PISTON-APPARATUS. 



261 



shuts off at the free end of the keel a hollow cone which is open at the apex 

 only, and the action of the stamens within the cavity is just the same as that of 

 the piston inside a pump. When the keel is depressed by a force acting in the 

 direction of the arrow (fig. 270^), the stamens, being fixed, are forced further into 

 the conical cavity and push part of the pollen stored in it through the small orifice 

 at the top. When the pressure ceases the keel returns to its former position. 

 It has been ascertained by careful observations that the process of pumping pollen 

 from a particular flower may be repeated eight times, provided that the keel is 

 not pressed down too low, and that when the keel is depressed beyond a certain 



Fig. 270. — Apparatus for pumping pollen on to the bodies of insects. 



' Lotus comiculatus. 2 Single flower of the same x 2. s The same flower with the standard removed. ♦ The same flower 

 with the standard and the wings removed so as to expose the keel. ' One component leaf of the keel removed ; in the 

 interior of the keel are seen the stamens, the longer ones clavately thickened towards their free extremities ; the conical 

 cavity above the anthers (which are empty) is full of pollen, and the style and stigma are embedded in the mass of pollen. 

 « Depression of the keel in the direction indicated by the arrow, in consequence of which pollen is pumped out of the 

 orifice of the conical cavity by the bundle of clavate filaments. ' The keel still further depressed in the direction of the 

 arrow so that the stigma is extruded through the orifice at the apex of the hollow cone. 



extent the end of the style also protrudes through the opening (see fig. 270^) and 

 comes into contact with the abdomen of the bee which is visiting the flower at the 

 time. 



This kind of pump-apparatus appears to be confined to papilionaceous flowers. 

 On the other hand, the mechanism to be described next, which does its work by 

 means of impact, occurs in flowers belonging to the most diverse families. In 

 every case of the kind the movement of the filaments, which results in the transfer 

 of pollen to the body of an insect, resembles the striking of the hammer on a 

 bell, although the cause of the movement is not the same in the different flowers. 

 Sometimes a two-armed lever is set in motion; sometimes there is a sudden recoil 

 of the stamens due to their liberation, as it were, from a vice, and in a third class 



