2 BULLETIN 844, U. S. DEPAKTMENT OF AGRICULTURE. 
merit, (2) to trie use of machinery not adapted to the handling of the 
crop, (3) to the shedding of immature pods, and (4) possibly to the 
lack of pollination. As the first two have been overcome, mainly 
because of a better understanding of the requirements for handling 
this crop, the subject matter of this bulletin is concerned primarily 
with the factors which produce the third and fourth causes. 
Where the production of seed was disappointing although the 
plants produced an abundance of flowers, it has been observed 
that many apparently were not fertilized, or if fertilized the pods 
aborted. In order to obtain data in regard to the causes of the 
failure of sweet clover to produce a normal seed yield, a study was 
made of the insects which were most active in pollinating the flowers, 
the source of the pollen necessary to effect fertilization, and the 
conditions under which the flowers must be pollinated in order to 
become fertilized. The relation of environmental conditions to the 
shedding of immature pods was also investigated. In order to 
overcome local environmental factors as much as possible, the 
experiments were conducted on the Government Experiment Farm 
at Arlington, Va., and in cooperation with the botanical department 
of the Iowa State College at Ames, Iowa. 
PREVIOUS INVESTIGATIONS OF THE POLLINATION OF SWEET CLOVER. 
Since Darwin (4, p. 360) 1 published the statement that a plant of 
Melilotus officinalis protected from insect visitation produced but a 
very few seeds, while an unprotected plant produced many, other 
scientists have investigated this subject. Knuth (19, v. 1, p. 37), in 
giving a list of the best known cases of self-sterility in plants, men- 
tions Melilotus officinalis. The same author (19, v. 2, p. 282) states 
that since the stigma projects beyond the anthers, automatic self- 
pollination is difficult, and for the same reasons Miiller (29, p. 180) 
believes that self-fertilization is not apt to occur. 
In 1901 Kirchner (18, p. 7) covered a number of Melilotus alba 
racemes with nets. On one of the plants 12 protected racemes 
produced 187 seeds and on another plant only one seed was obtained 
from 10 covered racemes. This experiment was duplicated in 1904, 
with the result that 40 netted racemes produced an average of 38 seeds 
each. Kirchner concluded from this experiment that spontaneous 
self-pollination occurs regularly even though the stigma projects 
above the anthers. He (18, p. 8) also performed an experiment with 
Melilotus officinalis in 1901. At this time 16 isolated racemes pro- 
duced a total of 11 seeds. This experiment was repeated in 1904, 
with the result that 16 protected racemes produced an average of 14 
seeds each. As the racemes on one of the plants that was protected 
1 The serial numbers in parentheses refer to " Literature cited," pages 36-38. 
