1. The Spotted Persicaria (_Polygonum Persicaria_).--A very common wayside plant, and a weed of almost all cultivated soils, growing from one to two feet high, and flowering from July to October. The leaves have, usually, a dark-coloured patch in the centre; and the stipules are fringed at the top with fine, stiff hairs. The flowers are rose-coloured, with more or less green, arranged in short axillary or terminal spikes without any leaves; and the nuts are rather thick, but flattened, smooth, and glossy.
2. Pale-flowered Persicaria (_P. lapathifolium_).--Very similar to the Spotted Persicaria, and sometimes regarded as a variety of that species; but it differs in that its leaves are never spotted, and the lower stipules are not fringed with hairs. The peduncle and perianth, which are smooth in _P. persicaria_, are here rough, being dotted with small, projecting glands; and the styles, which are united to about half way up in the last species, are quite free in the present one. The flowers are pink, with more or less green, and do not usually bloom after August; and the plant often attains a length of three or four feet.
3. Knot-grass or Knot-weed (_P. aviculare_).--A very common procumbent weed, with wiry stems from one to two or three feet long. The leaves, seldom as much as an inch in length, are narrow, oblong, and flat; and the stipules are white, membranous, more or less cut at the edges, with a few veins. The flowers are small, very variable in colour, arranged in short-stalked clusters of about three or four in the axils of nearly all the leaves; and the fruit is a triangular nut, shorter than the segments of the perianth. This plant flowers from July to September. An erect variety, growing to a height of two feet or more, may be seen in cornfields.
The same order includes the well-known Docks (_Rumex_), which differ from _Persicaria_ as follows:--The root is very thick, and grows to a great depth; the stems are erect and furrowed; and the thin membranous stipules, though never fringed with hairs, often become more or less torn. The flowers are small, green, in axillary clusters or terminal racemes, often turning red as the fruit ripens. The perianth is deeply divided into five segments, three of which become enlarged and close over the triangular nut. Two species of this genus are abundant on waysides. They are:--