XVl] , PROTEROGYNY AND PROTANDRY 159 



various arrangements which ensure effective crossing. In 

 Aristolochia, Eremurus, Scrophularia, Plantago, Antho- 

 xanthum, Alopecurus, Nardus, and a relatively few other 

 plants, the stigmas are mature and receptive before the 

 anthers have matured their pollen, and by the time the 

 pollen is shed are no longer capable of receiving it : 

 consequently, only pollen from another flower can be 

 effective in these proterogynous flowers. 



But it is far commoner to find the stamens matured 

 first, and that the anthers have dehisced and shed their 

 pollen while the stigmas are still immature and not yet 

 receptive, as occurs in most Papilionaceae, Umbel life rse, 

 Campanulacese and Composite, Epilobium, Gentians ; and 

 such flowers are termed protandrous. 



In the typical cases given above the separation of 

 pollen and stigma is practically as complete as if they 

 were in separate flowers, and this state of affairs known 

 technically as Dichogamy amounts to physiological dicli- 

 nism, for it is obvious that a proterogynous flower is 

 physiologically a female flower during its first phases of 

 anthesis and male afterwards, and conversely with prot- 

 androus flowers. The Dichogamy is therefore complete. 



In a vast number of flowers, however, the Dichogamy 

 is incomplete. For instance, in most Crucifers, Poppies, 

 Buttercups, Roses, &c., the stigmas are immature but 

 some anthers are dehiscing when anthesis begins, and the 

 flower is for a while male only ; then, before all the 

 stamens have shed their pollen, the stigmas mature 

 and the flower is hermaphrodite and capable of self- 

 pollination (Autogamy). Then comes a period during 

 which, all the anthers having shed their pollen, the 

 stigma remains capable of receiving pollen from other 

 flowers, and the flower in question is physiologically 

 female. 



