The flowers may be considered as constructed upon the monocotyledonous type. _Pterostegia_ has a perfectly monocotyledonous flower with 5 trimerous whorls. _Rheum_ likewise, but here the external staminal whorl is doubled (Fig. 254 _A_). _Oxyria_ has a dimerous _Rheum_-flower (4-leaved perianth, 6 stamens, 2 stigmas). _Rumex_ has a _Rheum_-flower with the suppression of the internal whorl of stamens (Fig. 354 _B_); _Emex_ is a dimerous _Rumex_. _Polygonum_, to which _Coccoloba_, _Muehlenbeckia_ and others are related, differs from _Rheum_ chiefly in having one of the leaves, which in the latter takes part in the formation of the perianth, developed in this case into a bracteole (so that the perianth is reduced to five members), and several or all the stamens in the inner whorl become suppressed.--The perianth in _Coccoloba_ and _Muehlenbeckia_ is more or less perigynous and becomes fleshy, enclosing the fruit. _Muehlenbeckia platyclada_ has flat branches with rudimentary leaves; sometimes branches with normal, arrow-shaped leaves are found. _Atraphaxis._ [Illustration: FIG. 355.--_Polygonum fagopyrum_: _A_ branch with flower and fruits (nat. size); _B_ flower; _C_ the same in longitudinal section; _D_ anterior and posterior view of stamen; _E_ gynœceum; _F_ fruit (mag.); _G_ fruit in longitudinal section; _H_ transverse section, showing the curved cotyledons embedded in the endosperm; _I_ the embryo.] POLLINATION. _Rumex_ is wind-pollinated, the stigmas are therefore large and brush-like (indicated in Fig. 354 _B_). _Rheum_ and _Polygonum_ are insect-pollinated and have therefore capitate stigmas, etc.; honey-glands are situated at the base of the stamens (_d_, in Fig. 354 _C_, and _n_ in Fig. 356); a few small-flowered _Polygonum_ species are self-pollinated; Buckwheat (_P. fagopyrum_) is dimorphic and has long-styled and short-styled flowers (Fig. 356). _Pol. bistorta_ is protandrous and homostyled.