vi.J POLYGONUM, AMENTACE^E. 159 



When the flower opens the stamens are ripe, while 

 the stigmas do not mature till the anthers have 

 shed their pollen, and shrivelled up. P. Fagopyruin, 

 on the contrary, is dimorphous ; some plants having 

 short stigmas and long stamens : others, on the con- 

 trary, long stigmas and short stamens'. 



The curious arrangement by which cross-fertiHsa- 

 tion is secured in Aristolochia, has been already de- 

 scribed in the introductory chapter {antk, p. 27). 

 Asarum, according to Delpino, is also proterogynous. 



Ruppia is an aquatic genus. At the time when 

 the pollen is shed, the female flowers are immature, 

 and the flower-stalk is short and submerged ; when, 

 however, the pollen has all escaped, the female 

 flowers mature, the flower-stalk elongates and as- 

 sumes a spiral form, so that notwithstanding any 

 slight change of level, the flower rests on the sur- 

 face of the water. A similar arrangement occurs 

 in Valisneria. 



Potamogeton is proterogynous (Delpino — Ult. Os- 

 serv. Part ii. p. 22). 



In the Amentaceae (oak, beech, willow, poplar, 

 hazel, hornbeam, birch, alder, &c.) the flowers are 

 unisexual, and generally moncecious ; the males are, 

 in some species — as, for instance, in the hazel — visited 

 by insects for the sake of the pollen. As, however, 

 they scarcely ever produce honey, the female flowers 

 offer no attraction to insects, which consequently take 

 no part in the fertilisation. 



