Adaptation Characteristics of Aquatic and Marsh Plants

Typical for many tropical and subtropical marsh plants living in temporary waters or in flood areas of stagnant or flowing waters, are their symptoms of adaptation to the periodic change of rain-intensive and rain-deficient seasons. During the dry season, the plants grow more or less on the edges of waterways, either completely emersed or in only a few centimeters of water. In case of the waterway drying up, this can also lead to the plant completely dying off after some time. During high-water levels, plants lead a partial or entire existence under water. Plants found in such locations are obviously in a position to adapt to an emersed and submersed existence over the course of annual seasons. They display a changing life cycle and have to have blossomed and borne fruit within a certain period of time, i.e., from the start of the rainy season up to their possible withering stage at the end of the dry period, in order to ensure their reproduction.

At the onset of the wet season many aquatic and marsh plants will initially form typical juvenile forms. From the vegetation organs which have remained in the soil from the dry period (rhizome, tuber, bulb), some mostly delicate juvenile leaves will emerge which are often narrow, strap-shaped, soft and pellucid. In the course of plants developing from seeds, these typical juvenile leaves will also form, following the formation of one or two cotyledons. Only with increasing age and progressing plant growth will those submersed and floating leaves develop which are typical for every individual species. These leaves, through their anatomical structure which is characterized by more or less absent stomata, thin outer epidermis as well as wide air ducts, display a special adaptive quality to water as a medium. The aquatic leaves are thus able to absorb oxygen, carbon dioxide and nutrients directly from the water. The leaves will shrivel rapidly and wither in dry air due to the lack of support tissue and transpiration protection. If the water level begins to slowly drop again, amphibious growing plants will change over to their emersed stage, i.e., they will cease to form submersed leaves but will increasingly form air leaves above the water surface and will then proceed to mature to their full size. These leaves growing above the water surface will assume a totally different appearance in many species compared to their submersed state (hetero-phylly = having different leaves). Whereas the underwater leaves are often delicate, thin, transparent and ribbon-like or dissected in order to achieve a larger surface, entire, hard, leathery and often hairy leaf blades are typical features of air leaves. The formation of differently structured leaves in different mediums can be observed especially well in Hy-grophila difformis, as well as Limnophila and Myriophyllum species. During sinking water levels during dry periods, flower and seed formation will commence. If the locations are beginning to dry out, the plants will develop an increasingly squat appearance until the foliage will often entirely dry up in line with the reduced moisture within the soil. Whereas in some species only the survival organ will survive in the dry soil, other species will die off entirely, and only their seeds will remain in a state of suspense until thé onset of the next wet season, when advantageous environmental conditions will initiate germination. Once they reach the water surface, many genuine aquatic plants will form floating leaves

Hygrophila corymbosa with submersed and emersed shoots in a river on Sulawesi.

Hygrophila Corymbosa

Hygrophila corymbosa with submersed and emersed shoots in a river on Sulawesi.

which lie flat on the surface and which also display typical features of adaptation in their anatomical structure. They are especially rich in intercellulars which aid photosynthesis. At the same time, the floating leaves of some plants, for example, water lilies, display countless hydropotes ("water drinkers," gland-like cells of the epidermis) on the leaf's underside which assist in the absorption of water and mineral nutrients. Such hydropotes can also be found on the leaves of many aquatic plants.

Using their roots and rhizomes, some aquatic plants, such as Nymphaea and Nu-phar species, are in a position to grow in badly aerated and oxygen-deficient soils. As a form of morphological adaptation to this environment, they have formed a system of cavities through which oxygen from the floating leaves can be transported to those plant segments existing within the soil. These cavities are so large that it is possible to actually blow air through the petiole of a water lily.

In especially thick soils, roots growing above the surface, bulbous roots on shoots, and, as an extreme adaptation to this environment, pneumatophores can be found occasionally on plants in tropical waters. The presence of such pneumatophores in several Ludwigia species is well-known. Pneumatophores generally develop in a very oxygen-deficient environment and can be distinguished from normal roots by their white, spongy and bloated tissue, incorporating large intercellular cavities (aerenchyma) which serve aeration and air storage purposes. These pneumatophores grow vertically into the air, probably absorb oxygen from the atmosphere and transport it to the submersed shoots via the aerating tissue.

Floating plants, too, display adaptive morphological and anatomical features. In the Salvinia species, for example, the two floating leaves are equipped with numerous intercellular spaces, whereas the submersed leaf hanging in the water is divided into many

Flowering Nymphaea lotus in temporary waters in Senegal.

Flowering Nymphaea lotus in temporary waters in Senegal.

Aquatic Adaptation Lotus Nymphaea

filiform pubescent lobes and has adopted the functions of the absent roots. The bloated, spongy petioles, which display a comprehensive aerating tissue and thus enable the plants to float on the surface, are conspicuous in Ceratopteris pteridoides, Eichhornia cras-sipes and Trapa natans. The underside of the leaf in Limnobium is conspicuously bloated and thickened, which also enhances the float-ability of the plant.

Specifically worth mentioning is the often distinct moisture protection found in floating plants. This protection serves to keep the transpiring upper leaf sections above the water surface dry from rain or dew. The fact that it is impossible to moisten the leaf of Ne-lumbo nucifera, which is covered with papilla, is well-known. But in the Salvinia species, too, the floating leaves are effectively protected from moisture by papilla arranged in rows and on which hairs are located. At the same time, the boat-shaped floating leaves enable a quick run-off of rainwater. Moisture protection in other species (e.g., Pistia stra-tiotes) is achieved through a strong pubescence. The bulging in floating leaves in some aquatic plants (e.g., Phyllanthus fluitans) is also notable. It also has the purpose of ensuring a rapid flow-off of rainwater from the surface.

The occurrence of strap-shaped and especially smooth, undulate or bullate leaves, for example, in some Aponogeton, Cryptocoryne and Vallisneria species, is a conspicuous feature in some aquatic plants. Such leaf shapes and structures can be explained both as measures of adaptation as well as protection, since the leaves, due to their shape or surface structure, offer the least possible resistance to any water current. Narrow or ribbon-like leaves are especially formed by those species that exist in flowing waterways. They are also

Ceratopteris pteridoides forms spongy, thickened petioles which increase the plant's floatability (photographed on the Rio Yanayacu, Peru).

Ceratopteris pteridoides forms spongy, thickened petioles which increase the plant's floatability (photographed on the Rio Yanayacu, Peru).

Lake Tanganyika Underwater Plants

typical of rheophytes, plants that are found between high and low water zones and are often subjected to short-time flooding. Their leaves are firm, leathery or coarse, and have at least a length to width ratio of 4:1.

Other features of adaptation to the surrounding environment can be observed in several other aquatic plants originating from Lake Malawi and Lake Tanganyika. These two East African lakes are well-known to aquarists because of their rich content of fish fauna. In the transitional zone between gravel and sand, and within the reed zone, there exist some aquatic plants which have created living conditions for themselves with the help of special adaptive features geared to the special living environment within these waters. Val-lisneria spiralis var. denseserrulata, for example, features conspicuously short and very hard leaves through which the plants can better resist any wave movement. Ceratophyl-lum demersum and Myriophyllum spicatum, too, display a variation in appearance totally different from other habitats by having short internodes as well as hard leaf and stem structures, which can also be interpreted as a protective and adaptive measure against wave movement. Furthermore, the shoots in C. demersum are of such compact growth that they do not move freely in the water as is common in other populations, but, due to their greater density, will drop to the bottom where they are subjected to a lesser wave movement compared to that found on the water surface.

The giant water lily, Victoria amazónica, too, has developed an optimum adaptive and protective characteristic to the natural living environment. The plant produces imposing floating leaves up to 2.5 m in diameter, on the surface of which the necessary stoma, which are important for respiration and assimilation, can be found. These floating leaves feature a strongly ribbed leaf venation as well as an up to 10-cm high curved margin, thereby setting a strong resistance against the water's wave movement and preventing the leaf's destruction.

Other features of adaptation to life in the water are the animal traps in Aldrovanda vesiculosa and the utricles in Utricularia species with which the plants catch, digest and exploit smaller animals as an additional source of nitrogen. The formation of hibernacles (winter buds, turions) in numerous aquatic plants is to be seen as an adaptation to disadvantageous periods of vegetation. Many endemic aquatic plants develop such hiber-naculums which drop to the bottom during autumn where they survive the cold season. During the following spring they will again rise to the surface and develop new shoots.

Interesting in this context is the behavior of some Lemnaceae. During late summer they develop modified small limbs with greatly retarded intercellulars and increased firmness which leads to an increase in their specific weight. The plants consequently lose their ability to float and drop to the bottom. In spring the limbs will recommence growth and again rise to the surface.

Apart from the typical features of adaptation of life within water mentioned here, the numerous pollination mechanisms of aquatic plants have to be mentioned (see also "Flower Biology") but will not be dealt with at this stage.

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Responses

  • alberica
    What are the characteristics of the aquatic plants?
    5 years ago
  • Marmaduke
    What are qualities of aquatic plants?
    4 years ago

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