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.
Water has a more uniform temperature than the soil, and thus aquatic vegetation is more or less constant, for the chief zones of latitude. But adaptation to an aquatic life necessarily involves particular habits, etc. Many water plants are found all over the world. This may be due in part to the migration of water-fowl.
Aquatic plants form open associations, but are not in this case so subject to the encroachment of other plants, except riparial types; and it is here that the invasional factor comes into play. Some plants in fact are amphibious, and can live in water and on land.
Water plants derive their air through the water, so that they have wide air-spaces. The leaves are thin and much divided, because the light coming through the water is already diffused or broken.
Some aquatic plants float freely upon the surface, as Duckweed; others are rooted in the mud, as Water Lily, and raise their leaves above water; whilst others are submerged. There are those that grow half in and half out of water, or at the margin. Thus the vegetation of the water exhibits every transition to that of the land.
Aquatic plants are distinguished from land plants in being submerged wholly or in part in water. There are also moisture-loving plants (see below), from which they are distinguished, hence the term Hydrophytes. Certain modifications, briefly mentioned in Section I, are required by water plants to fit them for life in water.
The watery element necessitates the increase, in size and number, of the air-spaces. The chlorophyll granules are aggregated in cells nearer the surface than in land plants to accomplish photosynthesis more effectively.
Water plants have a thin cuticle which is not cutinized or waxy, so that absorption takes place at all points, and stomata are not required as a rule, or where present are on the upper surface, and do not open and close.
The stems of water plants, owing to the support received from the water, do not require thickening, or a series of vessels of woody elements (xylem) for the conduction of water from the root to the rest of the plant, as absorption is possible at all points, so that the supporting tissues are reduced; but as it is necessary for the plant to convey food from the leaves the phloem is well developed.
Since absorption is not confined to the roots, these are also not well developed, and may be absent, as in Bladderwort, or where present be merely organs of attachment, or serve to maintain equilibrium.
Whilst many aquatic plants have primitive characters, some have highly-organized flowers, as Water Buttercups, and these are probably derived from earlier land types and have adapted themselves vegetatively to life in the water. A large proportion of Monocotyledons (over 30 per cent) are aquatics.
Between land plants that do not need a considerable amount of water and those that are aquatic there are intermediate types, which are distinguished owing to their demand for a greater or less amount of moisture. Those that are intermediate, such as meadow plants, woodland plants, cornfield plants, are Mesophytes, requiring a medium amount. There are others that require a larger amount of moisture, such as marsh plants and wet meadow types, and the riparial types of aquatic plants.
Marsh plants, like water plants, are not able to transpire readily, and have emergency exits for the expulsion of water in drops at special points.