiscus moscheutos L. is of interest. Under Short day condi¬ 
tions this plant is unable to make appreciable growth (7, p. 578). Seed¬ 
lings were grown in buckets during the summer under outdoor conditions 
and were left out of doors during the following winter. Beginning 
March 27, one series was allowed to receive only 10 hours of light daily 
while a second series continued to receive the full seasonal period of 
daylight. On these control plants new shoots apeared April 8 and 9 and 
growth continued in normal fashion. Under the io-hour day a feeble 
development of new shoots was noticeable by May 18, but these soon 
died when still less than an inch long. The condition of the plants under 
the two conditions of light on June 21 are shown in Plate 17, B. Simi¬ 
larly, Aster linariifolius was transferred to the greenhouse October 17 
and on November 27 was placed beneath a 100-watt electric light provided 
with reflector, at a distance of 1 foot, the light being on each day from 
4.30 p. m. to 12.30 a. m. New shoots appeared during the third week in 
December. Under control conditions the rest period was not broken 
till spring. In the case of second-year beets, previously described 
(p. 888), although development of flowering stems began under a io-hour 
day, the plants were never able to flower. A number of small trees and 
