[2] The writers wish to acknowledge their indebtedness to Mr. Carl Kurtzweil for assistance in conducting part of the field experiments at Ames.
Experiments were conducted to determine, if possible, the effect of various types of artificial manipulation of sweet-clover flowers when in full bloom on the production of seed. Only healthy, vigorous plants growing on well-drained soil were selected for these experiments. Before any of the flowers were open, the individual racemes were covered with tarlatan and labeled. ([Fig. 5.]) As soon as part of the flowers opened, the racemes were uncovered and after removing all flowers that were not open the open flowers were pollinated and the racemes re-covered. If the flowers of sweet clover are not fertilized they will remain open for two to three days, then wither, and in a short time drop. But after being fertilized the ovules enlarge very rapidly, and the corollas usually drop in about seven or eight days. Therefore, all fertilized flowers can be distinguished a few days after fertilization has taken place. Counts were made of the number of pods which formed in 10 to 12 days after pollination. An outline of the experiments is given in Table II.
Fig. 5.—Individual racemes of white sweet clover covered with cheesecloth to protect them from insect visitation.
Table II.—Treatment of sweet-clover flowers in the artificial-manipulation experiments.
| Experiment. | Method of pollinating the flowers. |
| A | Check—covered. |
| B | Check—open to insect visitation at all times. |
| C | A separate toothpick was used to spring the keel of each flower on the raceme. |
| D | One toothpick was used to spring the keels of all the flowers on a raceme. |
| E | Cross-pollinated. |
| F | Raceme rolled several times between thumb and finger. |
As insects, and especially honeybees, usually visit all recently opened flowers on a raceme, experiments C and D were conducted to determine whether more seed would be produced when pollen from other flowers on the same raceme was placed on the stigmas of the flowers than when only the pollen produced by each flower was placed on its own stigma. The effect of pollination when only the pollen produced by an individual flower was placed on its own stigmas was also obtained in experiment F, as by this method of pollination no pollen was transferred from one flower to another. It can not be stated definitely that the seed produced by the cross-pollinated flowers was the result of fertilization with foreign pollen, as the anthers were not removed from the flowers pollinated because it would be necessary to remove the anthers when the flowers were not more than two-thirds mature, and in doing this the flowers would be so mutilated that only occasionally would pollination at this time or at a later date be effective. The flowers listed in experiment E were pollinated a short time before they opened, and in each case pollen taken from flowers of other plants was placed on the stigmas. The petals of the cross-pollinated flowers were not mutilated, and in each case they returned to their original positions soon after pollination. The results obtained in experiment B, where the racemes were simply labeled and left open to the action of insects at all times, serve for comparison with other experiments where the flowers were protected from insect visitation and were artificially manipulated.
Martin ([25]) found the setting of alfalfa seed and Westgate ([40]) found the setting of red-clover seed to be affected by an excessive quantity of moisture in the soil or atmosphere. In order to overcome the possible effect of this or of other detrimental factors, in each experiment only the flowers on a certain number of racemes were pollinated at one time. All of the experiments were repeated a number of times during the months of July and August, 1916, and the results given in [Table III] show the total number of flowers pollinated and the number of pods that formed during the two months.
The results presented in [Table III] show that flowers fertilized with pollen transferred from another plant produced a higher percentage of pods than any of the other treatments. The results obtained in experiment D, where the same toothpick was used to spring the keels of all the flowers on a raceme, show that this method of pollination produced an average of 7.24 pods per raceme more than the racemes in experiment C. where a separate toothpick was used for each flower. These results indicate that pollen transferred from one flower to another on the same raceme is more effective than when the pollen produced by an individual flower is used to fertilize its own stigma. However, the results of experiment C prove that self-pollination is effective in Melilotus alba. In experiment B. which was the open check, 4.3 per cent more flowers set seed than on the racemes where the same toothpick was used to spring all the keels, but 11.57 per cent more seed was obtained than in experiment C. Spontaneous self-pollination occurs to only a very small extent, as will be seen from the results of experiment A, in which an average of only 2.9 per cent of the flowers set seed.