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.
The aquatic plants have different conditions to contend with in regard to the height to which they usually grow. A land plant, whether it be a shade plant or a sun plant, obtains the necessary light, and other factors essential for growth and nutrition, more or less directly. It has no great period of subterranean existence to outlive after the seed has germinated before it reaches the surface and the light. Nor is the light variable in the character of the rays absorbed, or in intensity to any considerable extent.
Aquatic plants, on the other hand, have to struggle upwards in the dark after germination, and the young shoots have to make some growth before they can obtain even a moderate degree of light. Hence they must attain a certain height under water before their chief functions can be adequately performed. This tends to make the submerged types generally of a uniform height in each zone, and those that struggle upward to put forth floating leaves also attain a more or less uniform length or height. Except in the case of the half-submerged types of the reed swamp, few become erect either below or above water.
The tallest plants, ranging from 3-10 ft., grow in the reed swamp, and are erect or nearly so. But the floating plants and submerged types may attain a great length, though the visible height above water is trivial in comparison.
As in the case of all moisture-loving plants, the season of flowering of aquatic plants is on the whole late. The riparial plants, the least immersed in water, flower relatively early in the year. The immersion of aquatic plants in water, colder than air, is responsible for the late stage at which the respective thermal constants of each plant are reached. It is an interesting fact that a large number of marsh and aquatic plants have the leaves and stem coloured red by anthocyan, which transforms the light rays into heat.
The earliest flowering aquatic or riparial types are Coltsfoot and Butterbur, and Snakes-head Fritillary, which flower in March. In April a small number of others first commence to bloom, as Water Fennel, Scorpion Grass, Crack Willow; and these, save the first, are riparial, or but slightly submerged, if at all.
In May, Meadow Rue, Water Cress, Water Betony, Brooklime are in flower. The following are found in flower in June, when the temperature is much higher, viz.: Yellow Water Lily, Great Water Cress, Creeping Jenny, Musk, Skullcap, Amphibious Knotgrass, Yellow Flag, Sweet Flag, Duckweed, Water Plantain, Flowering Rush. July is the principal month for aquatic plants to flower, and then one may find the blooms of the White Water Lily, Great Water Chickweed, Purple Loosestrife, Great Hairy Willow Herb, Marsh Bedstraw, Three-lobed Butterbur, Water Ragwort, Mint, Gipsywort, Frogbit, Reed Mace, Bur Reed, Arrowhead, Bulrush, Wood Club Rush, Reed. In August, Hemp Agrimony and Fleabane first come into flower.