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 character of the habitat introduces new features into the mode of dispersal of the seeds. The seeds of submerged and floating plants must in these cases germinate in the mud at the bottom. Heavy seeds are more likely to be deposited not far from the plant, sinking to the bottom. But in some the seed-coat contains air vessels and the fruit or seeds float along, some even germinating at the surface of the water. In this case wind and water currents enter into their mode of dispersal. Seeds of half-submerged plants in the reed swamp, which may fall into the water, are of this type, and one may note sometimes a scum of small seeds that have been drifted together by capillary attraction, and driven to the margin to germinate in the mud on the banks. The Water Lily seeds are surrounded by a spongy aril with air spaces, and at first float up to the surface. Afterwards, when the aril has rotted, the seeds fall again to the bottom.
The fruits or seeds of the following are dispersed by special agencies of the plants themselves: Meadow Rue, Water Cress, Great Yellow Cress, Great Water Stitchwort, Purple Loosestrife, Marsh Bedstraw, Water Betony, Musk, Mints, Gipsywort, Skullcap, Fritillary, Wood Club Rush. Water Fennel, White and Yellow Water Lily, Brooklime, Amphibious Knotgrass, Bur Reed, Sweet Flag, Duckweed, Water Plantain, Arrowhead, Flowering Rush, Bulrush, are chiefly dispersed by aquatic agency, in some cases also by the wind. The wind assists Great Hairy Willow Herb, Hemp Agrimony, Fleabane, Three-lobed Butterbur (also, owing to its hooked fruits, dispersed by animals), Coltsfoot, Butterbur, Water Ragwort, Marsh Thistle; also Yellow Loosestrife, Creeping Jenny, Crack Willow, Frogbit, Yellow Flag, Reed Mace, and the Reed. Water Fennel, Scorpion Grass, Bur Reed, may be largely dispersed by animals.
Though true aquatics, especially free floating types, are independent of soil, they, like the submerged and floating types which are anchored in mud or sand or gravel, etc, are to some extent influenced by the character of the substratum.
Since the soil brought down by rivers and streams is of a particular character, it is natural that plants that live in the reed swamp, or upon the banks or low-lying ground, should be dispersed elsewhere where the conditions are similar.
At the same time there is a considerable amount of peat or humus formed along a river margin, which is usually black and amorphous, and this is another requirement of such plants. Others are addicted to a clay soil. Meadow Rue, Water Cress, Great Water Stitchwort, Purple Loosestrife, Great Hairy Willow Herb,
Marsh Bedstraw, Hemp Agrimony, Fleabane, Coltsfoot, Butterbur, Water Ragwort, Marsh Thistle, Yellow Loosestrife, Creeping Jenny, Scorpion Grass, Water Betony, Musk, Mint, Gipsywort, Skullcap, Amphibious Knotgrass, Crack Willow, Yellow Flag, Fritillary Rush, Reed, require either sand, clay, or peat.