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



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 Arlington, 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 approximately of the same size. These two plants 

 produced a total of 544 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- 

 termine the relative efficiency 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 



