14 BULLETIN 844, U. S. DEPARTMENT OE AGRICULTUBE. 
production with the thickness of stand is probably due to the shading 
and partial prevention of insect visitation to part of the racemes on 
the lower branches. Most of the flowers upon the lower branches 
of isolated plants are directly exposed to sunlight and to insect visits : 
therefore the racemes on these branches produce as large a percent- 
age of seed as the racemes on the upper branches. In a thick 
stand, little seed is produced by racemes on the lower branches. 
A plant approximately 3 feet high growing close to the center of a 
field at Aldington. Va., in which was an average stand of four sweet- 
clover plants to the square foot was selected in order to determine 
the number of racemes produced and the average number of seeds 
to the raceme. This plant produced 196 racemes, which contained 
an average of 20.4 pods each. The racemes varied from 2 to 10 cm. 
in length, and the number of pods to the raceme ranged from to 75. 
The racemes on the upper and most exposed portions of the plants 
were larger and the flowers produced a much higher percentage of 
pods than the racemes close to the bases of the larger branches. 
Many of the small racemes on the lower branches produced less than 
five pods each. 
The data obtained from the two plants at Arlington that were 
protected from night-flying insects may also be cited here, as the 
results of that experiment show that night-flying insects are not an 
important factor in the production of sweet-clover seed, and. further. 
because they were growing under the same conditions, in the same 
plat, and were appro xim ately of the same size. These two plants 
produced a total of 541: racemes, with an average of 20.9 pods each. 
The number of pods to the raceme varied from to 86. 
EFFICIENCY OF CERTAIN KINDS OF INSECTS AS POLLINATORS OF SWEET 
CLOVER. 
In order further to test the self-sterility of sweet clover and to de- 
temiine the relative efficiencv of night-flying and of different 
kinds of day-flying insects as pollinators of the flowers, a number of 
cages covered with cheesecloth, glass, or wire screen having 14 
meshes to the linear inch were placed over plants at Arlington. Va., 
and at Ames. Iowa, in July and August. 1916. The bases of the 
cages were buried several inches in the ground, so that insects could 
not pass under them. Cheesecloth was used to cover most of the 
cages and was made into sacks of such a size that they could be put 
on or removed from the frames of the cages without difficulty. It 
was stretched tightly over the frames and fastened to their bases 
with laths. 
A cage having two sides and the top of glass but with ends covered 
with cheesecloth to permit ventilation was used at Ames to protect 
a number of plants from insect visitation at all times. The purpose 
