2 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 



ment, (2) to the 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. 



