Mu 17,1923 
Further Studies in Photoperiodism 
893 
daily only 1 or 2 blossoms opened on each plant, the remaining flower 
buds being suppressed. The leaves assumed a dark green color, growth 
of the vines ceased and above ground the general appearance was sug¬ 
gestive of a sort of dormancy. In September both lots of plants died 
back to the ground level. The underground portions were harvested 
October 28. The control plants had produced a network of small tubers 
throughout the soil of the boxes, but the mother tubers had enlarged 
but little and their resting buds were small. The plants exposed to a 
io-hour day, on the other hand, had enlarged the mother tubers to 
several times their original size, and they bore a large number of promi¬ 
nent resting buds but only a very few new tubers were formed. The 
new tubers, however, were noticeably larger than those formed under 
the full day length. The total weights of all tubers under the long-day 
and short-day conditions were 625 and 383 gm., respectively. These 
results are of special interest in bringing out the difference in mere 
enlargement of a storage organ, on the one hand, and the organization 
of new units, on the other hand. Apparently under the shortened 
illumination period there was a dearth of some formative material other 
than storage forms of carbohydrate. The original or mother tubers 
when grown a second season under a io-hour day continued to enlarge as 
in the first season, attaining an average weight of 7% ounces. It is of 
interest to note that the behavior under the io-hour day closely approaches 
that of the related species Apios priceana Robinson, occurring in Ken¬ 
tucky. Under the natural day length of summer the latter species 
forms only a single tuber, which attains large size. 
A bean from South America (Phaseolus multiflorus Willd). planted 
May 13, was exposed to regulated day lengths beginning June 3. Under 
the full day length of summer only moderate growth was made and there 
was no flowering. In October these plants were transferred to the 
greenhouse, electrically illuminated from sunset till midnight, and under 
these conditions growth was more vigorous than in summer but there 
was no flowering. Under a 12-hour day the plants behaved much like 
the preceding controls and there was no flowering. These plants were 
transferred to the greenhouse not provided with electric lights October 15. 
A few blossoms appeared November 29 and two seed pods matured 
Under a io-hour day flower buds were showing by July 15 and a number 
of blossoms opened, though no seed developed. Sparse flowering again 
occurred in September and in November. The plants were transferred 
to the greenhouse in October with the 12-hour day plants. The plants 
of the three series were taken up and photographed January 18. As 
shown in Plate 6, A and B, the roots of the plants exposed to 10 hours 
of light in summer and the short day length of winter were strongly 
tuberized, while under the long day length of summer and the artificially 
lengthened daylight period of winter there was no tuberization at all. 
The plants exposed to a 12-hour day in summer and the natural-day 
length of winter showed only moderate tuberization of the roots. With¬ 
out going into details it may be stated, also, that cuttings of two varieties 
of dahlia made May 10 were grown during the summer months in the 
greenhouse with and without electric illumination at night. Under the 
lengthened illumination period there was almost complete elimination 
of the usual formation of tubers. 
Cinnamon vine, Dioscorea divaricata L., was grown under regulated 
day lengths, both from aerial tubers and from 1-year-old underground 
27976—23-6 
