CHAPTER XII. POLLINATION AND FERTILIZATION. 



93 



from a flower in the earlier to one in the later stage of develop- 

 ment, will be likely to transfer pollen to the latter flower and 

 fertilize it. 



Proterogynous flowers are much less common, though inter- 

 esting instances of them are found in the Birthwort, in Arum, 

 in Scrophularia nodosa, and in some other plants. Fig. 284 is 

 a diagram of the inflorescence of the Arum. The large enveloping 

 spathe is contracted near its middle, leaving but a narrow passage- 

 way to the cavity below, which encloses the separate masses of 

 staminate and pistillate flowers. This passage-way is obstructed 

 by stiff" hairs, which point downward. These, being flexible at 



Fig. 282. 



Fig. 283. 



Figs. 282 and 283. — Staminate and pistillate stages, respectively, of flowers of Epilo- 

 bium angustifolium. 



their base, are readily bent downward and afford but a slight 

 obstacle to the entrance of insects, but they are not so easily 

 forced upward from below, because pressure in that direction 

 brings the distal ends of the hairs into contact with the side walls 

 of the tube. The insects which enter are, therefore, imprisoned. 

 The pistillate flowers are clustered at the base of the spadix, and 

 reach maturity considerably earlier than the staminate ones 

 which are clustered above them. After the stigmas have passed 

 maturity, a drop of nectar is secreted in the bottom of the tube 

 to compensate the flies for their imprisonment ; the anthers, now 

 ripened, shed their pollen in abundance ; the insects' bodies 

 become thoroughly dusted with it ; and, lastly, the hairs that pre- 

 vented their exit, wither, permitting them to fly away to some 

 other inflorescence of the same kind, carrying with them the 

 fertilizing pollen. 



