Table V.—Summary of the insect pollination studies conducted at Arlington, Va., and Ames, Iowa, in 1916.
| Location. | Number of plants. | Method of treatment. | Number of— | ||
| Racemes. | Pods produced. | Pods per raceme, average. | |||
| Arlington. | 1 | Check—subject to insect visitation at all times. | 196 | 4,013 | 20.47 |
| Ames. | 1 | do. | 239 | 9,943 | 41.60 |
| Arlington. | 3 | Protected from all insects. | 904 | 577 | .63 |
| Ames. | 12 | do. | 2,370 | 653 | .27 |
| Arlington. | 3 | Visited by night-flying insects only (cage 1). | 723 | 2,720 | 3.76 |
| Do. | 1 | Visited by night-flying insects only (cage 2). | 227 | 152 | .67 |
| Ames. | 1 | Visited by night-flying insects only. | 486 | 8,024 | 16.51 |
| Arlington. | 2 | Visited by day-flying insects only. | 544 | 11,397 | 20.95 |
| Ames. | 1 | do. | 418 | 17,186 | 41.11 |
| Do. | 9 | Protected from all insects. | 1,594 | 502 | .31 |
The results in [Table V] show that an average of 0.37 pod to the raceme was obtained from the plants protected from visitation by all insects during the flowering period. As the racemes of Melilotus alba will average approximately 50 flowers each, less than 1 per cent of them set seed without being pollinated by insects. The results obtained in the cages in which only night-flying insects had access to the flowers show that these insects pollinate sweet clover to a slight extent, but that the number of pods produced by them is so few that it may be assumed that these flowers would have been pollinated by day-flying insects. This assumption is borne out by the results obtained in the cages where only day-flying insects had access to the flowers, as the results obtained in these cages at Arlington and Ames, respectively, are approximately the same as those obtained on the plants subject to insect visitation at all times. It will be noted that the yield of seed on the plants visited by insects at Ames is much higher than that of the plants subjected to insect visits during the same period at Arlington. This difference in seed yield may be attributed to the fact that isolated plants were used in the experiments at Ames, and at Arlington the experiments were conducted with plants growing under field conditions.
RELATION OF THE POSITION OF THE FLOWERS ON MELILOTUS ALBA PLANTS TO SEED PRODUCTION.
Observations of sweet-clover plants grown under cultivation, and especially when the stands were thick, showed that the flowers of the racemes on the upper and exposed branches produced a larger percentage of seed than those on the lower branches which were less exposed. It is thought by some that the failure of the flowers on the lower racemes to be fertilized is due to shading; but the results obtained in the cheesecloth and glass covered cages do not warrant this belief, as it is doubtful whether the shading of the flowers on the lower racemes is more than that caused by the cheesecloth. It is probably the lack of pollination that causes this decrease in seed production on the lower branches of plants growing close together, as a vast number of flowers open each day on portions of the plants which are exposed directly to visitation by insects and are therefore more accessible to them.
In order to obtain information upon the number of flowers that produce seed on the upper and lower portions, respectively, of sweet-clover plants when grown under field conditions and where the stand contained four to five plants to the square foot, a number of racemes were labeled on different portions of the plants at Ames in 1915 and 1916. When the pods were partly mature, records were made of the number of flowers that produced pods. The results obtained are given in [Table VI].
Table VI.—Relation of the position of sweet-clover flowers on the plants to seed production, at Ames, Iowa, in 1915 and 1916.
| Year. | Position of the flowers. | Number of flowers. | Pods formed. | |||
| Number. | Percentage. | Average. | ||||
| 1915 | Upper half of plants | 812 | 357 | 43.9 | } | 42.6 |
| 1916 | do | 261 | 101 | 38.7 | ||
| 1915 | Lower half of plants | 344 | 44 | 12.7 | } | 18.3 |
| 1916 | do | 216 | 59 | 27.3 | ||
The flowers on the upper racemes of the plants produced 31.2 per cent more pods than those on the lower racemes in 1915. and 11.4 per cent more in 1916. These results prove that insects more frequently visit the flowers that are directly exposed and are therefore more accessible.