Most seaweeds (apart from the giant kelps) have no vascular system because they need no strengthening tissue, being supported by the water around them. Seaweeds have a thick cuticle that covers a thin photosynthetic layer. The inner layers are formed from densely packed, thick-walled filaments, with a few thin-walled medullary cells between them. These cells may serve to transport water through the plant.

Aquatic plants (hydrophytes) survive in two main habitats—saline and freshwater. Saltwater areas include the shoreline and salt marshes; freshwater plants survive in rivers and streams, and on their banks, in lakes and in ponds. Both types are subjected to permanent immersion or frequent flooding, strong winds and water movement, and unstable soil conditions, which few other plants can tolerate. The major difference between the two kinds is that saltwater plants (halophytes) have to endure high concentrations of salt, whereas freshwater plants do not.

The elongated ribbonlike leaves of aquatic plants, such as eelgrass (Zostera sp.), offer little resistance to moving water and are, therefore, an ideal shape in their periodically turbulent environments. This plant has submerged flowers, which are pollinated under water (one of the few flowering plants in which this occurs).

General adaptations

The fact that the density of completely immersed aquatic plants is similar to that of the water around them means that they have little need of support; submerged leaves and stems therefore contain little strengthening tissue. Leaves and stems underwater also have no cuticle, so that they readily absorb dissolved carbon dioxide, oxygen, and mineral salts from the water.

Freshwater plants have a vascular system but with few woody lignified vessels in the xylem. In these plants the whole vascular system, particularly the xylem, is simplified and in some species is replaced by a cavity, or lacuna. Several aquatic species have many lacunae within their tissues. In floating leaves they are gas-filled and maintain buoyancy, raising the plant to the light so that it may photosyn-thesize— chloroplasts occur in the epidermal cells of the upper surface, unlike those of land plants. The lacunae also allow oxygen to diffuse rapidly from the surface to the submerged parts.

Marine habitats

Most plants that grow in the less-shallow areas of the shore are seaweeds. Those that are not tend to live in estuaries or in rock pools. Brown algae dominate in cool temperate to Arctic waters, while red algae predominate in subtropical and tropical waters.

All seaweeds require light for photosynthesis and cannot grow in water where sunlight does not penetrate. Most species have gas-filled bladders that keep them buoyant when the tide is high so that they remain near the light. All species contain chlorophyll, although red and brown seaweeds also contain other pigments (fucoxanthin and phycoerythrin respectively). Chlorophyll is not particularly efficient in trapping the bluish light that reaches deeper water, whereas the red-brown pigments (which absorb blue) gain additional energy when the plants are submerged. Brown and red seaweeds, therefore, have an advantage over green species and, in fact, only red seaweeds are found in deep water.

Apart from the algal seaweeds, there are a few flowering plants that dominate shallow marine waters, such as eelgrass (Zostera spp.), and species of cord grass (Spartina spp.) that dominate the saltmarshes on coastal mud flats. Species of cord grass have vertical roots for anchorage, horizontal roots for obtaining nutrients, and stolons by which they spread, forming a compact mat within the intertidal zone.

Rocky shores and dunes are extremely unstable habitats, where plants may be buried under sand and stones. Many plants, therefore, have extensive root systems, such as the yellow horned poppy (Glaucium flayum), so that they can make new growth despite being deeply covered.

Shore zonation

Seaweeds are often abundant on rocky shores where a clear zonation, or ecological gradient, can be seen. From the low-tide level upwards, the plants increasingly experience desiccation (drying out) and high temperatures. Many of them have developed rubbery cell walls and produce a coating of mucilage to meet these environmental stresses. The cell walls can contract and expand without damage according to the water content of the plant, and the mucilage reduces desiccation when these plants are exposed to the air.

At the lowest levels are the brown kelps (Fucus spp. and Laminaria spp.), whose long, pliable stipes (stems) make them resilient to violent wave action. Wracks dominate the middle and upper levels of the intertidal zone.

The algae that are characteristic of the lower shore often grow in rock pools at higher elevations, where they remain submerged even at low tide. These pools suffer extreme variations in temperature and salinity. Some red seaweeds, such as the thin-fronded, delicate laver bread (Porphyra sp.) and sea lettuce (Ulva spp.) that grow high on the shore, survive desiccation and recover when the tide returns.

The intertidal zone of coastlines is most frequently inhabited by seaweeds, such as oarweed (Laminaria digitata) and thongweed (Himanthali elongata). The long, flexible, but resilient, stems of these plants allow them to survive the pounding of the waves.

Salt-marsh, dune, and rocky shore plants

Salt-marsh plants are subjected particularly to changing salinity levels because the salt content of the soil is increased by incoming tides and by evaporation, and then reduced by rain and dew. Flalophytes, such as glasswort (Sali-cornia fruticosa), have a high osmotic concentration maintained by a high cellular concentration of amino acids (proline, for instance), which enables them to absorb water directly from the sea. Some species, such as sea lavender (Limonium sp.), have special salt glands to pump out excess salt onto their leaf surfaces.

Plants that grow on sand dunes and rocky shore are not true aquatics, although they may sometimes be inundated by the sea. They often have a need to conserve water, because the loose soil particles hold little water, and they are frequently buffeted by strong winds. Some have xeromorphic adaptations to reduce their transpiration rate: the prickly saltwort (Salsola ka/iJ has tiny leaves reduced to spines; in some species the leaves roll up in unfavorable conditions, as in marram grass (Am-mophiia arenaria); other plants have a thick cuticle, often with a waxy coating or a thick covering of hairs. Low growth, frequently in the form of rosettes or cushions, also avoids the harmful effects of wind and evaporation. Another feature is succulence, in which leaves or stems swell when water is stored in the tissues, as in the leaves of the shrubby seablite (Suaeda fruticosa).

Succulence is also seen in the mangrove shrubs—so typical of the coastal swamps of tropical areas. The succulent leaves retain the water that the plant has absorbed and excrete the salt. The salt is then washed away in the frequent rains. Most mangroves have stilt roots, anchoring the plant in the mud and preventing the waves and tides from moving it. The lack of oxygen in the mud is overcome by the presence of protruding roots, or pneu-matophores, which absorb oxygen from the air and take it down to the roots embedded in the mud. The zones of different mangrove species trap mud and silt, extending the land seaward. Inland from them lies a region of brackish swamps, flooded by the sea only at times of very high tide—a habitat for many tropical aquatic plants.

Freshwater habitats

Freshwater plants have to cope with bodies of water that change rapidly in their chemistry and rate of flow. The chemical composition of the water depends on the rocks over which the water has passed, the water collected from several individual springs, shallow seepage, and surface runoff. In arid regions particularly, there is a great annual fluctuation in the size and chemical properties of river water.

The variation in volume and speed of flow of river and stream water, and strong currents, can damage and uproot plants. Mountain streams are fast-flowing and, therefore, usually contain few plants; those that do survive these conditions include willow moss (Fontinalis squamosal which is anchored to stones. Sluggish rivers, however, permit plant growth in their shallows and along their margins. Still or slow-moving water in ditches, ponds, and lakes also support far more species.

Freshwater plants in fast-moving river water are usually rooted to the stones at the bottom of the riverbed. Water buttercup (Ranunculus aquatilis) is one of the few plants that survives this turbulence and has submerged leaves that are long and narrow and flow with the current without damage.

Freshwater adaptations

As in deep saltwater, light does not reach the bottom of deep freshwater, and rooted plants cannot grow there. In addition, little light penetrates water that is stained with peat, mud, silt, or microorganisms. The only plants in such habitats, therefore, are free-floating species, notably algae.

Where light does reach the bottom, aquatic plants root in the mud. Some, such as stone-worts (Chara spp.) and Canadian pondweed (Elodea canadensis), grow completely submerged. Others, such as waterlilies (Nympha-eaceae) are rooted in the bottom, but their leaves, which are attached to stalks up to 10 feet (3 meters) long, float on the surface.

Emergent aquatics grow on pond and lake edges in water less than 6 feet (1.8 meters) deep. They include reeds (Phragmites spp.) and bulrushes (Scirpus spp.), which send stems and leaves above the water surface. When these plants are the dominant species, the habitat becomes a swamp. Freshwater hydrophytes show various adaptations to avoid damage by swamping.

Common duckweed (Lemna minor) is a well-adapted, free-floating freshwater plant. Each plant consists of a small green thallus 3-4 millimeters across, which is kept buoyant by air-filled lacunae. It is not differentiated into stems and leaves. New cattail (Typha) plants are also seen here.

Many have a water-repellent cuticle on the upper surface of their floating leaves. (This also reduces water loss by evaporation.) In some species of waterlilies, the leaf margins grow vertically upward to reduce the chance of flooding. The petioles are long and, in some species, are corkscrewlike, which allows the leaves to stretch and contract to accommodate changing water levels. Diaphragms at the internodes of submerged stems prevent internal flooding if the plant is damaged.

Many aquatics have several types of leaves to cope with their watery conditions. Water buttercup (Ranunculus aquatilis), for example, has divided submerged leaves and lobed floating leaves, whereas arrowheads (Sagittaria sagittifolia) also have aerial leaves. The floating leaves of aquatics have stomata on the upper surface, whereas the submerged leaves are usually long and narrow, offering minimum resistance to currents. They are often finely divided to provide a large surface for absorption. The aerial leaves are usually like those of terrestrial plants.

During the day, when freshwater hydrophytes photosynthesize, they use up the dissolved carbon dioxide, which may become scarce. Some plants, therefore, such as the quillworts (Isoetes spp.), have a special mechanism for absorbing carbon dioxide at night, when the respiration of aquatic animals and plants causes a carbon dioxide build-up.

The carnivorous bladder-wort (Utricularia sp.) augments its nutrient supply by sucking insects into bladderlike traps in its submerged leaves and absorbing nitrogen from their bodies. This underwater plant sends up shoots above the surface when it blooms, so that the flowers can be pollinated by insects or the wind.

The roots of freshwater plants do not take in water, but function mainly in extra nutrient absorption. They also serve as means of anchorage in bottom-rooting species and balance in free-floating species.

Most hydrophytes have flowering stems that project above the surface. The stems are usually supported by floating leaves, but such plants as waterlilies have floating flowers. Aerial flowers are pollinated by the wind or insects. A few species, however, have flowers that are adapted for pollination by water. The free-floating hornworts (Ceratophyllum de-mersum), for example, bear tiny flowers, which open underwater. Their stamens become detached, float, and burst, releasing pollen grains that sink slowly, reaching the submerged stigmas. During floods, when aquatic plants cannot easily produce aerial flowers, submerged, cleistogamous (non-opening) and self-pollinating flowers may be produced. The seeds of hydrophytes are usually dispersed by water. Some emergent aquatics, such as the bur reed (Sparganium sp.), often have inflated seeds that float to the edges of lakes where they germinate.

Vegetative reproduction is also common. New fronds bud from the side of duckweed (Lemna sp.) and break off to form separate plants, and detached pieces of Canadian pond-weed will root and grow independently. The aquatic fern, Salvinia, reproduces by the breaking up of old stems.

Frost damage is a winter hazard to freshwater plants. Water starworts (Callitriche spp.) and water soldiers (Stratiotes aloides) avoid it by sinking to the bottom in winter and rising again in spring. Waterlilies survive by deciduousness and by storing food in their stout rhizomes, buried in the mud. In the spring new leaves grow, and food is manufactured once more. Frogbit (Hydrocharis morsus-ranae) produces special winter buds (turions) on stolons that sink to the bottom when the plant dies in autumn. After remaining dormant in winter, they rise to the surface and form new plants.

In the freshwater tape grass (Vallisneria spiralis), the male plants flower underwater; the buds float up to the surface where they open. The flowers on the female plants rise above the water where they are pollinated by the floating male flowers. Once fertilized, the female flower is drawn underwater where the ovule develops. The mother plant dies away and the new seed develops into a new plant.