Mar. 17. 1923 
Further Studies in Photoperiodism 
897 
light period for sexual reproduction which tends to direct the energies 
of the plant quantitatively toward flowering and fruiting (see p. 881), it 
seems possible that in some species exposure to this optimal light period 
will be found to favor successful fruiting without the aid of fertilization. 
In other words, the duration of the light period may be a factor in par¬ 
thenogenesis and in the development of seedless fruits. 
As the length of day is removed still further from the optimum for 
stem elongation, flowering and fruiting tend to give way to formation 
of the usual forms of vegetative reproductive structures in which tuberi- 
zation is a characteristic feature. Tuberization marks a further step 
toward complete elimination of stem elongation. There is no hard and 
fast line between flowering and fruiting, on the one hand, and formation 
of tuberized vegetative reproductive structures on the other; and an 
intermediate day length may permit of both types of development just 
as vegetative stem elongation and sexual reproduction may proceed 
simultaneously. Various steps in the complete separation between 
sexual reproduction and the formation of tubers, however, are readily 
traced in the experimental data which have been given. In the soybean 
a sufficiently reduced light period which marks the end of successful 
flowering and fruiting likewise marks the beginning of tuberization in 
the stem. With Irish potato a daily light period of 18 hours caused 
vegetative development almost exclusively, for, although flower buds 
appeared, they were unable to unfold, and tuber formation was com¬ 
pletely inhibited. Under the natural seasonal day length of 14 to 15 
hours the plants flowered freely and there was moderate tuber formation. 
With 13 hours of light there was a decided increase in quantity of tubers 
formed, while with 10 hours of light there was very intense tuber forma¬ 
tion, the ratio of tubers to “ tops ” being increased sevenfold as compared 
with the plants exposed to a light period of 13 hours. With a light 
period of only 5 hours there was a sharp decline in the ratio of tubers to 
tops, indicating, perhaps, that the income of carbohydrate by photo¬ 
synthesis was but little in excess of immediate requirements for the 
limited growth which the plant could make. The results obtained with 
Apios tuberosa and with Helianthus tuberosus show, also, that the type of 
tuber formed may be markedly affected by the duration of the light 
period. 
Associated with the change from flowering and fruiting to tuber forma¬ 
tion there is a tendency toward the transfer of growth activity from aerial 
to underground parts of the plant. Thus, underground tuber-forming 
stems tend to replace aerial flowering stems. As the light period is 
further shortened even the elongation of underground stems is checked, 
with the result that the number of tuberized reproductive structures is 
reduced while their size is correspondingly increased. The final stage is 
reached in those cases in which tuberization is confined to the mother 
stem, as in the beet and radish when exposed to the action of a light 
period of short duration. In this case stem shortening has been pushed 
to the limit and the plant consists essentially of a rosette of leaves con¬ 
nected with the root system by means of an enormously thickened stem 
entirely lacking in power to elongate or to form aerial or subterranean 
branches. 
The available data indicate that there is an optimal light period for 
tuber formation just as there are optimal periods for stem elongation 
and for sexual reproduction. The highest proportionate production 
