VI.] POLYGONUM, AMENTACE^E. 167 



These species, however, also differ considerably ; P. 

 Bistorta is proterandrous. 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. Fagopyrum, on the contrary, is dimorphous ; 

 some plants having short stigmas and long stamens : 

 others, on the contrary, long stigmas and short 

 stamens. In Polygonum amphibium the stems, if grow- 

 ing in water, are smooth : while if on dry land they 

 are provided with a certain number of glandular hairs. 



The curious arrangement by which cross-fertilisa 

 tion is secured in Aristolochia, has been already de- 

 scribed in the introductory chapter (ante, p. 31). 

 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 monoecious ; 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. 



