May 3, 1924 
Photoperiodism of Tephrosia Candida 
451 
or too short; the impulse to blossom is much more pronounced when the preced¬ 
ing days have been long rather than short, long days followed by shorter days 
quickly bringing into expression the reproductive process. 
The explanation of the phenomenon of spring budding not being followed by 
flowering is that the reproductive stage is initiated by the spring days of favor¬ 
able length, but that its further development is inhibited by the longer days 
of summer which induce a return to vegetative development. The starting and 
then the arresting of the reproductive stage, followed by the resumption of 
vegetative growth, are especially interesting in the Tropics, where winter cold 
cannot be considered as a possible factor in* the arrested development of the 
reproductive process. 
The growth of the plants is decidedly affected by the day length. 
In figure 2, the height of the plants in the various rows in the field is shown for 
three different dates. The growth made during the short days from November 
to March, inclusive, amounted to very little for the plants in the older rows, rows 
No. 1 to 12, inclusive, increasing less than a foot in height, while the growth of 
the plants in the younger rows was much less than that made in the following four 
*otrs 
^ \ \ \ N N NN N N M 
Fig. 2.—Growth in field 
months with longer days. As the rainfall in this latter period is much more 
plentiful than it is in the earlier period, moisture, as well as day length, doubtless 
accounted for the increased growth. The cans were protected at this time from 
rainfall, but were well and frequently watered at all seasons. The height of the 
plants in cans No. 1 to 13, inclusive, B series, averaged 28.9 inches at 12 weeks 
from planting, while the correspondingly numbered plants, which were kept at a 
10-hour daily exposure, averaged only 19.1 inches. The same plants at 24 weeks 
average 63.4 and 44.1 inches, respectively, in height. 
Plants in cans No. 1 to 13, inclusive, B series, were measured at 12;weeks from 
planting. With the exception of one, all the plantings made between December 
21 and April 12 and measured between March 15 and July 5, showed a steady 
increase in height (fig. 3). The day length began to shorten before the middle 
of July. Plants attaining the age of 12 weeks after July 15 showed a decrease in 
height, which was probably attributable to the inception of the reproductive 
stage. Measurements of height at 24 weeks showed the tallest growth on plant¬ 
ings of late March and early April, measured just prior to the blossoming season 
(fig. 4). The maximum difference in height between controls and treated lots was 
96035—24t-4 
