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 plants which are found in cornfields are a motley assemblage. Some are pure aliens and of merely sporadic occurrence, as Larkspur, Gold of Pleasure, Venus's Looking Glass. Charlock is a denizen. Corn Buttercup, Red Poppy, Fumitory, Candytuft, Corn Cockle, Venus's Comb, Corn Marigold, Cornflower, Snapdragon, Wild Oat, and Darnel are colonists, which were defined by Watson to be weeds of cultivated land, by roadsides or about houses, only existing here as long as human agency provides suitable conditions. The denizen is apparently indigenous, but liable to some suspicion of having been originally introduced by man, as the Horse-radish and Melilot. The alien plants are certainly or very probably of foreign origin, though now more or less distinctly naturalized, as among trees the Sycamore and the Bird Cherry. The casual was accidentally imported or strayed from cultivation, not truly naturalized, and generally unable to maintain itself from year to year, as the Caraway, and perhaps Larkspur.
Native plants are believed to be truly aboriginal species, and amongst cornfield plants perhaps the following are native: Scarlet Pimpernel (this grows also in woods and on shingle), Corn Gromwell (Common Gromwell grows by the roadside), Hemp Nettle, also a wayside plant.