Thousands of plant species live in freshwater habitats around the world: along edges, on the surface, or at the bottom of shallow lakes and ponds; in temporarily flooded low areas and meadows; at seeps and springs (cienegas) in hill or montane regions; in flowing water of streams and rivers; rooted in waterlogged soils; and along any other natural or human-produced drainage system. "Freshwater wetlands" occur from below sealevel to some very lofty alpine habitats, where water may persist throughout the year or where it can be very ephemeral. Normally we classify a freshwater wetland as a place where at least half of the species found there are truly aquatic plant species.
Many species of aquatic plants are essentially cosmopolitan, meaning that they are widely distributed around the world. Some of the widest distributions are attributable to human activities. Humans have accidentally (sometimes intentionally) transported seeds, fruits, or vegetative clones from one pond or watershed to another, but many of the cosmopolitan distributions are attributable instead to birds, particularly waterfowl, which inadvertently transport the plant propagules when lodged in their features or trapped in mud on the feet.
Many of the designs exhibited by plants living in water were obvious to early botanists. For example, Agnus Arber published a book in 1920 on aquatic plants, documenting many of the strategies that we still talk about today.
All accounts discuss three basic types of leaves:
Submersed leaves receive low levels of sunlight (PFD) because light energy diminishes rapidly while passing through a water column. Light penetration is especially poor in turbid water with dense surface populations of algae. Such underwater leaves are often so highly dissected that the segments may appear superficially to be macroscopic green algae (e.g., Chara and Nitella). This is a strategy to maximize surface-to-volume (S/V), permitting rapid diffusion of carbon dioxide into the chloroplasts of the cells by having proportionately greater surface area. Certain aquatic species have very high ratios of surface to volume (S/V) by having one- or two-cell layer construction. These leaves have a very thin cuticle (wax), but the wax is porous enough to permit easy diffusion of gases through the surface. On these leaves, stomates are generally absent, and would be useless for submerged plants, where water, not air, continually surrounds the photosynthetic organ. Such leaves have very poor development of xylem tissue (water transport), appropriate inasmuch as shoots are bathed in water. Intercellular air spaces are not well developed, thereby enabling this plant to remain submersed by having greater specific gravity. The highly dissected underwater shoot can be tugged at and pulled by water currents without damaging the segments (i.e., little mechanical resistance to current). In swiftly running streams, these shoots and leaves wave and dance wildly.
Floating leaves tend to be much broader, without major lobing, and remain flat on the water, taking advantage of full sun. Stomates are present for gas exchange, especially on the upper (adaxial) leaf surface. The upper leaf surface tends to have a very prominent cuticle, thereby permitting water to roll off, and not interfering with photosynthesis or promoting growth of epiphytic algae. Epidermis may be rich in chloroplasts, and a bifacial mesophyll (palisade and spongy layers) is formed. Floating leaves often have well-developed air chambers (lacunae), which provide buoyancy, and they may also have hard cells, sclereids, within the mesophyll that provide some toughness for the leaf and prevent the layers from becoming collapsed.
Emersed (aerial) leaves are essentially like typical leaves of herbaceous angiosperms that inhabit full-sun environments. Such leaves are emergent from the water and, consequently, have a waxy cuticle on both surfaces. Many are also amphistomatic (stomates on both surfaces and in nearly equal densities) and have well-developed leaf mesophyll, to take advantage of the abundant light.
Among the many species that are required to inhabit fresh water, there are a number of plant designs or lifeforms:
Floating plant with no attachment to the mud or bottom and with inflated portions of leaves or stems or special hairs that enable the plant to remain floating
Free-floating, submersed plant (in most cases, due to fragmentation of rooted plants)
Plant rooted in the bottom mud and muck but with two distinct types of leaves, i.e., submersed and emersed leaves or submersed and floating leaves (amphibious plant)
Examples of amphibious plants with dissected submersed leaves and unmodified emersed leaves:
Examples of amphibious plants with dissected submersed leaves and broader floating leaves
Plants rooted in soil or mud (also sand) along edge or bank of freshwater. This is essentially terrestrial (e.g., along a stream bank or lake shore, in freshwater marsh or a very wet mountain meadow) or with stiff shoots that arise from the water, i.e., where a plant shoot does not require support by the water. A number of shoreline herbs have a land form and water form, where the individuals formed in standing water have more highly dissected leaves than the land form (e.g., Marsilea and Ranunculus sceleratus, R. aquatilis).
An aquatic plant may experience abundant soil moisture during the entire growing season, but water levels drop during the dry season or summer months, when these types of plants commonly experience severe water stress and dormancy if water recedes or soil around the root system becomes very dry.
One or a few species of emergent aquatic plants can dominate the freshwater community. Most of these grow aggressively via rhizomes or stolons, crowding out other species. Rhizomes permit these plants to endure periods of environmental stress, and the rhizome (or corm) is the overwintering bud of plants growing in cold climates.
A number of floating aquatic species are excellent organisms in which to study logarithmic population growth. Under full sun and nonlimiting nutrients, a single individual can be introduced into a pond and multiply rapidly via vegetative means. For example, duckweeds (Subfamily Lemnoideae of Family Araceae) clone by forming plantlets on the mother plant, doubling in surface coverage approximately every two days. Water-lettuce, Pistia stratiotes, forms new plants around the mother plant via underwater stolons. Water-hyacinth, Eichhornia crassipes, and floating fern species of Salvinia and Azolla also show explosive population growth. In the tropics and heated quiet waters of ponds and lakes, such species can completely cover the water surface within several months, and for that reason are considered pernicious aquatic weeds, which are removed at great expense and trouble because they clog channels and choke out other forms of life in the body of water.
Plants that normally are submersed typically form their flowers raised above the water surface. This is true, e.g., of Myriophyllum, Elodea, Hippuris, and Utricularia. There are some bizarre plants that have underwater pollination mechanisms, most notably Vallisneria.
An important adaptation for many freshwater aquatic plants is the formation of aerenchyma, which is parenchyma tissue having large intercellular air spaces. Aerenchyma functions both to store oxygen and to transport that gas to living tissues. This gas collection is important in leaves for buoyancy. In addition, the system of lacunae is a diffusion pathway for oxygen; the oxygen is, of course, made in the chloroplasts during the light reaction of photosynthesis. Oxygen, when released via photosynthesis, diffuses preferentially into the lacunae, because it cannot diffuse as rapidly into water and comes out of solution in the intercellular air spaces, where oxygen concentration of trapped air there may be one-third or greater. Here it can be used in constructive ways by aquatic plants. A leaf midvein, petiole, or stem develops an internal pressure, which enables oxygen to be transported via bulk flow in a lacunar network to rhizomes and roots located in the anaerobic mud and muck, permitting these organs to grow more rapidly. Gases can also move in bulk to young tissues, where the pressurized air helps expansion of developing lacunae near the growing tip. The cut end of an aquatic plant will give out bubbles (underwater, of course) from lacunar gas under pressure.
Woody species that also may line palustrine and riverine habitats generally do not show the same adaptations of leaves found in the herbaceous species that actually live in the water. The most interesting case of convergence is the willow-type leaf. Willow, Salix (Family Salicaceae), has relatively long lanceolate to narrowly ovate leaves with tapered tips, and the branches tend to be very flexible, so that in running water the leaves can be dragged through the water with relatively little resistance and no tearing. Many totally unrelated woody shoreline plants from around the world have evolved this type of leaf, e.g., in seep-willow (Baccharis salicifolia) and arrow weed (Pluchea sericea, both Family Asteraceae) of California and Australian willow (Geijera parviflora, Family Rutaceae) and Australian willow myrtle (Agonis flexuosa, Family Myrtaceae) of Australia.
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