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.
Bog and marsh plants are amongst the most truly native of our wild plants. The vegetation they make up is essentially primitive, derived very largely from arctic or northern regions, where few or no annual plants are known. As has been seen the period of flowering is indeed very late, and the interval between this period and winter is relatively very short; the uniformly perennial character of bog and marsh vegetation is therefore a natural characteristic. A few, as Cranberry, Rosemary, Bog Myrtle, are shrubs, and the Alder and the Willows are trees. The only annual is Marsh Lousewort, a hemi-parasite on Grass roots.