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.
If the growth habits of the plants are studied it will be seen that though there are three zones of plant societies, as in woods and meadows, and though several types of habit may be distinguished, yet, considered generally, a dominant habit is resolved from the manifold conditions of the cornfield which is in all particulars analogous to that of the chief characteristics of meadows and pastures. This is the grass habit.
The cereals themselves are Grasses, and are associated, like meadow-grasses, in close rank. It is one feature of a cornfield that it should make for this manner of growth in order that there may be the greatest possible result in the least possible space, and the exact balancing of the conditions most favourable to attain this end.
Consequently this factor has the most profound effect upon the associated cornfield weeds. All the plants that may be said to belong to the tree type or zone, such as Gold of Pleasure, Flax, Corn Cockle, Chicory, Corn Sow Thistle, are of the grass habit more or less. Many, too, in the two lower zones (scrub and ground flora) adopt the same habit, as Spurrey, Shepherd's Needle, Lamb's Lettuce, Mousetail, etc.