THE PHOTOPERIODISM OF TEPHROSIA CANDIDA 1 
By T. B. McClelland 
Horticulturist , Porto Rico Agricultural Experiment Station 
INTRODUCTION 
Darner and Allard 2 through their observations and discussions on the effect of 
length of day on plant growth have opened a new and broad field for investigation. 
Variation in the duration of exposure to daylight would seemingly have small 
influence on plant growth in the Tropics, where the range in day length is much 
;morerestricted than it is in the temperate regions. Such, however, is not the case, 
tany change in day length being sufficient to exert a marked influence on plant 
tgrowth. A knowledge of this influence as a factor in the growth of plants is of 
relatively greater importance in the Tropics, where plantings may be made 
»every day in the year, than it is in the temperate regions where, at least in the 
higher latitudes, the planting season is more or less narrowly restricted by temper¬ 
ature. 
The observations recorded in this paper were made at Mayaguez, P. R., 
which is little less than one-fourth degree north of the eighteenth parallel of 
north latitude. As is graphically shown in figure 1,* the time between sunrise 
rand sunset in Mayaguez ranges from 11 hours in December to 13.2 hours in June. 
PRELIMINARY OBSERVATIONS 
Tephrosia Candida , a tropical legume which is used as a cover crop, was selected 
for observation of the effect of variation in length of day on blossoming and 
growth. T. Candida is a woody plant which attains a height of 9 to 11 feet. 
It makes a good stand for about 18 months, after which it gradually weakens and 
-dies. In some instances, however, the plants outlive the second year. 
Plantings were made in duplicate rows at intervals of 14 days between Decem¬ 
ber 21, 1921, and December 20, 1922. The rows were uniformly 25 feet in length, 
:and were numbered consecutively from 1 to 27 with the date of planting, each 
two rows that were planted on the same day bearing the same number. The 
plants were thinned to 50 to the row. 
The inflorescence is terminal and at first looks like a tiny, green, erect tassel. 
As soon as the tassel appears, the plant is considered as having budded. An 
examination in the field of 100 plants, which were considerably older than those 
in the experimental rows, showed the presence of two budded plants on April 12, 
and again on May 9, 1922. Five plants were in blossom, or had developed young 
pods, and 10 were budded May 23; 6 bore young pods and 17 others were budded 
June 2. No further observations were made of these plants as budding was 
soon afterwards noticed in the experimental rows. Between June 13 and June 
26, 108 plants were budded in the 8 rows of the four earliest plantings, which 
was over one-fourth of the total number of the plants in these rows. No buds 
were observed, however, in the younger rows. The budded plants were labeled 
and dated for observation. About a month later, it was observed that the 
1 Received for publication Mar. 6,1924. 
* Garner, W. W., and Allard, H. A—effect of the relative length of day and night and 
OTHER FACTORS OF THE ENVIRONMENT ON GROWTH AND REPRODUCTION IN PLANTS. Jour. Agr. Research 
18: 553-606, illus. 1920. 
3 Data furnished by the U. S. Dept. Agr., Weather Bureau, San Juan, P. R. 
Journal of Agricultural Research, 
Washington, D. C. 
( 445 ) 
Vol. XXVIII, No. 5 
May 3,1924 
Key No. B-20 
