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. [Illustration: FIG. 356.--Flower of _Polygonum fagopyrum_ in longitudinal section: 1, long-styled; 2, short-styled; _a_ the anthers; _st_ the stigmas; _n_ nectary.] Order 2. =Piperaceæ (Peppers).= Shrubs or herbs, often with nodose, jointed stem; leaves simple, entire, often with curved veins; stipules wanting (_Peperomia_) or intrapetiolar and cap-like, often enclosing the terminal buds (_Piper_). The flowers in the group _Pipereæ_ (_Piper_, Fig. 357, and _Peperomia_) are borne in spikes with fleshy axes (_club-like_), seldom in racemes, the outer ones are crowded and are ☿ or unisexual, always small, _naked_ and without bracteoles; ~generally stamens 3 + 3, and gynœceum 3, but the number of the stamens may be reduced by suppression to 2, and the carpels to 1~. The flowers are situated in the axils of the small, generally shield-like floral-leaves. The ovary is always _unilocular_ and has _one upright, orthotropous_ ovule. Fruit a berry or drupe. Both endosperm and _perisperm_ are present, the latter being especially well developed (Fig. 359). _Piper_; generally shrubs with scattered leaves, and terminal inflorescences which are crowded to one side by the development of the highest lateral bud, so that they are situated opposite the leaves (Fig. 357). Many species have stems with an abnormal anatomical structure.--_Peperomia_; chiefly succulent herbs, often epiphytes, with opposite or verticillate leaves having aqueous tissue on the upper side. [Illustration: FIG. 357.--_Piper nigrum_: branch with fruit (½)]