Fig. 354.—A Diagram of Rheum; B of Rumex; C of Polygonum fagopyrum; D of P. lapathifolium. The ovules are indicated inside the ovaries; bracts and bracteoles are not shown.
Rheum (Rhubarb, Fig. [354] A) has a 6-leaved, petaloid perianth (Pn 3 + 3) and 9 stamens (A 32 + 3). The 3-winged nut is not enclosed by the perianth.
Rumex (Dock, Fig. [354] B) has 6 stamens (A 32 + 0); the perianth is 6-leaved (Pr 3 + 3), green or red, and the triangular nut is enveloped by the 3 interior perianth-leaves, which point upwards and continue to grow after flowering. These perianth-leaves often have warts on their outer surface. The following are monœcious: R. acetosa and R. acetosella.
Polygonum (Knot-grass, Figs. [354] C, D; 355). The petaloid perianth is most frequently 5-merous (2/5 spiral); 5–8 stamens. The nut is triangular (Fig. [354] C, 355), or lenticular (Fig. [354] D). There are two whorls of stamens, the external with introrse, and the internal with extrorse anthers. The gynœceum is often bicarpellate (Fig. [354] D).
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.
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.
About 750 species, most of which are found in the temperate regions of the Northern Hemisphere, some reaching as far as the snow line or into the Arctic regions (Oxyria, Kœnigia). Trees and shrubs are found in the Tropics: Coccoloba, Triplaris. Rheum is Central Asiatic.—The thick rhizomes of R. officinale (Rhubarb) are officinal. The rhizomes of the ordinarily cultivated species, R. undulatum and rhaponticum, are used in veterinary medicine. The following are cultivated as culinary plants for the sake of their leaves:—Rumex acetosa (Sorrel), R. patientia, R. scutatus, and Rheum undulatum (petioles). Several species of Polygonum (P. hydropiper and others) have a sharp, pungent taste. “Buckwheat” is the mealy fruit of Polygonum fagopyrum (Central Asia) and is of value as a farinaceous food. P. cuspidatum (P. sieboldi, Japan) is an ornamental plant.—Calligonum in sandy and stony deserts.
