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 great difference between land and water plants has an important effect upon the duration of the latter and their mode of seeding.
It is much more difficult, in fact, for an aquatic plant to germinate, put forth aerial shoots, branches, leaves, flower, and finally fruit in a single season, flowering late as such types do, and beginning to grow upward late.
The marsh plants and Hygrophiles generally are similarly retarded. It is therefore unlikely that many aquatic plants should be annual. For the possible success of the flowering cycle, and maturing of seed, may not eventually result in the propagation of a new plant from seed next spring. Therefore the provision of perennial underground organs or roots or rhizomes is a great assistance to aquatic plants. The time taken in developing these must equal the ordinary life of an annual.
Moreover, the reproduction of aquatic plants is very largely vegetative. Resting buds or hibernacula are formed in winter by Frogbit, Pondweeds, etc. Naturally the aquatic element is for these purposes a distinct advantage. Normally, of course, growth is going on during the summer months, and the winter is, as in the case of most other plants, a period of rest. The vegetative cycle is broken in winter by the elaboration of such special structures. Many plants simply sink to the bottom, and in the spring rise again to the surface. The hibernacula drop off and sink to the bottom till the spring.
Water Lilies die down to the rhizome. Only a small percentage of aquatics are annual or biennial, as Duckweed, Horned Pondweed and Awl-wort.
The aquatic habitat of water plants affects all the phases of their life history. This applies not only to their vegetative organs, but also to the reproductive processes, or devices for the continuity of the species.
Some few aquatic plants hardly raise their flowers above water, or not at all as in the case of Zostera, and in Vallisneria, a sub-tropical plant which has been found in the Canal at Manchester. The rest are more or less normal, the reproductive parts of aquatic plants being less altered than their vegetative portions. But there are some features in which they differ from land plants. Thus a number of them have very small (reduced) flowers, and few are sweet-scented. A large proportion have white or yellow flowers. The plants in the reed-swamp association are the most diverse, and most closely allied to land plants.
Beetles help to pollinate the White Water Lily, and many are pollinated by small flies, as Water Plantain. Whilst cross-pollination is effected by insect agency in the majority of cases, a number are more liable to be self-pollinated, as Great Yellow Cress, Great Water Stitchwort, Three-lobed Butterbur, Yellow Loosestrife, etc. Heterostylism is found in Purple Loosestrife and Yellow Loosestrife, which are trimorphic. In Great Hairy Willow Herb and Creeping Jenny the anthers and stigma are ripe together. The Marsh Bed-straw, Great Water Stitchwort, Brooklime, Amphibious Knotgrass, Flowering Rush, ripen their anthers first, before the stigma. Butterbur, Frogbit, and Reed Mace are dioecious. Coltsfoot, Bur Reed, Bulrush, and the Reed mature their stigma first. Duckweed and Arrowhead are monoecious, and the former is pollinated by aquatic insects. The following are wind-pollinated: Crack Willow (visited also by bees), Reed Mace, Frogbit, Bur Reed, Sweet Flag, Bulrush, Wood Club Rush, and the Reed.