u.] ARUM, ARISTOLOCHIA. 33 



As, however, in Aristolochia, so also in Arum, small 

 insects which, attracted by the showy central spadix, 

 the prospect of shelter or of honey, enter the tube while 

 the stigmas are mature, find themselves imprisoned, 

 by the fringe of hairs (Fig. 29, k], which, while per- 

 mitting their entrance, prevent them from returning. 

 After a while, however, the period of maturity of the 

 stigmas is over, and each secretes a drop of honey^ 

 thus repaying the insects for their captivity. The 

 anthers then ripen and shed their pollen, which falls 

 on and adheres to the insects. Then the hairs 

 gradually shrivel up and set the insects free, which 

 carry the pollen with them, so that those which then 

 visit another plant can hardly fail to deposit some of 

 it on the stigmas. Sometimes more than a hundred 

 small flies will be found in a single Arum. In these 

 two cases there is obviously a great advantage in 

 the fact that the stigmas arrive at maturity before 

 the anthers. 



Our common Scrophularia nodosa, some species of 

 Plantago, &c., are also proterogynous, but such cases 

 are comparatively rare. 



The advantage to Scrophularia of being proter- 

 ogynous, as Mr. Wilson (" Nature," September 5, 1878) 

 has ingeniously pointed out, arises from the fact of its 

 being fertilised by wasps, which generally begin with 

 the upper flower and work downwards, while bees begin 

 below and work upwards. The lower flowers are the 

 older. Hence a bee coming from another plant of the 

 same species fertilises the lower flowers, and then carries 

 off a fresh supply of pollen from the upper and younger 

 ones. On the other hand, as wasps commence from above 



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