98 ALFALFA FARMING IN AMERICA. 



close the pistil. A set of interlocking processes for the keel 

 further assist in forming this spring-trap arrangement. When 

 an insect of sufficient weight alights upon the keel, it depresses 

 the latter, together with the enclosing wing petals; the trigger- 

 like processes are pushed down past the upcurved column of the 

 pistil and stamens, releasing them, and allowing the whole col- 

 umn to spring up with considerable explosive force against the 

 erect standard. At the time of pollination the style with the 

 stigma has grown up above the stamens, and when released the 

 stigma precedes the stamens, striking the insect's body first, in 

 case the latter rests upon the keel, bearing its deposit of pollen 

 brought from another flower. The burst anthers in turn dust off 

 a new deposit of pollen as they are driven past the insect, which 

 is thus equipped with a fresh supply of pollen to become avail- 

 able for the next flower. Sometimes the shaking of the flower 

 stems by the wind, or by the pelting of rain, many accomplish 

 the same result. Self-fertilization may be secured also by visits 

 of insects not yet loaded with pollen, which may, by setting off 

 the explosive mechanism, bring about self-pollination. Since the 

 pollen is shed before the stamen-pistil column is released, it 

 happens that the stigma is already partly covered with pollen. 

 Nevertheless, self-fertilization seems to occur but seldom in en- 

 closed plants protected from insect visits. 



The explosive mechanism of the alfalfa flower has long 

 been known, having been discovered as early as 1832 by A. P. 

 De Candolle. 



In 1894, Burkill found it impossible to make seeds set in the 

 unexploded flower, even though pollen were in contact with the 

 stigma. He considers this fact to be due to the circumstance 

 that the stigma does not become receptive to the pollen until its 

 cells are injured by violent contact with some object. In proof 

 he adduces the fact that he had caused unexploded flowers to 

 set seed by pinching the stigma, by cutting off the tip of the keel 

 or by rubbing the stigma with a stiff brush. It appears, there- 

 fore, probable that insects secure the fertilization of alfalfa flow- 

 ers largely by accidental injury to the stigma while endeavoring 

 to cause the proboscis to enter; or else by exploding the flowers 

 and causing the stigma to be dashed against the standard, the 

 necessary amount of injury may be accomplished to enable the 

 pollen to become effective, in which case it may either be the 

 already present pollen of the same flower, or foreign pollen 

 brought by the insect that is utilized. 



Thus it is plain that insects play a large part in 



