ALFALFA SEED PRODUCTION. 9 
thus serve to strengthen the keel. Contrary to Miller’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 accomplished by the 
insect’s proboscis separating ever so slightly the upper posterior 
edges of the keel. This may be done directly, but more 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 little 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 earlier by Delpino, the elastic 
tension lies entirely in the coalesced filaments of the nine anthers 
and not at allin the style. With care the style can be severed atthe 
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 yellow or sickle 
alfalfa the stamineal column is relatively much shorter, but the 
mechanism is thesame. It is also very well developed in Medicago 
scutellata, M. rugosa, M. turbinata, M, rigidula, M. ciliaris, and WM. 
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 
