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.
There is a marked contrast between the longevity of the meadow and the cornfield. In the former there is but a slight disturbance of the main conditions when a meadow is converted into pasture, or vice versa. But in a cornfield there is the yearly ploughing, harrowing, sowing, drilling, rolling, hoeing, cutting, reaping, and harvesting, and a repetition of this process for the next crop.
It is important to remember that the plants of especial interest to the botanist in a cornfield are treated as weeds, and are there on sufferance, their eradication (total or partial) depending upon the diligence or laxity of the farmer. In the meadow no such artificial selection or division into weeds or crops takes place, if one excludes thistles and a few other plants that are distasteful as hay, or useless, e.g. Yellow Rattle. Hence one reason for the ephemeral character of the denizens of the cornfield, or colonists as they are called.
Another equally important factor in regulating the permanence of the cornfield flora is the rotation of crops, of which more is to be said later. Owing to the exhaustion of the soil by certain crops of a highly-specialized character, such as wheat, the plants that grow in cornfields are rendered unstable, because each different type of crop brings with it in the sowing, and by reason of the different method of tillage, a different set of weeds.
A further reason for the short life of cornfield plants is the possible return of arable to pasture or fallow. All these facts should be studied in detail.
Cornfields and Woodlands: some Similarities. - In describing the tiers of plants in a meadow attention was drawn to the analogy between this and that noticed in woodlands. An even more marked similarity exists between this arrangement of crops and other plants in a cornfield pure and simple and that observed in woods. In this case the tree zone is represented by the corn itself and those giants of the cornfield, such as Corn Cockle, Corn Sow Thistle, etc., which tower above their fellows. This, of course, obtains only in the summer, when the stalks are tall and close set. They provide an ample shade for the lower zones of plants.
Intermediate between the tree zone and the lowest zone (ground flora) is a middle zone, comparable with the scrub of the woods, which consists of such medium plants with a widespread pyramid habit as Corn Buttercup, or the later Fool's Parsley. These have narrow leaves, adapted to growth amongst close, erect plants such as cereals, an adaptation not noticed definitely so far. Below come the trailers, dwarf plants, such as Scarlet Pimpernel, and short erect plants (with linear leaves) as Least Spurge. Rosette plants, as Great Plantain, are found like the Daisy covering the surface. There are some climbers, too, as Corn Bindweed or Small Convolulus.