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Wetland Plant Types and Adaptations

Arkansas Natural Heritage Commission - Thursday, November 15, 2018
by Brent Baker

Wetland plants, called hydrophytes, are adapted to living in water or on saturated soil all or part of the year. Wetland plants are generally classified into three main types: emergent, floating, or submerged. Emergent plants are rooted in soil under water, but at least some or most of their stems and leaves extend above the water (e.g., rushes [Juncus spp.]). Floating plants have leaves and sometimes stems that float at or on the water surface. Floating plants are further classified as floating-leaved, rooted in soil under water (e.g., fragrant white water-lily [Nymphaea odorata]), or as free-floating, unattached and suspended on the water surface (e.g., duckweeds [Lemna, Landoltia, and Spirodela spp.]). Submerged plants grow completely under the surface of the water, either attached or rooted to a substrate (e.g., riverweed [Podostemum ceratophyllum]) or unattached (e.g., coontails [Ceratophyllum spp.]), although some may have reproductive structures that occur at or just above the water surface. Photo top left — Fragrant white water-lily (Nymphaea odorata), photo by Eric Hunt.

Wetland plants are presented with unique challenges for surviving in their wet environments. Thus, they have developed special adaptations to meet these challenges. It is worth noting that wetland plants exist in a wide array of unrelated families and many lineages have independently evolved similar or identical adaptations to face the same challenges.

One major challenge for wetland plants is getting oxygen (which plants require for respiration) since wetland soils are naturally low in oxygen. While terrestrial plant stems and roots can simply take up oxygen from the air or form air pockets in the soil, wetland plants have to adapt special ways to get oxygen. One such adaptation is called aerenchyma, special soft tissue containing air spaces through which oxygen can travel within plants. Aerenchyma may not always be externally visible, but sometimes it may be obviously evident as spongy tissue. For floating and submerged plants, aerenchyma also provides buoyancy.

Wetland plants have evolved other methods of getting oxygen as well. Stems of some woody wetland plants (e.g., corkwood [Leitneria floridana]) contain hypertrophied lenticels, oversized pores that allow for greater exchange of gases. Some wetland plants produce adventitious roots or water roots, which sprout off stems under water or at or just above the water surface, increasing the surface area through which oxygen can be taken in.

Photo at left — The light colored dots on the stems of the wetland shrub corkwood (Leitneria floridana) are oversized pores, called hypertrophied lenticels, that allow for greater gas exchange. Photo by Brent Baker.

Some wetland plants grow in conditions that are so low in nutrients that they have adapted to getting their nutrients by feeding on insects and other arthropods (absorbing nutrients from them). Venus flytraps (Dionaea muscipula), and pitcher plants (Sarracenia spp.), utilize snap-trap and pitfall trap mechanisms, respectively, to capture insects. They are among some of the most famous of such carnivorous plants in the Southeast, though apparently absent from Arkansas. However, we do have a few wetland carnivorous plants in the state. Sundews (Drosera spp.) use a mechanism referred to as a flypaper trap in which a sticky substance is secreted by special glands to trap insects. Bladderworts (Utricularia spp.) use a bladder trap mechanism to capture their prey. In this type of mechanism, tiny traps are attached to runners at the base of the plant by slender stalks that are set under negative pressure in relation to their environment.

Photo above right — Sundew (Drosera brevifolia), absorbs nutrients from insects it traps with a sticky substance. Photo by Eric Hunt.

Some wetland plant adaptations are structural in nature. Many emergent plants have elongated stems and leaves (e.g., Typha spp. [cattails]), which increases the odds that at least some portions of the plants reach above variable water depths for photosynthesis and reproduction. Such elongated vegetation also offers less resistance to wind and water movements, reducing the odds of tissue damage.

Photo at left — Cattails (Thypha spp.) have long, narrow emergent leaves and stems. Photo by Eric Hunt.

Many submerged plants, or submerged portions of some floating or emergent plants, have thin, ribbon-like or finely dissected leaves (e.g., water-starwort [Callitriche heterophylla]). This increases the surface area for absorption of gasses and nutrients and for photosynthesis. Additionally, these narrow or dissected leaves, along with limited strengthening tissues in underwater stems of such plants, allows for greater flexibility with water movements, also reducing the odds of tissue damage.

Photo at right — Water-starwort (Callitriche heterophylla) has thin, ribbon-like submerged leaves. Photo by Brent Baker.

Floating-leaved plants often have long, flexible petioles (stem of the leaf) to allow for fluctuations in water depth. Some floating leaves (e.g., spatterdock [Nuphar advena]) have a thick waxy coating, which prevents water from covering them and inhibiting photosynthesis. It may also increase buoyancy.

Photo at left — Spatterdock (Nuphar advena), photo by Brent Baker.

Wetland trees are often shallowly rooted so as to increases exposure to oxygen. This makes them less stable, especially in the softer soils often found in wetlands. Thus, some wetland trees have buttressed and fluted trunks for additional support. Bald cypress (Taxodium distichum) has the further adaptation of knees, root protrusions above the soil and water surface. Although the true purpose of the knees is not known, they likely provide some structural support and may play a role in respiration.

Some wetland plants have also adapted their seed dispersal mechanisms for their water environments. This often involves fruits and/or seeds that float. For example, white swamp milkweed (Asclepias perennis), our most aquatic milkweed, has seeds that are widely winged for floatation and lack the silky hairs that all of our other milkweeds use for wind dispersal.

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