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 submerged parts of bog and marsh plants are exposed to the same conditions as aquatic plants, but the water in which they grow is deficient in some salts, especially nitrates, though rich in lime. Their aerial parts above water are also exposed to drought, and the bog plants have xerophil-ous adaptations, absorption of water being difficult.
Marsh plants are similarly protected and enabled to withstand drought, though less liable to alternations of wet and dry conditions than bog plants. Moorland plants show the most marked contrivances for resisting drought, and the vegetation is almost entirely xerophytic.