Mar. i7,1923 
Further Studies in Photoperiodism 
899 
period which is so far removed from the optimum as to permit only the 
leaf-rosette type of development, with complete inhibition of vertical 
elongation of the stem. Moreover, both the character and the extent 
of the branching appear to be influenced by the seasonal range in length 
of day. 
Under the influence of long days cosmos continues to elongate the 
major axis, more commonly with no branching and even with a minimum 
of leaf development. Shortening the day length sufficiently to induce 
flowering also invariably causes branching at the top of the plant, as in 
the experiment discussed on page 883. In Table II data are given on the 
extent of stooling noted in the small grains. It is clear that in the spring 
types of wheat, tye, and barley stooling was greatly increased under the 
relatively short, natural day length of winter as compared with the 
artificially lengthened light period. This condition is reversed in the 
winter wheat and rye. This difference in response doubtless depends on 
fundamental differences in light requirements between the winter and 
the spring types of the small grains. The Honey variety of sorghum 
was sowed May 21 in the greenhouse. Under the natural day length of 
summer the piants began shedding pollen August 19, and as the days 
shortened there was marked stooling, many of the new basal shoots 
attaining the height of the original stem. A similar lot of plants receiving 
electric light from sunset till midnight began shedding pollen August 6. 
In this case the branching was confined chiefly to the tops of the plants, 
a behavior resembling that of cosmos. These differences in habit of 
branching are shown in Plate 11, A and B. 
Rosettes of Oenothera biennis L., transplanted and placed under a 
io-hour day beginning March 29, showed marked basal branching while 
the controis exposed to full daylight developed only a primary axis. 
This difference in behavior is shown in Plate 12, A. In the earlier stages 
the basal branches showed a prostrate type of growth but eventually 
showed a decided apogeotropic form of growth, as may be seen in the 
illustration. Returning to the Anal stages of development of the radish 
plant which had been long maintained under short-day conditions, as 
described on page 888, the influence of the short-light period on the growth 
of the branches is striking. Ordinarily the flowering stem of the radish 
branches chiefly at the top, as did the present plant at the outset. (PI. 
3, A.) As a result of the shortened day length to which it had been 
recently transferred the basal branches gained the ascendency and the 
plant gradually assumed the general shape and the type of branching of a 
shrub. The light-darkness ratio was such that the accumulated nutrient 
material could be used only for forms of growth involving a minimum of 
apogeotropic response. Finally, the pendent form of elongation of the 
primary and secondary stems assumed unusual prominence, producing a 
sort of ‘ ‘ weeping-willow ’’ effect, and marking the last phase of develop¬ 
ment. Apparently, there was at the end almost complete loss of the 
power for apogeotropic development. (PI. 3, B.) 
The duration of the daily light period also is a factor in the elongation of 
underground stems. As stated on page 876, Rumex is unable to develop 
flowering stems when exposed to a short day length. Aerial development 
is limited to the formation of leaf rosettes, as shown in Plate 12, B. In 
this case, however, there is very active underground development of 
stems, indicating, perhaps, a further step toward complete loss of the 
power of growth against the force of gravity. It may be surmised, 
