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.
Since marshes are areas where plants are in various stages of submergence, it is not unnatural that they should show various degrees of connection with plants that are entirely submerged, or aquatic plants. Marsh vegetation and aquatic vegetation are, in fact, intimately connected. The former is often marginal to the latter, and the plants of the one are frequently common to the other; thus a river or lake with aquatic vegetation may give rise to a marsh at its edge. The difference again between marsh formations and fen or carr vegetation is less marked than the difference between the former and moorland vegetation, the alkaline nature of the water of the second resembling that of a marsh. Marsh vegetation may, on the other hand, become transitional to wet meadows by drainage. Marsh plants are also largely common to salt marshes where the conditions are suitable.