This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The marsh plants as a whole are of Arctic origin, a large proportion of them being of northern type. They are ascending species, which, owing to the milder conditions since the Glacial epoch, have gradually tended to reach a higher altitude more suited to their primitive or original requirements.
The marsh flora in its unaltered phase demands a lowland habitat, where water of a stagnant character may rise to a certain level, so that the plants may be more or less submerged. The water is, however, not as in aquatic vegetation able to circulate freely. The marsh vegetation is thus largely above the river level, and may rest upon a certain amount of peat.