As the name implies, these plants are characterized by the possession of conducting tissues (Xylem and Phloem) for transport of materials throughout the plant body.
Most exhibit structural specialization into true roots, stems, and leaves. The development of these organs is associated ecologically with life in a terrestrial habitat.
These higher aquatic plants normally grow, or at least start their life cycle, in the water. Because they require light, they are mostly limited to shallow water where they grow toward the light. Often they produce floating leaves.
Many grow completely submersed throughout their life and some may have both submerged and exposed leaves that may be quite different in form. Generally, the stems that bear flowers reach to or above the surface of the water.
They nearly all grow anchored in the muddy or silty bottom. Through roots and root hairs, they absorb mineral nutrients to be used in metabolism and growth. However, some can absorb nutrients from surrounding water through their leaves. When the upper part of the plant dies, part of the nutrients in organic combination are released to the water.
As the plants photosynthesize below the surface, they contribute to the oxygenation of the water and take part in the consumption of carbon dioxide.
Those parts that are above water may utilize the carbon dioxide of the atmosphere to make food matter that may become part of the aquatic food supply.
A body of water that produces many aquatic plants is usually considered a rich habitat for many forms of aquatic life because furnishes shelter and food. For instance, the plants serve as bases of attachment for sessile protozoa and algae and thus increase the capacity of the lake to support such organisms. They afford shelter or protection for insects, crustacea, and fish thereby enabling many of these organisms to maintain a breeding population.
The distribution of aquatic plants is frequently considered rather cosmopolitan. They are found in waters subject to much variation in dissolved salts, nutrients, color and transparency as well as physical and chemical variation of the bottom.
However, certain species of aquatic plants are rather exacting in their requirements and may be somewhat restricted in their range of habitat. Their limited distribution may be due to such factors as the temperature or depth of water, the physical properties of the bottom, the quantity or quality of salts dissolved in the water and the competition of other plants. Depending upon their tolerance and aggressiveness and also upon their mobility, aquatic plants contain many restricted species as well as many cosmopolitan ones.
In their natural environment, angiospermthi aquatic plants reproduce and spread by seeds and in many species also by vegetative propagation. Many of them blossom and fruit in abundance in shallow water, but seldom produce mature seeds in deeper water or where they are continuously submersed.
In numerous species vegetative propagation is accomplished by special organs where in others any part of the stem ma break off and take root.
The most common propagating parts consist of rhizomes, runners, tubers, corms, terminal leafy axes or winter buds. The seeds and tubers of some aquatic plants are rich in stored foods, such as starch, and are frequently sought by animals.
In fresh waters, the number of species of lower vascular plants is relatively small. Some of the noteworthy ones are water horse-tail (Equisetum) and the grasslike quillwort (Isoetes).These are frequently common along the shores of fresh waters and sometimes grow submerged in a lake or stream.
Among the ferns, the water shamrock (Marsilea) inhabits the shallow zones where it’s four broad leaflets arise from rhizomes growing in the substrate. The water fern (Azolla) is a small floating fern with overlapping leaves and fine roots hanging from the undersurface. The leaves, usually no more than 5 mm long, are green during summer, but turn reddish in the fall. Where abundant, these plants may completely obscure the water surface, the shading effect thereby inhibiting production in the water below.
Of some 200,000 known species of flowering plants, relatively few grow as “true” aquatics in fresh waters. We may define “aquatic plants” as those whose seeds germinate in either the water phase or the substrate of a body of water and which must spend part of their life cycle in water.
This ecological grouping includes plants which grow completely submerged (except when flowering) as well as a variety of emergent types. In the United States, about 50 families of flowering nonwoody plants may be considered primarily aquatic.
The family Najadaceae is said to be the largest of the family of aquatic plants. The two genera of pondweeds, Potamogeton and Najas, include over 80 percent of the species of Najadaceae. One or another of the species occurs in nearly all types of fresh waters. These plants are mostly submersed, some with floating leaves, exhibiting a variety of leaf forms, ranging from linear to broadly ovate.
Ecologically, the pondweeds are of great importance in the cycles of nutrients and respiratory gases, and in often providing very dense habitats which supply food and shelter to numerous small organisms. Many potamogetons serve as a major item of food for ducks and geese.
Many other plants such as grasses (Gramineae), sedges (Cyperaceae), and rushes (Juncaceae) typically inhabit shore zones. These plants normally grow in very shallow waters, but usually in profusion. Upon death and decay, they contribute to the richness of the body of water.
A number of flowering plants have become adapted for floating on the water surface. Their role in community activities is interesting because the plant roots dangle in the water phase. The roots extract nutrients from the water in competition with phytoplankton. As the plants die, nutrients are returned to the water. where these forms are abundant, they shade the water from sunlight thereby inhibiting production.
An example of this is Pistia, which resembles garden lettuce and is common in the Gulf Coast States. Others are in the family Lemnaceae that includes the duckweeds.
These are minute plants of flattened or spherical body form. Lemna and Spirodela are duckweeds that are widespread in the United States and are often found in association with the liverwort Ricciocarpus and the fern Azolla.
Another floating species, but of the family of pickerelweeds, the water hyacinth (Eichhornia), was introduced in the United States from South America because of its showy flower. In favorable areas, however, the plant quickly becomes an ecologically disruptive pest.
Members of the water lily family are also important flowering plants. To this family belong the white lily (Nymphaea), the yellow lily (Nuphar) and the lotus (Nelumbo). In many lakes and streams patches of lilies are favorite nesting areas for certain sunfishes.