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 papillse, 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 satisfac- 
torily, indicating that a high percentage of humidity in the atmosphere 
does not check the germination of the poUen 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. 
