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 groups of plants found in cornfields are as a whole deficient in adaptation for seed dispersal by animal agency; for except the Field Bu-gloss, Corn Buttercup, and Wild Oat, there are none which are likely to catch in the wool or fur of animals.
Only a few have special devices of their own apart from the external agency of the wind for dispersal to a distance, such as Shepherd's Needle and Heart's Ease. The seeds of Gold of Pleasure and Corn Gromwell may be eaten by birds.
A large number are dehiscent, dry fruits, which split open when ripe, as Charlock and Larkspur, and the Scarlet Pimpernel has a special type, or pyxis, with a lid which opens when the seeds are ripe. The bulk of the plants have fruits of the pepper-box type, as the Poppy, or censer fruits, which scatter their numerous small seeds, when the wind jerks the flowering stems, around the plant itself. In a high gale they may be driven some distance. A few, as Corn Sow Thistle, have a feathery pappus which assists in their dispersal by the wind.
The closeness of the cornstalks, by its shielding effect, rather counteracts the action of the wind. And the seeds of the majority of the plants are thus as a rule scattered over a small area. Hence the patches of the same plant in a cornfield in addition to the even dispersal of most as a result of harrowing, etc.