10 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 



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 papillae the pollen must overcome a high resist- 

 ance offered by the sap of the papillae, a resistance that maybe 

 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 satisfac- 

 torily, 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-pollina- 

 tion and self-pollination of sweet clover do not agree. The experi- 

 ments 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-polli- 

 nation 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 neces- 

 sary, whether the flowers must be cross-pollinated, and (3) what 

 insects are active agents as pollinators of sweet clover. 



ARTIFICIAL MANIPULATION OF SWEET-CLOVER FLOWERS. 1 



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, vigor- 



iThe writers wish to acknowledge their indebtedness to Mr. Carl Kurtzweil for assistance in conducting 

 part of the field experiments at Ames. 



