The germination of the pollen of Melilotus alba permits considerable variation in moisture, as is illustrated in [Table I].
Table I.—Germination of the pollen of Melilotus alba in water and in solutions of cane sugar of different strengths.
| Melilotus alba. | Pure water. | Cane sugar in solution (per cent). | |||||||
| 8 | 12 | 18 | 24 | 30 | 35 | 45 | 55 | ||
| Germination of pollen per cent | 33 | 23 | 64 | 46 | 60 | 46 | 31 | 22 | 0 |
The results given in [Table I] represent the average of 12 tests. Some of the pollen grains burst in pure water and in the weak cane sugar solutions, the percentage of bursting being greatest in pure water and decreasing as the percentage of sugar in the solution was increased. There was considerable variation in the percentages of germination in both water and in the solutions of different strengths, and at times there was very little bursting which was not accompanied by a high percentage of germination. The pollen tubes grew as rapidly in water as in any of the sugar solutions, some reaching a length of 100 microns in six hours. As the pollen tubes made no more growth in the solutions of sugar than in water, it is evident that the sugar is not used as food, but helps in germination by reducing the rate at which water is absorbed.
To judge from [Table I], the pollen of sweet clover can be effective not only under ordinary conditions but also when the flowers are wet with rain or dew or when the stigma is so dry that in order to obtain water from the papillæ the pollen must overcome a high resistance offered by the sap of the papillæ, a resistance that may be equal to the osmotic pressure of a 45 per cent solution of cane sugar. This is in accord with results obtained under field conditions; as flowers that were pollinated while rain was falling set seed satisfactorily, indicating that a high percentage of humidity in the atmosphere does not check the germination of the pollen sufficiently to interfere with fertilization. Neither was the setting of seed affected when the soil about the roots of plants was kept saturated with water, showing that the excessive quantity of water in the stigmas resulting from an abundance of water in the soil did not interfere with the fertilization of the flowers.
No definite counts were made of the germination of the pollen of Melilotus officinalis in the solutions of cane sugar of different strengths, but observations show that the moisture requirement of the pollen of this species is approximately the same as that of Melilotus alba.
CROSS-POLLINATION AND SELF-POLLINATION OF SWEET CLOVER.
Results published by previous investigators on the cross-pollination and self-pollination of sweet clover do not agree. The experiments of Darwin ([4]) show that the flowers are self-pollinated to only a small extent. On the other hand, Kirchner ([18]) and Kerner ([17]) find that self-pollination occurs generally and that cross-pollination is not necessary for the production of seed. However, all investigators agree that many different kinds of insects are able to pollinate sweet clover.
Because of the diverse opinions as to the pollination of sweet clover, a number of experiments were conducted to determine (1) whether insect visitation was necessary to pollinate the flowers, (2) if necessary, whether the flowers must be cross-pollinated, and (3) what insects are active agents as pollinators of sweet clover.