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.
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 produced 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 in 1904 died, Kirchner concluded that the flowers of M. officinalis were especially sensitive to inclosure in nets and that the failures to obtain more than a very few seeds on protected racemes in Darwin's experiment and in his first experiment were due to this cause.
According to Kerner ([17], v. 2, p. 399) the peas and lentils (Pisum and Ervum) and the different species of horned clover and stone clover (Lotus and Melilotus) as well as the numerous species of the genus Trifolium and also many others produce seeds when insects are excluded from the plants, and only isolated species of these genera gave poor yields without insect visitation.
OUTLINE OF POLLINATING EXPERIMENTS.
The yield of sweet-clover seed varies greatly from year to year in many parts of the United States. It has been assumed that this variation was due to climatic conditions, as excellent seed crops were seldom harvested in seasons of excessive rainfall or of prolonged drought just preceding or during the flowering period. The lack of a sufficient number of suitable pollinating insects also was thought to be an important factor in reducing seed production. This was especially true where the acreage of sweet clover was large and where few, if any, honeybees were kept.
In order to obtain data upon the factors influencing the yield of seed, a series of experiments was outlined to determine (1) whether the flowers are able to set seed without the assistance of outside agencies, (2) whether cross-pollination is necessary, (3) the different kinds of insects which are active agents in pollinating sweet clover, and (4) whether a relation exists between the quantity of moisture in the soil and the production of seed.