ALFALFA SEED PRODUCTION. 27 



A number of flowers were left untripped on these plants. Less than 

 5 per cent of the untripped flowers produced pods. 



The results of both experiments, as tabulated, show that there is no 

 definite relation between the age of the flowers when tripped and the 

 proportion of pods developed. Clearly there is no diminution in the 

 ability of the flowers to become fertilized as long as the flowers remain 



open. 



FORCE NECESSARY FOR TRIPPING. 



Burkill's results in measuring the force necessary to trip alfalfa 

 flowers are thus reported: 



That tlie separation of the basal processes is the legitimate and almost only natural 

 method of exploding the flower is obvious from the following consideration. By 

 means of a fine wire hung onto the alae, weights to a known extent were suspended 

 from them. In September, 1892, flowers obtained near Poulton (Gloucestershire) 

 were found to explode with an average weight of 1.68 grams (maximum and minimum, 

 2.37 and 0.93). Now, an insect visiting the flower rests its weight on the points whence 

 these weights were hung. The worker of Apis I find to weigh about 0.096 and Bombus 

 hortorum (large specimens) 0.199 grams. The mere weight of these two insects is 

 therefore quite insufficient to explode the flower. Moreover, the pedicel of the 

 flower bends under a weight insufiicient to explode the flower, so that in these experi- 

 ments I found it necessary always to fix the flower by a wire hooked into the standard; 

 and, again, the hive bee so settles as to hold the parts of the flowers together with 

 its feet. 



By the same method of experiment I discovered that the flower is not always in the 

 same degree of explosiveness; the hotter the weather the more explosive is the flower. 

 In cold weather the flower frequently remains unexploded for eight or nine days, after 

 which it withers, but in hot, sunny weather I found three days to be the maximum 

 duration, for explosion is brought about often within 24 hours from the opening of the 

 bud. We must remember in this connection that M. sativa is of Persian origin and 

 has only traversed Europe. northward by slow degrees. 



Shaking by the wind can not explode the flowers. Pieces of paper with a surface 

 of 18^ and 22 square inches were tied to stalks of this plant in order to give more power 

 to the wind, but no effect was observable from the shaking it produced.^ 



Table XIX gives the results of experiments carried out by West- 

 gate with an apparatus similar to that used by Burkill. His results 

 confirm those of Burkill in showing that the force required is much 

 greater than the mere weight of bumblebees or other insects which 

 trip alfalfa flowers. They also show clearly that the force required 

 diminishes as the flowers become older. There is also a considerable 

 range of variation in the weight required to trip different flowers of 

 approximately the same age. 



1 Burkill, I. H. On the fertilization of some species of Medicago L. in England. Proceedings, Cambridge 

 Philosophical Society, v. 8, pt. 3, p. 146, 1894. 



