895 
Mar. 17,1923 Further Studies in Photoperiodism 
Tests were made with two varieties of yam (.Dioscorea alata L.) fur¬ 
nished by the office of Foreign Seed and Plant Introduction, Bureau of 
Plant Industry. In each case two plants were grown in a box 3 feet 
long, 10 inches wide, and 10 inches deep. For plantings of the variety 
S. P. I. 46801 small root tubers were used, and for the variety S. P. I. 
47001 aerial tubers were employed. All plantings were made May 14 
and all tubers germinated between May 23 and June 4. The different 
light exposures began at time of planting. The yams were harvested 
October 6. Under a day length of 10 hours the yam S. P. I. 46801 
developed at first a very marked chlorosis, the leaves taking on an 
almost pure yellow color, but by June 25 there had been a decided 
change toward the normal green. None of the plantings of either yam 
flowered in these tests. The variety S. P. I. 46801 formed no aerial 
tubers under the io-hour exposure, but with 12 hours of light daily and 
under the full day length these tubers appeared. The variety S. P. I. 
47001 formed no aerial tubers in any case. The relative development of 
vines and underground tubers under the different conditions are shown 
in Table VII. 
Table VII .—Behavior of two varieties of tropical yam under daily illumination periods 
of different duration 
Duration of illumination period. 
Combined 
weights of 
vines and 
tubers. 
Weight of 
tubers. 
Weight of 
vines. 
Percentage 
of tubers. 
S. P. I. No. 47001: 
Ounces. 
Ounces. 
Ounces. 
12 hours. 
126 
81K 
44 % 
64.7 
Control; full day length. 
81 
11X 
69 K 
14. 1 
Do.... 
79 
II 
68 
13-9 
Do.. 
67 
9 
58 
13-4 
S. P. I. No. 46801: 
10 hours. 
117 
86 X 
73*9 
12 hours.. 
117 
88 
29 
75*2 
Control; full day length. . .. 
109 
4 i 
68 
37-6 
Do. 
91 
39 
52 
42.8 
Do.. 
106 
55 X 
50 K 
52.3 
It is quite clear that the shortened length of day caused a pronounced 
increase in tuber formation. The relative sizes of tubers formed by 
variety S. P. I. 47001 under a 12-hour day and the full summer day 
length are shown in Plate 10, A. It should be mentioned that while 
the plants exposed to the shorter day lengths formed only single thick¬ 
ened roots the controls tended to form more than one thickened root 
each. 
Tubers of artichoke, Helianthus tuberosus L., were planted April 25 
and were up May 5. Under a io-hour day flower buds could be seen 
by June 21. The first evidence of flowering was a branching of the stem 
followed by formation of the flower buds. These buds, however, never 
developed to the flowering stage and by July 28 had been absorbed. 
By August 15 the plants were dead, having reached a height of about 
2 feet. On the two control plants flower buds were showing August 30, 
and on one plant the first open blossoms were seen on September 22, 
while on the second plant the blossoms had not opened on October 1, 
when all plantings were harvested. Under the io-hour day the plants 
