Aquatic plant
AQUATIC PLANTS
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.
Characteristics of a Freshwater Environment
- Water is plentiful, at least during the growing season.
- PFD (wavelengths of sunlight used for photosynthesis) is low for submerged leaves, because light penetration through the water column is very much reduced. At the water surface there often is unobstructed full sun for a photosynthetic organ floating, and an emergent canopy may intercept high PFD.
- Concentration of carbon dioxide dissolved in water is low (higher in water strongly acidic or strongly basic than in neutral pH solutions).
- Oxygen concentration of oxygen in the water and in thick tissues of the underwater plant is low.
- Minerals and nutrients are scarce or dilute within the water medium, as compared with drier soil.
- Moving water (currents and waves) can be damaging to the organs of the plant.
Types of Leaves
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, which are very thin and narrow, often highly dissected and very flexible
- floating leaves, broader leaves that are firm or leathery but flexible enough to resist tearing by wave action
- emersed leaves (aerial leaves), i.e., similar to typical leaves of terrestrial plants living nearby
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.
- Examples of highly dissected submersed leaves
- Parrot's feather (Myriophyllum aquaticum) and European milfoil (M. spicatum)
- pondweed (Potamogeton pectinatus, P. filiformis, P. crispus, P. foliosus)
- thread-leaved water-nymph (Najas gracillima) and rice-field water-nymph (N. graminea)
- hornwort (Ceratophyllum demersum, C. submersum)
- Examples of slightly wider but very thin submersed leaves
- elodea (Elodea canadensis, E. nuttallii, Egeria densa)
- mare's-tail (Hippuris vulgaris)
- Examples of thread-like, undivided leaves:
- horned-pondweed (Zannichellia palustris)
- ditch-grass (Ruppia cirrhosa, R. maritima)
- quillwort (Isoetes spp.)
- Pilularia americana
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.
- Examples:
- water lily (Nymphaea odorata)
- yellow pond-lily (certain species of Nuphar)
- cape-pondweed (Aponogeton distachyon)
- pondweed (e.g., Potamogeton nodosus, P. natans)
- Victorian water lily (Victoria regia, V. cruziana)
- water-shield (Brasenia schreberi)
- floating heart (Nymphoides)
- water-chestnut (Trapa natans
- frog-bit (Hydrocharis morsus-ranae)
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.
- Herbaceous perennial examples:
- cattails (Typha)
- wetland irises (Iris)
- buttercups (e.g., species of Ranunculus and Caltha palustris)
- hundreds of grass species (Poaceae), including the giant reed, Phragmites australis
- sacred lotus, Nelumbo nucifera, N. lutea, and in some species of yellow pond-lily (e.g., Nuphar polysepala)
- skunk-cabbage, Symplocarpus foetidus, as well as aquatic aroids from the tropics
- arrowleaf (Sagittaria spp.) and water-plantains (Alisma)
- some species of knotweed (Persicaria or Polygonum)
- lizard's tail (Saururus cernuum) and yerba mansa (Anemopsis californica)
- bur-reed (Sparganium)
- Thalia geniculata (Family Marantaceae) and pickerelweed (Pontederia cordata, Family Pontederiaceae)
- umbellifers, e.g., cutleaf water-parsnip (Berula erecta) and species of Hydrocotyle
Lifeforms of Aquatic Plants
Among the many species that are required to inhabit fresh water, there are a number of plant designs or lifeforms:
- Plant rooted in the mud or muck
- with emersed leaves (see Types of Leaves)
- with photosynthetic stems and relatively small leaves
- species of tule (Scirpus) and other species of sedges (e.g., Carex, Eleocharis, and Cyperus) with or without noticeable leaves, e.g., papyrus (Cyperus papyrus, Family Cyperaceae)
- most species of rushes (Juncus, Family Juncaceae)
- horsetails (Equisetum, seedless land plants)
- with plant body modified as submersed thallus, which is attached to a rock substrate (Family Podostemonaceae); species in this family have photosynthetic roots
- with floating leaves only (see Types of Leaves)
- with submersed leaves only (most examples in Types of Leaves)
- water-lettuce (Pistia stratiotes) with nonwettable leaves
- water-hyacinth (Eichhornia crassipes) with inflated petioles
- duckweeds (Subfamily Lemnoideae of Family Araceae, Lemna, Spirodela, Wolffia, Wolffiella), among the tiniest vascular plants
- bladderworts (Utricularia) with a modified, submersed stem system for photosynthesis and catching invertebrate prey and, sometimes, a rosette of inflated stems floating at the surface of the water (U. inflata)
- featherfoil (Hottonia inflata) with dissected submersed leaves and at the surface inflated, leafless stems
- water soldier (Stratiotes aloides)
- Phyllanthus fluvitans
- Salvinia with nonwettable leaves by possessing special hairs
- mosquito fern (Azolla filiculoides, A. mexicana)
- Rorippa amphibia
- Myriophyllum heterophyllum
- Prosperinaca palustris
- water-marigold, Megalodonta beckii
- Potamogeton natans, P. amphibium, P. vaseyi, P. spirillus
- Cabomba caroliniana and water-shield (Brasenia schreberi, Family Cabombaceae)
- Examples:
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:
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.
Contents
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Freshwater plants
Characteristics
Characteristics of aquatic plants:- A thin cuticle. Thick cuticles reduce water loss; thus most hydrophytes have no need for thick cuticles.
- Stomata that are open most of the time, because water is abundant and there is no need for it to be retained in the plant. This means that guard cells on the stomata are generally inactive.
- An increased number of stomata, which can be on either side of the leaves.
- A less rigid structure: water pressure supports them.
- Flat leaves on surface plants for flotation.
- Air sacs for flotation.
- Smaller roots: water can diffuse directly into leaves; thus large root systems are not required for water uptake.
- Feathery roots: no need to support the plant.
- Specialized roots able to take in oxygen.
Wide flat leaves in water lilies (family Nymphaeaceae) help distribute weight over a large area, thus helping them float near surface.
Many fish keepers keep aquatic plants in their tanks to control phytoplankton and moss by removing metabolites.
Many species of aquatic plant are invasive species. Aquatic plants make particularly good weeds because they reproduce vegetatively from fragments.
Adaptations
- Floating plants: In an outdoor body of water, these receive more sunlight than submerged plants do. They also rarely have to compete with one another for sunlight
- Submerged plants: The leaves of submerged plants receive lower levels of sunlight because light energy diminishes while passing through a water column.
All floating plants
- Either have air spaces trapped in their roots, or else air spaces in their bodies (aerenchyma) to help them to float, thus receiving adequate sunshine
- Have hair on their leaves that traps air
- Structural adaptations
- Have hollow stems to float on water.
Duckweed, water cabbage
- Chloroplast found on the top surface of the leaves
- Upper Surface has a thick, waxy cuticle to repel water and to help keep the stomata open and clear
- Structural adaptation
- Small and light
Water lily
- Structural material to reach higher points and receive more sunlight
- Structural adaptation
Floating heart, water lily, lotus, yellow pond lily, water-shield
- Their leaves tend to be broader without major lobing, to remain flat on water surface, to enlarge their surface area, and to make use of as much sunlight as possible. Their chloroplasts are found on the tops of their leaves.
- Structural/ behavioral adaptations
Most partially-submerged ("emersed") plants
- Air spaces within their tissues to keep them buoyant so that their leaves can reach the top of the body of water, in order to receive an adequate amount of sunlight
- Structural adaptation
Dissected: Parrot's Feather, Hornwort
Thread-like: ditch-grass, quillwort
- Highly dissected/ divided leaves or thread-like ones, allows for a bigger surface area (surface to volume ratio)
- Structural adaptation
Hydrilla
- Elongates rapidly to reach water surface and branches out at water surface; more light can be obtained at water surface
- Structural/ behavioral adaptation
- Xylem tubes are absent
Saltwater plants
Several aquatic plants live or are able to survive in brackish, saline, and salt water. Some are also grown commercially at or near shore. These include Salicornia, Aster tripolium, and Crambe maritima.Human nutrition
Many aquatic plants are used by humans as a food source. Note that especially in (South-east) Asia edible but uncooked hydrophytes are implicated in the transmission of fasciolopsiasis.See also Fasciola hepatica.- Wild rice (Zizania)
- Water caltrop (Trapa natans)
- Chinese water chestnut (Eleocharis dulcis)
- Indian Lotus (Nelumbo nucifera)
- Water spinach (Ipomoea aquatica)
- Watercress (Rorippa nasturtium-aquaticum)
- Water mimosa (Neptunia natans)
- Taro (Colocasia esculenta)
- Rice (Oryza) is originally not an aquatic plant.
- Bulrush, reedmace, or cattail (Typha)
- Water-pepper (Polygonum hydropiper)
- Wasabi (Wasabia japonica)
- Kjosco
- Totora (Scirpus californicus)
Animal nutrition
Some examples of aquatic plants- Water hyacinth (Eichhornia)
- Duckweed: Lemna, Spirodela and Wolffia
- Trichanthera gigantea
Some examples of aquatic plants
- Most algae, and all seaweed and kelp
- Riella, the only genus of liverwort to grow as a submerged aquatic.
- Utricularia (from the Latin, utriculus, a little bag or bottle) is a genus of slender aquatic plants, the leaves of which contain floating air bladders. They are called bladderworts.
- Water lettuce
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