Mar. 17. 1933 
Further Studies in Photoperiodism 
891 
The original bulbs in most instances gave rise to several new individuals, 
each of which eventually formed bulbs of considerable size. Under the 
full-day length only single stalks were formed, while under a io-hour 
day each unit gave rise to several stalks. Under the io-hour day there 
was permanent failure to form bulbs although the plants split into two 
or more new individuals. The appearance was that of spring onions, 
and the tops remained green for an indefinite period. Though the plants 
have remained green for more than 12 months there has never been any 
indication of flowering or bulb formation. With only 5 hours of light 
daily, weak shoots of pale green color developed and no bulbs were formed. 
A surprising fact is that flower buds were formed, though these were not 
able to open. This behavior possibly indicates that for plants of this 
type, having an intermediate length of day as optimum for vegetative 
growth, there may be two-day lengths favorable for flowering. The 
plants under the 5-hour day were short-lived. The effects of the expos¬ 
ures on formation of tops and bulbs are well shown in Plate 9, A. These 
results indicate that in the onion bulb formation as well as flowering and 
the resultant rest period are induced by the long days of summer while 
a shorter day length favors a period of vegetative activity of indefinite 
duration, thus eliminating bulb formation and the rest period which 
normally follows. The shorter day length does not interfere with the 
splitting into new units. The onion affords an instance in which the 
optimal day length for vegetative growth is relatively short while a long 
day results in dormancy following the development of a resting organ 
representing extreme shortening of the primary axis. Referring to Plate 
9, A, it is interesting to note that the high temperature of the greenhouse 
greatly reduced the size of the plant as a whole but did not alter the 
general morphology. The addition of the electric light of very low 
intensity shows a slight attenuating effect, the plants being somewhat 
taller than those exposed only to natural daylight. 
Tubers of the McCormick, a late variety of potato, were cut in the 
usual fashion and planted in the greenhouse May 21. One lot of the 
potatoes was exposed to natural daylight only while a second lot received, 
in addition, artificial illumination of 3 to 5 foot-candles from sunset 
till midnight in the manner previously described. As previously stated, 
the temperature in the greenhouse averaged io° to 20° F. above the 
outside air temperature. In the house in which no artificial illumination 
was used the first blossoms opened July 3 when the plants were about 27 
inches high. When harvested November 1 the average weight per plant 
of green tops and tubers combined was 1,127 gm. and that of the tubers 
alone was 340 gm. The average height of the plants was about 6 feet. 
The tubers were of small size. In the electrically lighted house flower 
buds formed, but none of them were able to unfold. When harvested 
November 1 the average height of the plants was feet and the average 
green weight was 1,528 gm. In this case no tubers whatever were 
formed, so that under the influence of the increased illumination period 
both sexual and vegetative forms of reproduction were suppressed. All 
energies of the plant were directed toward vegetative development. 
Tubers are resting organs, and the longer light period did not permit of 
slowing down in vegetative activity or any tendency toward dormancy. 
This is illustrated by the fact that an inderground bud which otherwise 
might have given rise to a tuber germinated and, in effect, developed 
an offset, the usual rest period of the tuber being eliminated (PI. 8, A). 
Similar plantings of the McCormick variety also were made in boxes on 
