The Climbing Bistort (_Polygonum convolvulus_--order _Polygonaceæ_), also known as the Climbing Buckwheat, Climbing Persicaria, and Black Bindweed, is a very troublesome corn-field weed, with the climbing habit of the Convolvulus, often strangling the plants round which it twines its angular stem. It varies from one to four feet in height; and its alternate leaves are heart-shaped or arrow-shaped, pointed, with short membranous stipules at the base of the stalk. The flowers are small, pale green, in little loose clusters of from four to twelve. The lower clusters are stalked in the axils of the leaves, and the upper ones form irregular, terminal racemes. The five segments of the calyx are bluntly keeled, and occasionally winged; and the three outer ones closely envelop the fruit--a triangular nut. The plant flowers from July to September. At least two or three of the Spurges (_Euphorbiaceæ_) are commonly seen in cultivated fields, but one in particular--the Dwarf Spurge (_Euphorbia exigua_)--is common in corn fields. It is a slender, smooth plant, usually from two to ten inches high, with several ascending stems diverging from near the base. The little yellow flowers are in terminal umbels of from three to five rays, sometimes very much contracted; and their glands (see p. 207) are crescent-shaped, with their fine points turned outwards. The time of flowering is July to October. Our last example of the corn-field plants is the Wild Oat Grass or Havers (_Avena fatua_)--an erect grass, two or three feet high, with rough leaves, and stem hairy at the joints. Its flowers form a loose, spreading panicle, from six to nine inches long; with three-flowered spikelets, about an inch long, on very slender stalks, erect at first but afterwards drooping. The outer glumes are about three quarters of an inch long, tapering to a bristly point, often tinged with purple; and the inner ones, two or three in number, are a little shorter, cleft at the top into pointed lobes, and covered outside with yellowish-brown hairs. The awn is about twice as long as the spikelet, twisted at the base, and usually bent near the middle. This grass flowers during June and July. XVIII ON THE CHALK While some flowers are so universally distributed that they may be described as existing almost everywhere, others are restricted to certain kinds of localities, outside which they seldom occur. This restriction is sometimes merely one of light and shade, the same species growing almost equally luxuriantly in open spaces, or, in shady places, regardless of other conditions. Some plants, however, are particularly partial to certain conditions of soil, situation, or climate, and are consequently more strictly confined to limited districts.