Part I.--POLLINATION STUDIES OF SEED PRODUCTION. UNSATISFACTORY YIELDS OF SWEET-CLOVER SEED. In some sections of the country much trouble has been experienced for a few years past in obtaining satisfactory yields of sweet-clover seed. This difficulty has been due for the most part to the following causes: (1) To cutting the plants at an improper stage of development, (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, mentions _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 Müller (29, p. 180) believes that self-fertilization is not apt to occur. [1] The serial numbers in parentheses refer to "Literature cited," pages 36-38.