This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The cornfield as a complete whole is a composite type of vegetation. Apart from the portion of each field under cultivation, there is usually a grassy strip along the borders, varying in width according to circumstances, and in some cases as extensive as the grass rides in a wood. Beyond this zone or border comes, as a rule, a ditch with banks on either side. Then there are also the boundary hedges in most districts, or in some areas dykes or ditches alone, with no other boundary, and in the north of England, Wales, and Scotland, stone walls.
Each of these extensions or limits to the cornfield is of particular importance in studying a cornfield flora, and special attention should be devoted to them. For not only do they form a transition from the grass-field type of vegetation upon which they abut, but they also serve in the case of the grass fringe to stabilize the cornfield flora, and in the case of the ditch and hedge to retain certain of its constituents. The weeds that can subsist under such conditions (more intense than on the open ground of the cultivated area) are thus selected as the dominant and sturdier types, e.g. White Campion and Hemp Nettle.