Lifeforms of Aquatic Plants

Among the many species that are required to inhabit fresh water, there are a number of plant designs or lifeforms:

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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