rs) BULLETIN 75, U. S. DEPARTMENT OF AGRICULTURE. 
essential parts of the mechanism (fig. 1) are the tension of the stamin- 
eal tube, which is held from contracting by two opposite restraining 
lateral processes on the inside of the keel. These processes are really 
om A A 

Fic. 1.—Alfalfa flower (much enlarged). The left-hand figure shows an optical section of the flower, 
indicating the position of the stamineal column before and after tripping. The upper right-hand 
figure gives a view from above of an untripped flower with the calyx and standard removed; the 
lower right-hand figure, the same after tripping. 
invaginations, on the outside occurring as depressions. Each of the 
wing petals is provided with two fingerlike processes, one extending 
forward and the other backward. The anterior process of each wing 
fits into the depression on the same side of the keel, and the two wings 
