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 are two distinct features of the cornfield that are compensative in effect in relation to each other. The struggle of the cornfield weeds amongst the corn towards the light has already been emphasized, and this may be said to mould their characters more than any other factor. The difficulties that cornfield plants have to contend with in this connection are very great; and they would be even more so if the plants were perennial, and under necessity of storing up reserves to help them over the resting season and to make a fresh start in spring. But this duty is not required of them. Hence the light requirements are not so vital as might seem to be the case.
If this were so, however, the plants have not to struggle for their existence against overcrowding or lack of nutritive elements derived from the soil, for the latter is open and not thickly colonized, so that the intensity of the struggle for the light is directly counteracted by the favourable conditions in the soil. This is shown by the luxuriance of such plants as Red or White Dead Nettle growing in a turnip field, compared with their growth in a cornfield, and Selfheal is a good example of the same kind of thing. Wall Speedwell, which sometimes grows in a cornfield, is much more luxuriant than when growing on a wall, and so is Creeping Speedwell, which grows usually in meadows.