ALFALFA SEED PRODUCTION". 9 



thus serve to strengthen the keel. Contrary to Miiller's statement, 

 both of the wings can, by exercising great care, be removed without 

 tripping the flower, thus showing that their function is purely sec- 

 ondary. The posterior processes of the wing meet on top of the 

 stamineal column. They can have but little, if any, effect in confin- 

 ing the column in position, as Henslow supposed, for the reason above 

 stated, namely, that their removal is not necessarily followed by 

 explosion. The keel is not purely passive, but its basal tissues are 

 under a lateral tension which tends to pull it open, as Henslow first 

 observed. This tension is restrained by the pressure of the stamineal 

 tube against the two internal knobs. If both the apex and the base 

 of the stamineal column are severed by a razor, so that pressure is 

 removed from the keel, the latter will open automatically. If the 

 edges of the keel are again brought together, they open as soon as 

 the restraining force is removed. In an uninjured flower a very 

 slight separation of the edges of the keel, and consequently of the 

 restraining knob, will release the tense stamineal column. Heavy 

 insects, like bumblebees, may do this by their combined weight and 

 pressure on the tip of the keel, but usually it is accomphshed by the 

 insect's proboscis separating ever so shghtly the. upper posterior 

 edges of the keel. This may be done directly, but jnore commonly 

 by spreading apart the two posterior processes of the wings and thus 

 indirectly spreading the keel. The terminal part of the keel, not- 

 withstanding the cohesion of the two petals, has httle influence to 

 prevent tripping, as, with the inclosed stamineal tube and style, it 

 can be cut off with a razor without releasing the explosive mechanism. 



As shown by Henslow, and perhaps earUer by Delpino, the elastic 

 tension lies entirely in the coalesced filaments of the nine anthers 

 and not at aU in the style. With care the style can be severed at the 

 base without affecting the tripping movements, as Henslow pointed 

 out. 



After tripping, the opening to the nectary is almost closed by the 

 upcurved stamineal tube, but insects continue to visit tripped flowers 

 until the wilting of the petals makes the closure complete. 



The occurrence of this explosive mechanism is not unique in the 

 alfalfa flower, but is found in at least 20 other species of Medicago 

 which have been examined in this connection. In yeUow or sickle 

 aHaHa the stamineal column is relatively much shorter, but the 

 mechanism is the same. It is also very well developed in Medicago 

 scuteTlata, M. rugosa, M. turhinata, M. rigidula, M. ciliaris, and M. 

 echinus. It is less noticeable in some other annual species, because 

 the stamineal column is shorter and not exposed when tripped, as in 

 alfalfa and the species just mentioned. Other genera in which 

 tripping mechanisms occur are Alysicarpus, Trigonella, Indigofera, 

 and Genista. 



28437°— 14 2 



