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 vegetation of the land is of one type, that of the water of another. This physical distinction, indeed, has a marked influence upon the forms of plants. Those that grow in water are aquatic or Hydrophytes. While land plants are directly exposed to the air, water plants are not, and the air dissolved in water sometimes contains a larger proportion of oxygen and carbon dioxide than has atmospheric air. This is important, because plants respire by aid of the former and assimilate by aid of the latter under the action of light upon the chlorophyll, manufacturing their carbohydrates by its means. Obviously some waters contain more or less oxygen than others, and some stagnant waters are devoid of any aerating agent.
The influence of water upon light, also essential to plants containing chlorophyll, is great. This influence is least in clear, greatest in dirty water. Depth here is of importance, and is connected with the absorption of different rays of light, red rays being absorbed at the surface, ultra-violet in the lower layers. Water is more uniform in temperature than a land surface, but the different depths of water have different temperatures, hence the zonal distribution as in the case of light.
The constituent salts and nature of water have a great influence on the flora of an aquatic formation. This, again, is dependent on the basin which is drained or the soil of the bed. Some water has lime-salts in solution, the carbonic acid in water dissolving the calcium carbonate, and so on. Then, lastly, the movement of water is a great factor in deciding its constituent flora. For some plants are floating, some submerged or attached, and some are attached to rock, some to a soil. If water is in a state of rapid motion it abounds in oxygen, if slowly moving it contains less, if still or stagnant still less. It is a means of dispersal of the seeds and the plants, and as the first plants, like the first animals, undoubtedly arose from a liquid solution, plants have been largely spread over the earth by water.
There are certain plants like Hydrocharis, giving its name to this type of flora, which float on the surface, unattached, in fresh water, swamp-plants, in still water, sailing about, and with erect stems and leaves. Here, besides Frogbit, we may further include Bladderwort, Duckweed, etc, and Water Violet. These plants have the same specific gravity as the water.
The shoots have long internodes, thin stems, stalkless or stalked leaves, with threadlike segments, as in Bladderwort and Water Violet, when submerged. Floating leaves are shield-shaped, egg-shaped, heart-shaped, as in Hydrocharis, Lemna, and the division is well shown in different types of Water Buttercup which have both types of leaves. The plant is secured by its root. Nutriment is largely absorbed by the stem and leaves.
Many plants growing in water reproduce by division vegetatively, as Frogbit and Duckweed. The pollination of the Frogbit, Water Violet, and Bladderwort is effected by insects, and the Hornwort opens its flowers under water. Frogbit and others are perennial, and survive the winter by forming winter-buds or hibernacula, which sink in the autumn and rise again in the spring.
The plants that grow on the loose soil of aquatic formations where the soil is quartz-sand are differentiated by the movement and salinity of the water, and chiefly flowering plants grow upon it. The roots are chiefly attachments, and the Mare's Tail has few or no root hairs, nor has the Water Violet. Zostera forms meadows on account of its long, creeping rhizomes or underground stems in purely saline waters, forming maritime vegetation. Most maritime aquatic plants are Algae.
Belonging to what is called the Enhalid formation are the colonies of Zostera growing in salt water, and with Naias, unique amongst the flowering plants, Ruppia and Zannichellia are found in brackish water. In Zostera the leaves are ribbon-like and long, and the roots rhizome-like. Along some shores it forms a regular zone. In brackish water Chara, Water Buttercup, Potamogeton, and Myriophyllum grow.
The chief aquatic formations of flowering plants are known as the Limnaea formation, so called from the prevalence of the fresh-water pond snail in it, and are submerged or have floating leaves. The chief types belong to the Pondweed, Water Pepper, Bur-reed, Water Lilies, Water Buttercup, Starwort, Water Celery, etc. The Hydrophytes altogether number some 700, of which we describe 41, and of the Hydrophytes some 120 are lacustral. Here we include littoral, of which there are 20, and we have also included many riparian species which form a link between Hydrophytes and marsh plants and are hygrophilous.
Photo. L. R. J. Horn - Reed Swamp At The Margin Of A Norfolk Broad
Most are herbaceous perennials. The majority have creeping stems, and are of clustered, crowded growth, as Mare's Tail, Water Lily, Water Buttercup, Starwort, with creeping stems. A few, such as Naias, are annual. There are three types of plant-shoots, the rosette type, as in Lobelia; the Nymphaea type, as in Water Lilies, with long-stalked, floating leaves; the long-stemmed type with erect stem and internodes, completely submerged, as in Pondweeds, with floating leaves, as in Starwort (Callitriche verna).
The leaf type is floating, as in Water Lilies, Polygonum am-phibium, Pondweeds, with broad entire leaves and bent margin, pores on the upper surface; and the blade is dorsi-ventral, red below, to retain heat, and the petiole or leaf-stalk adapts itself to the water, growth ceasing when the surface is reached. The Batrachian Ranunculi are heterophyllous, and able to adopt a terrestrial existence if need arise.
The submerged leaf may be zosteroid or ribbon-like, as in Bur-reed, some Pond-weeds, caused by deep or running water, some marsh plants acquiring them if necessary, and the Bulrush has current leaves half a yard in length. The elodioid leaf is narrow, flat, stalk-less, entire, as in Elodea, Mare's Tail, etc. The isoetoid leaf is linear, undivided, rounded, and tubular, as in Isoetes. The myriophylloid leaf is whorled, as in Water Milfoil, or consists of leaves divided into threadlike or linear segments, as in Dropwort and Sium.
Most aquatics are pollinated above water. Water Lilies and Water Violet are pollinated by insects; Mare's Tail, Water Milfoil, and Pond-weed by the wind or water, and by water in the case of Zannichellia, Starwort, Naias. Subularia, Limosella, and some Batrachian or Water Buttercups, etc., are cleistogamic. The fruits or seeds are dispersed by the water. Reproduction is largely vegetative. Many plants develop hibernacula, as Pondweeds, Starwort, etc.
Associations arranged in zones may be recognized as Algae, Chara-ceta, Nymphaeeta, Nuphareta, Batrachieta, Limnaea, rosette forms, Lobelia, etc. The Hydrocharis and Limnaea formations merge into each other, and the ease with which water plants become helophilous is shown by the amphibious Polygonum amphibium, by Water Cress, and the Water Plantain.
Along the margins of the rivers tall clumps of sulphur blooms of Meadow Rue line the waterway. Water Fennel dangles its lace-like form in the water. The white and yellow Water Lilies lend their floating tables to the nymphs of the pool. Water Cress grows in the smaller streams and ditches, in clear running water. Here Great Yellow Cress rears its tall, erect heads of yellow bloom in the canal or river. There Great Water Chickweed, late in the year, fills up the ditches with its brittle stems. Down by the banks of the river-side the Purple Loosestrife, with its trimorphic blooms, gives honey and pollen to the bees. Where the Great Hairy Willow Herb fills the bed of the stream with its tall stems with purple blooms comes the scent of apple-pie or lemon curd, delicious on a sweltering summer's day. Water Bedstraw makes a lovely show of white flowers by the pond-margins. The Hemp Agrimony grows in the wet copses or by the water-side, tall and fleecy-flowered. The Common Fleabane and Three-lobed Butterbur up and down the country line many a river-side close to the bank. Butterbur makes dense brakes with ample cover, where the streams have meandered and formed hollows along their course. Coltsfoot, too, grows on the clay banks, being equally proter-androus. Water Ragwort, with its handsome and large-rayed flowers, is scattered over most water meadows where the purple lances of the Marsh Thistle tower, in close rank, on the lower ground. Great Yellow Loosestrife and Moneywort, their golden-yellow blooms large and brilliant, are both Hygrophytes.
Scorpion Grass hangs its pretty blue-and-yellow spotted flowers above the watery mirror of the pool, surveying its own rich beauty. Water Figwort, less striking in colour, towers by its side. Here and there the straggling clumps of Musk have their wide, yellow, trumpet-mouthed blossoms adorned with rose-pink honey-guides. Brook Lime with light-blue flowers, Water Mint with whorls of lavender-like bloom, and the shy Gipsy Wort adorn the marshy strips by the water-side or grow half-submerged, and close by is the dainty Skull-cap, blue-flowered and neat. Amphibious Knotgrass in the water yields a bright-pink flower, but on land is roughly hairy, long-rooted, and difficult to dispose of. Alder and Crack Willow give grateful shade by the waterside.
Frogbit floats in the still pools and rarely blooms, its margin, rustics tell one, nipped by frogs! Flanking the sides of the pool the mellow Yellow Flag forms a fair girdle almost everywhere. In the water meadows Snake's Head Fritillary is found in dainty clumps with chequered flowers of a rare purple or white tint. Along the borders of the river Reedmace, Bur-reed, Sweet Flag (how sweet the leaves smell crushed in the hand!), form a thick avenue.