In this article, we will discuss about hydrophyte plants and their classification.
Vascular plants inheriting from remote marine ancestors a multicellular body with unlimited apical growth and a dimorphic life cycle slowly conquered the land and spread themselves over much of the land surface. Yet, despite this flourishing conquest of the land, some angiosperms, most of them herbs and a few pteridophytes, ventured back into fresh-waters and even further,’ to their ancestral habitat, the oceans.
They carried with them the relics of their terrestrial heritage and the advanced reproductive methods of their terrestrial relatives which, to say the least, seem – more of an anachronistic obstacle to them now—for from whatever depth at which they grow, most angiospermic hydrophytes still aim at raising their flowers above the surface of water for wind or insect pollination.
Hydrophytes are plants normally growing in water and also include plants inhabiting swampy or marshy habitats containing a quantity of water which would prove much more than optimal for the average land plant. It will be evident that hydrophytes are subject to less extremes of temperature than land plants for the watery habitat in which the plants grow certainly takes longer to be heated and also longer to cool.
Conditions thus are more uniform compared to plants growing in soil and as a result, many of the hydrophytes are very widely distributed geographically. Submerged aquatics, because they are screened from too high intensities of light show many of the characteristics of shade plants, sciophytes.
Habitat water per se is not harmful but because of extremely low solubility of oxygen in water or in water-saturated soils, (oxygen readily dissolves in water when the surface is in contact with air but subsequent diffusion downwards is so slow that a permanent oxygen deficiency exists all the time for submerged hydrophytes; on the other hand the availability of CO2 in water for photosynthesis in submerged plants is certainly as much or even somewhat more, compared to land plants; at 20°C, one litre of water can hold 0.3 ml of CO2) a complex of critical environmental conditions is produced and only specialised forms of plants can suitably cope with it.
The upper sunlit zone of the sea where the light intensity is great enough to permit photosynthesis, is referred to as euphotic zone, usually regarded as being to a depth of a hundred metres, though there might be differences depending on such factor as the amount of solid carried in suspension. River estuary obviously is less transparent than the mid-oceans.
Roots of vascular hydrophytes are evidently able to live and grow normally in very low oxygen tension.
The ability to respire anaerobically is especially well developed in certain hydrophytes growing in still water or in wet soil. The dispersal of seeds and fruits by water is a common characteristic of hydrophytes, for seeds of most xerophytes or mesophytes are killed when wetted (due to lack of aeration). The streams and ocean-currents carry the spores of hydrophytic cryptogams, the floating fruits of Cocosnucifera and Xanthium, the floating seedlings of mangroves and entire floating plants such as Lemma, Salvinia, etc.
Land plants which live in situations in which atmosphere is permanently moist are spoken of as hygrophilous. Many hygrophilous plants, living in almost saturated atmospheric conditions of tropical forests, show considerable resemblance to hydrophytes in form and structure.
Without water, there is neither active metabolism nor any possible development. This is sometimes termed anabiosis, when no respiration or any other life-activities occur. In barley, seeds hardly consume any O2 in respiration, below 8% humidity of the environment.
Many hygrophytes show adaptations to excessive water absorption in these plants by developing hydathodes or similar structures as means of getting rid of surplus water (exudation) in the liquid form.
Among the principal morphological and anatomical characteristics of hydrophytes are the following:
(i) Because of the suboptimal aeration of tissues due to the watery habitat, one of the outstanding structural peculiarities shared by one and all hydrophytes is the sponginess of their tissues. In hydrophytes, owing to disintegration of groups of cells or their separation, large intercellular spaces or cavities develop in all parts of the plant which remain filled with air.
Vascular hydrophytes with a terrestrial ancestry became so adjusted to high oxygen content of the air during their sojourn in land that when they again take to water they must make special provision for aeration; hence the development of aerating tissue. On the other hand most thallophytes do not possess any special aerating tissue.
The remarkable absence of air cavities or lacunae from the thalli of angiospermic Podostemaceae, must be mentioned here. It is possible that lacunae may have been absent in Podostemaceae from ancestral stocks and the family has been necessarily restricted to well-aerated watery medium throughout its evolution.
Alternately, it may have been that the ancestral stocks had lacunae but the ability to develop them has subsequently been lost in the course of evolution, for the lacunae become superfluous in more favourable environment, where gaseous exchange is facilitated usually by a swift current and oxygen-saturated running water (Sculthorpe, 1967).
It is thus probable that because the thallophytes originated in water medium they have been fully adjusted to oxygen deficiency condition in water from the very beginning. An interesting case of the development of so-called aerenchyma (Figs. 766, 767, 768, 770, 772 and 773) is seen in the spongy and enlarged petioles of water hyacinth. The air storage in such cells gives buoyancy to the plants which helps to keep them afloat.
In lotus, even the fruits contain large air cavities rendering them buoyant and also facilitating their dispersal by water. On the other hand, Azolla owes its buoyancy primarily to the minute epidermal hairs on the upper surface of the dorsal lobes of the leaves; the layer of air trapped by the hairs keeps the surface of the leaves dry, at the same time preventing the plant from being immersed.
In Jussiaea repens and J. suffruticosa the bulk of the tissue consists of aerenchyma if growing in water but if J. suffruticosa be grown on land no development of aerenchyma occurs. In some marine brown algae, e.g., Fucus, special swellings, like bladder serve to keep the shoots erect in water. The air cavities, in the floating leaves of aquatics such as water-lily, generally form a continuous air-communicating system by means of which the submerged organs can easily exchange gases with the air outside through the ever open stomata.
The ability to develop air cavities (lacunae) is probably genetically endowed and in submerged hydrophytes, the seemingly apparent reaction to the aquatic environment may very well be no more than the full realisation of this ever present potential. The number of air cavities is sometimes specific and may vary in a characteristic way between different species of a genus; it may be of some value as a criterion for identification.
The fragility of lacunate tissue is counterbalanced by radial plates and in many species of vascular hydrophytes, e.g., Pontederia, Potamogelon, Sagittaria sp., etc., by the development of water-tight transverse diaphragms which interrupt the air cavities at intervals. The diaphragms are groups of very thin-walled cells, 1-3-cell thick, cell walls of which swell into the cavities, branching freely and rapidly, usually forming a spongy mass of loosely arranged cells.
Diaphragms are commonly found at the nodes of submerged stems and are usually scattered in the petioles. They are characteristic of Nymphaeceae and perhaps of all aquatic nonocotyledons. They are absent for some unknown reasons from the petiole of Nymphaea and are also very infrequent in its submerged leaves.
In some hydrophytes such as Trapa, Ceratopteris, Pistia, Hydrocharis, etc., lacunate tissue develops so excessively that bladder-like swellings, floats of spongy parenchyma are formed in Pistia, the underside of which often shows a conspicuous ovoid swelling, while in Eichhornia, the whole petiole may be swollen into a bulbous spongy float. The relation of this adaptation of forming floats to the buoyancy requirement is confirmed by the fact, that these plants when growing in the mud do not develop floats, the petioles or the leaf-bases then remaining slender and elongated.
(ii) Root system is often very poorly developed usually shorter and much less branched than land plants or as in some floating hydrophytes, (e.g., Wolffia, Salvinia, etc.) the roots may be entirely absent (in some species of Ceratophyllum root hairs have completely disappeared). In Jussiaea repens, however, two forms of root develop when growing in water; ordinary anchorage roots and erect very spongy negatively geotropic roots which grow upwards often till they reach the surface of water.
The above generalisation is, however, quite unjustified. Many hydrophytes, it must be apparent to even a casual observer, have exceedingly well-developed root systems or rhizomes which show few anatomical modifications compared with those of land plants, e.g., Nymphaea, Pontederia, etc., have stout spongy rhizomes. The tough and sometimes woody-rhizomes of Cyperus, Scirpus, Typha, etc., branch freely, covering vast areas in growing season. The root stock of Vallisneria which is bulbous in Sagittaria and hard and corm-like in Aponogeton and Isoetes, are well-known.
All free-floating rosette plants, e.g., Eichhornia, Pistia, etc., have supremely well- developed adventitious roots. Relieved of the function of anchorage (the floating plants do not need any anchorage), the roots of these plants are at least partly responsible in preserving the stability of their rosette leaves. The roots of these genera can develop chloroplastids in their epidermal and cortical cells when they get sufficient light and may contribute something, however small that may be, to the net assimilation of the plant.
Ceratopteris has a shallow system of quite freely branched roots. It has been found that a significant proportion of biomass is represented in vascular hydrophytes, by the underground parts, e.g., rhizome, roots, bases of the aerial shoots—in Typha sp. more than 50% of the biomass can be accounted for by the root system on fresh weight basis.
Biomass, is actually the quantity of organic substance produced in a given area, as in the weight of the vegetable matter, removed by a dipping quadrat (a sample plot from which the vegetation is removed) or the plankton of a given volume of water. Contrary to popular idea, the roots of most submerged hydrophytes do develop abundant root hairs.
The notion of wholly or mainly foliar absorption of water and nutrients by floating or submerged hydrophytes thus rests on quite unwarranted evidences. These include (a) rudimentary nature of the root system. We know that it cannot be true, for far from being rudimentary, root system of many species is remarkably well developed; (b) lack of root hairs; again this is certainly not applicable to the vascular hydrophytes for root hairs are not only present but may be conspicuously abundant; moreover, the absence of dearth or root hairs are not real indications of the roots’ ability to absorb water and nutrients; (c) vascular reduction; it is true that, there is reduction in the extent of xylem vessels, but is the uptake of water and inorganic ions by roots directly related to it?
An interesting anatomical anomaly must be mentioned here. Except in Nymphaea, and a few other hydrophytic species, the stele of roots is surrounded invariably, in all manner of watery habitats, by a very conspicuous endodermis, bearing typical casparian bands (Myriophyllum) or heavily thickened transverse, radial and inner tangential walls (Potamogeton sp.). This cannot conceivably be a trend towards structural reduction of roots—may not also be dismissed so easily as mere ancestral relics.
We know it for certain that endodermis is functional in regulating lateral movement of water and ions in terrestrial roots; how could we now reconcile logically the prominence of endodermis in aquatic roots with the classical notion that in vascular hydrophytes there is scarcely any significant absorption and transport? (Sculthorpe, 1967).
The root caps of vascular hydrophytes are typically elongated and sheath-like (root pockets) or may also be totally absent (e.g., Azolla). Root cell walls usually remain abnormally thin.
(iii) Epidermis usually lacks cuticle or periderm (suberised cork ceils) and stomata are generally absent in the submerged organs. If present they are usually functionless (Figs. 766 and 767). This certainly adds to the evidence of the terrestrial
ancestry of the vascular aquatics. In the floating leaves, stomata, if present, are generally restricted to the upper epidermis.
The lack of cutin and suberin from the cell walls of submerged organs theoretically makes these organs Capable of absorbing water and nutrients throughout their surface directly, rather than by means of root absorption.
(iv) Mechanical tissue, although very feebly developed, is amply compensated by the buoyancy due to the presence of aerenchyma and the support afforded by surrounding water, renders the formation of mechanical tissue seemingly unnecessary.
(v) Conducting tissue is poorly developed but there is evidence that a transpiration current operates in the xylem and there is also conclusive evidence that at least in vascular aquatics, root absorption is indispensable and which, as we have seen just now, may be quite considerable for optimal growth and development of vascular hydrophytes (Figs. 767, 768, 769 and 770).
Vascular tissues are usually found to- absence of cuticle and stomata and reduced wards the centre of the stele with very few vascular elements, bundles towards the periphery for the pulling forces, exerted in quickly flowing water, are more economically met by aggregation of vascular bundles towards the centre. The almost complete absence of secondary growth in thickness of stems and roots is also an important feature of hydrophytes (Figs. 769, 770 and 772).
(vi) In floating hydrophytes, rooted in the mud, e.g., water-lily, etc., with the usually rounded leaves lying flat on the surface of water, certain mechanisms, such as waxy surface causing water-drops to roll off quickly or as in Salvinia, leaf hairs holding water-drops above the surface of the leaf, protect the leaf surface from getting wet.
Closely packed hairs, particularly those with hydrophobic tips can also protect the leaf surface from getting too wet. A homogeneous wax covering on the leaf surface might also be expected to provide an unwettable surface. Much of the strength of floating leaves results, however, from their leathery texture.
The presence of copious mucilage accumulation on the aerial organs seems also an adaptation for protecting them from getting too wet. Mucilage may also prevent too rapid diffusion of water through the cell wall. This certainly is an obvious benefit to the plants, whose stomata usually are confined to the upper surface of leaf lamina.
(vii) In the ‘amphibious’ group of aquatics, growing in shallow water-logged soils with their shoots well above the surface of watery medium, striking dimorphism is sometimes exhibited by the leaves that diverge from the stem below the water level, compared to those that diverge above water.
(viii) Heterophylly—Heterophylly is the phenomenon which is commonly observed in some hydrophytes, as for example, Sagittaria sagittifolia, Limnophila heterophylla, etc. It
indicates in contrast to homophylly (only one form of leaf) production of completely different forms of leaves in the same plant (transition forms are also sometimes seen). In submerged aquatics with floating aerial leaves, the submerged leaves are generally linear, ribbon-shaped or finely dissected while the aerial leaves are usually entire and rounded or slightly lobed.
This difference in the two or more forms of leaf structure may be due to the characteristic physiological features of water plants, such as:
(1) Quantitative reduction in transpiration,
(2) Reduction of light intensity (in the completely submerged parts),
(3) Plants subjected to much hydrostatic pressure,
(4) Unaffected by drought and lastly
(5) Variation in life form and habitat.
In Sagittaria (Fig. 771) the heterophylly seems to be due to the difference in the intensity of light in the submerged and aerial parts (more light intensity in the aerial parts apparently favouring formation of entire leaves). In others, e.g., Proserpinaca palustris, the aquatic leaves are finely serrate on horizontal stem whether in air or in water while the aerial leaves are linear but entire on erect vertical stems. In the last case, the heterophylly is most probably determined by genotype.
In Limnophila heterophylla, the case is even more interesting. On aerial stems, transition forms of leaves are found, from finely dissected leaves to the fully aerial entire types; in the submerged leaves, however, only the veins of the aerial lamina are developed, the remaining development of lamina (the inter-venous mesophyll tissue) is totally arrested, (are these leaves vestigial ?) perhaps as an evolutionary adaptation (Fig. 771).
Heterophylly, it must be pointed out here, is not unique to hydrophytes. It also occurs in many terrestrial plants, both woody and herbaceous. Thus heterophylly has been attributed both in the internal causes, e.g., age, genotype and nutrient status of the plant as well as to the influence of environmental factors, notably the photoperiod, temperature and moisture conditions.
Heterophylly means the presence of two or more distinct types of leaf in a single individual. The leaf types may differ markedly in shape and in anatomical organisation or they may differ in habit and anatomy, yet be of comparable shape (e.g., the floating leaves of some Nymphaea). In extreme case of heterophylly, the leaf types differ in all the above three aspects and a full-grown plant may bear submerged, floating and aerial leaves at the same time. The change from one leaf type to the other may be quite abrupt or it may be more gradual with transitional forms.
In some hydrophytes, a heteroblastic sequence (an ontogenic sequence in which early formed leaves are markedly different from later-formed ones) is apparent regardless of the environment. In Sagittaria sagittifolia, the first formed ‘juvenile’ leaves are invariably of the dissected or ribbon-shaped submerged type, irrespective of whether the plant is in water or on land, and the later formed ones on the other hand, are the floating or aerial type (usually associated with the reproductive phase).
Heterophyllous polymorphism in many species has confused their taxonomic significance and nomenclature within several hydrophyte groups. The close morphological similarities of ecological forms, which really belong to different species, has brought out clearly the unreliability of any identification based wholly or primarily on leaf characters (attention should be diverted to fruit characters instead). Allsopp (1965) believes that the principle involved in the morphogenesis of hydrophytes, is the highly unrestricted water entry into the developing tissue.
Allsopp (1956) found that in spore-lings of amphibious Marsilea drummondi, the heteroplastic sequence, from the juvenile subulate first leaves through spathulate and bifid intermediate types to the quadrifid or quadrifoliate adult form could be affected by the composition of the media. Carbohydrate or nitrate starvation induces formation of only juvenile submerged type of leaf and also to a reduction from solenostele to protostelic condition.
Wardlaw (1965) also found that in ferns, higher sugar concentration favours the aerial adult type of leaf-formation while in low carbohydrate media; only the juvenile types are formed. Allsopp suggested that the structural differentiation, distinctive of land and water forms, is perhaps mediated through the carbohydrate balance of the plants.
The heteroblastic manifestation in many heterophyllous plants is certainly correlated with the progressive enlargement of the shoot apex, the degree of which is certainly determined by protein synthesis (Allsopp, 1954-1965). Further evidence of the importance of protein synthesis in the heterophyllous plants is afforded by White (1966).
He found that if thiouracyl, a long-known inhibitor of protein synthesis, is added to the media, the rate of leaf formation in some Marsilea sp., is retarded and leaves are only of one type, the typical quadrifoliate natural land-forms. Sometimes fully formed water-forms are converted into land-forms in presence of thiouracyl.
The linear and the finely divided forms of leaves in the submerged parts may be of considerable ecological importance to plants for the dissected leaves certainly offer less resistance to under-currents and the stress and strain of water which, if the leaves were entire and broad, might have been torn to shreds and permanently damaged. Dissection of the leaf is not a direct reaction of plant to flowing water, since it occurs in many hydrophytes of stagnant habitats.
Dissected leaves certainly have a higher surface area/ volume ratio than most entire aerial leaves but the advantage gained by this is debatable. Sometimes mechanical tissues are found along the margins of such finely divided or ribbon-shaped leaves. In the aerial leaf of Eichhornia, angular collenchyma along the margins of blade certainly offers resistance to tearing stress.
(ix) In lotus, the long petioles seem to adapt themselves to the depth of water where they grow, always keeping the leaf lamina on the surface of water. This is perhaps due to the fact that petioles of floating-leaved hydrophytes have always a great capacity for renewed growth, (as much as 17 mm/hr in Sagittaria) which is perhaps regulated by auxins. (1 p.p.m. IAA, IBA, IPA, NAA, etc., actually promoted cell expansion in Nymphaea and Sagittaria). An interesting observation is the geotropic growth curvature exhibited by some Nymphaea petioles.
(x) The propagation of most aquatics is essentially vegetative, e.g., by rapid fragmentation in Elodea, by runners in Eichhornia, by rapid production of thalli in Lemna, etc. They usually flower much less freely than land plants. When flowers are present fertilisation takes place at or above the surface of water. Setting of fruits and seeds and their dispersal are always uncertain in the watery habitat.
(xi) The majority of hydrophytes are perennial. The relative uniformity of the aquatic environment encourages good vegetative growth. The insulation against intense illumination, afforded by water, abundance of CO2 in the watery medium and freedom from violent fluctuations of temperature, all seem to maintain active photosynthesis of submerged hydrophytes at a high optimal level, leading to a sustained and sometimes luxuriant development of foliage.
This “excessive vegetative activity of water plants, acts in all probability as a deterrent to sexual reproduction” (Mrs. Arber). It may be that the increasing number of vegetative meristems makes a greater demand on the plants’ nutrients, leaving very little for the production of floral meristems.
There is thus very often a tendency against hydrophytes towards the replacement of sexual by vegetative reproduction. This is quite in line with the fact that the flowers of many angiospermic hydrophytes are singularly ill-adapted to aquatic life and have still to be raised into the air above the surface of water for pollination. A capacity for sustained vegetative propagation, evading the hazards of pulling up the flowers above the water level, may be advantageous especially to the plants living in deeper waters.
Although the frequency of vegetative reproduction is higher among the hydrophytes, the organs actually employed as propagules are essentially similar to those of terrestrial herbs. Fragmentation of the plant body, followed by regeneration from any small part, bearing a vegetative bud, is a common occurrence in some free-floating plants and also in submerged hydrophytes with long and delicate stems. Regeneration from any fragment of the body containing meristematic tissue is also common among members of the family Podostemaceae. Gemmipary the development of new plants from vegetative buds on the parent body occurs in many wild hydrophytes.
Colonisation by means of rhizomes, stolons, runners, etc., is widespread amongst hydrophytes and in numerous taxa, these organs also store food reserves and serves as hibernacula, enabling plants to survive through unfavourable environmental conditions.
Towards the ends of the growing season, some species of Sagittaria and Potamogeton form small tubers on the ends of stolons or lateral branches, whilst some species of Hydrocharis, Myriophyllum and Utricularia develop specialised dwarf-shoots known as turions. Among other specialised structures, Ceratophyllum has dense shoot apices, protected by mucilage or a thick cuticle which are full of food reserves.
These remain dormant through the winter, germinating again with the advent of favourable weather in the spring. Throughout large sectors of their geographical distribution. Elodea canadensis and Acorns calamus, propagate only by vegetative reproduction—in those regions they do not produce flowers at all.
There are numerous examples amongst hydrophytes of a phenomenon known as pseudovivipary (it also occurs in many terrestrial monocotyledons and dicotyledons). Here, the vegetative propagules replace some or all of the normal sexual flowers in the inflorescence. It has been known for a long time that if an inflorescence of Myriophyllum verticillatum became submerged, a turion was formed at the apex. In an Aponogeton sp., the digitately branched axis may bear at the same time the much reduced inflorescence as well as young plants (de Wit, 1958).
The hydrophytes of the family Alismaceae are rich in pseudo-viviparous species. Caldesia grandis, an Indian species has an inflorescence-axis, bearing whorls of turions instead of flowers in submerged plants growing in water of 50 cm or greater depth (Den Hartog, 1957). Several American species of submerged hydrophytes (e.g., Echinodorus) bear plantlets in place of flowers all along the axis of their inflorescence (de Wit, 1958).
Gemmiparous buds may also be produced from foliar tissue in some hydrophytes. These buds arise at specific sites of meristematic cell-aggregates in the leaf-lamina. Since a single leaf may bear several such buds, each capable of developing into a plantlet (or bulbil) which may either drop off or become independent when the leaf decays, gemmipary is certainly a prolific method of vegetative multiplication.
We do not know much as yet of the physiology and cytology of these sites of cell-aggregates and how they differ from the remaining cells of the leaf tissue. In the aquatic fern, Ceratopteris, which produces plantlets in the vicinity of vein-endings at the base of the marginal furrows of the mature leaf, the excised meristems of those specific sites will produce healthy plants on a nutrient medium, only in presence of either adenine or IAA (Gottlieb, 1963).
(xii) Physiology of sexual reproduction in vascular hydrophytes. There is some evidence that in some hydrophytes, the initiation of the flowering phase may depend upon nutrition. The carnivorous Utricularia flowers when grown aseptically in inorganic nutrient medium but only when supplemented with organic nitrogenous compounds, e.g., a mixture of peptone and meat extract (Harder, 1963). The carnivorous habit does seem likely to be indispensable in natural habitats for the fulfilment of the life cycle of Utricularia.
A photoperiodic response may also be involved in Utricularia. Pringsheim (1962) and Harder (1963) found that the plant flowered in artificial light in supplemented nutrient medium, under 11-12 hr. photoperiod.
Eichhornia crassipes, on the other hand, seems to be a day-neutral plant—flowering seems to be influenced by temperature rather than daylength. Species of the genus Lemna show opposite photoperiodic behaviour in that, some species flower in short days while others in long days or even in continuous illumination. Some do not flower under any photoperiodic condition. The floral effect in Lemna is indeed complex with complications arising from interaction of different external factors, e.g., temperature, CO2– concentration, chelators, micronutrients (e.g., Cu and Fe), etc., with intensity and duration of light.
Wolffia microscopica in artificial culture behaves as a short-day plant for flowering—a minimal dark period of 12-14 hr. of at least one cycle, seems to be the essential requirement. The recently isolated cytokinin, zeatin, when added to the medium, however, promotes flowering in Wolffia even under a long photoperiod (Maheswari and Venkata- raman, 1966).
Photoperiodic responses are probably involved in the promotion of reproductive phase in some aquatic pteridophytes such as Salvinia. Salvinia natans appears to be a short-day plant—in nature, sporocarps appear in daylength of 13 hr. or less and even a 7 hr. photoperiod seems to be sufficient to induce 100% response, if the treatment is given for a week (Nakayama, 1952).
(xiii) The seeds of an overwhelming majority of aquatic vascular angiosperms exhibit prolonged dormancy. In most species, this is due to the mechanical imprisonment of the embryo within a hard, heavily cuticularised testa or pericarp. Germination is extremely erratic and uncertain—some way remain dormant but viable for four or five years. Seeds of lotus show good viability, ranging from two to four years, irrespective of the temperature and humidity of the medium.
(xiv) In some hydrophytes, we have noted before, food is stored up in the rhizome, (e.g., water-lily) in others such as Sagittaria, tubers are formed.
Classification of Hydrophytes:
Mrs. Arber (1920) recognised two primary groups of aquatic angiosperms: rooted and non-rooted, which she again subdivided according to the type of foliage and inflorescence produced and the position of these organs with respect to the water level. She realised that this distinction cannot be a strict one as the lines of demarcation between them were blurred by transitional types.
Penfound (1952), following the outline of Hess and Hall (1945), split hydrophytes into wet-land types—in soils saturated with water and aquatic types—in soils covered with water for most of the growing season. He recognised three forms among the aquatic forms—emergent, floating and submerged.
Danserau (1945) developed a new classification of the biological forms of hydrophytes. This is a rather sensitive system with attributes of correlating the zonation of communities along streams and lakes.
Danserau classified all aquatic forms into two broad types—helophyta (paludous) and hydrophyta. The hydrophyta is further subdivided into natantia (not fixed to the substratum; e.g. Lemna, Ceratophyllum, etc.), radicantia (fixed to the substratum) and adnata (fixed on rocks, or on other plants; e.g., Podostemon, Fontinalis, etc.). Radicantia is divided into emersa (at least partially emerged) and subemersa (none or few leaves floating). Emersa is further divided into foliacea (with a grand leaf development; e.g., Sagittaria, Pontederia, etc.), Junciformia (reduced leaf development; e.g., Scirpus, etc.) and nymphoidea (leaves floating; e.g., Nymphoea, etc.). Subemersa is put into 3 classes vittata (with long stem, leaves soft; e.g., Potamogeton, Vallisneria, etc.), rosulata (leaves reduced, basal; e.g., Isoetes, Lobelia, etc.) and annua (therophytic annuals; e.g., Najas, etc.).
Hejny (1957, 1960) simply recognised three groups euhydatophytes—with vegetative organs completely submerged and the reproductive organs, submerged or aerial hydatoaerophytes—with partly submerged and partly floating vegetative body and aerial inflorescence, and tenagophytes—amphibious plants occurring in habitats with marked fluctuations of the water level.
On the basis of their attachment to the soil, Luther (1949) classified hydrophytes into: haptophytes—plants which are attached to, but do not penetrate much to the substrate, e.g., many algae, lichens, bryophytes and among angiosperms, only the Podostemaceae; rhizophytes—whose basal parts actually penetrate the soil or the substrate in which they grow and planophytes—freely floating plants with submerged or surface-floating photosynthetic organs. The third group includes microscopic planktophytes and macroscopic pleustophytes (e.g., larger floating algae, liverworts ferns and some angiosperms).
Luther further divided pleustophytes into three groups according to the level at which they float—a differentiation which can hardly be sustained as species of Lemna, Ceratophyllum, Utricularia, etc., seem to rise and fall according to the season or to their stage of development. Den Hartog and Segal (1964) further elaborated and then supplemented Luther’s division of rhizophytes and pleustophytes.
Eleven basic types of growth habit were recognised among rhizophytes, e.g., rhizophytes possessing a short stem and a rosette of stiff leaves, with or without stolons, distinguished as isoetids. (Isoetes, etc.); rhizophytes with long stems, entire submerged leaves, no floating leaves, and aerial or submerged reproductive organs as elodeids, (e.g., Elodea, Najas, etc.) and so on.
The British ecologists, Tansley (1949), Spence (1964) and Sculthorpe (1967) recognised principal life and growth forms of vascular hydrophytes and adopted a scheme of classification based on this recognition:
A. Hydrophytes Attached to the Substratum:
(1) Emergent Hydrophytes:
Occur on exposed or submerged soils; mainly rhizomatous or cormous perennials; in heterophyllous species submerged and/or floating leaves precede the mature aerial leaves; many submerged forms (usually sterile); all produce aerial reproductive organs, e.g., Typha, Phragmites, etc.
(2) Floating-leaved Hydrophytes:
Occur in submerged soils in water depths up to approximately 4 m.; some species may exist as reduced land forms; in heterophyllous species, submerged leaves precede or accompany the floating leaves; many species produce aerial leaves in crowded habitats; reproductive organs, floating or aerial;
(i) Rhizomatous or cormous type, with floating leaves on long flexible petioles, e.g., Aponogeton sp., Nymphaea, etc.
(ii) Stoloniferous type, with trailing stems and producing floating leaves on relatively short petioles, e.g., Potamogeton natans, Nymphoides, etc.
(3) Submerged Hydrophytes:
Occur on submerged soils at all water depths to a maximum of about 12 m.; foliage entirely submerged; leaves often filiform or ribbon-shaped or even finely divided; a few species may produce land forms; reproductive organs aerial floating or submerged.
(i) Caulescent type with or without a rhizome, the long leafy stems rooting from nodes, e.g., Elodea, Hydrilla, Najas, Potamogeton sp., etc.
(ii) Rosette type, with radical leaves arising often from a condensed, tuberous stock or a rhizome; often stoloniferous, e.g., Aponogeton sp., Isoetes, Sagittaria sp., Vallisneria, etc.
(iii) Thalloid type, with plant body reduced to a cylindrical or flattened, creeping or floating , polymorphic thallus, with erect or trailing secondary branches, e.g., the Podostemaceae such as species of Hydrobryum, Podostemon, etc.
B. Free-floating Hydrophytes:
All are unattached in slow-flowing water, but some species with extensive root system may become attached to the substrate in shallow water; numerous species produce land forms; very diverse in form and habit, ranging from stoloniferous plant, with rosettes of aerial and/or floating leaves and well-developed submerged roots (e.g., Ceratopteris sp., Eichhornia crassipes, Pistia, Trapa, etc.), to minute surface-floating or submerged plants, with reduced assimilatory thallus, having few or no roots (e.g., Lemna and Wolffia); reproductive organs floating or aerial, very rarely submerged (e.g., Cerato- phyllum and Salvinia); numerous submerged species rise to the surface to flower, and may sink to the bottom of the substratum to perennate (e.g., Lemna trisulca, Utricularia, etc.).
For convenience, the hydrophytic vegetation has been grouped here under the following five morpho-ecological groups:
(a) Floating Hydrophytes:
Include all plants that float or are in contact with water and air but not rooted in the soil. Examples—Pistia, Lemna (in the family Lemna ceae, where the leaf and stem are represented by a small thallus, the roots are in the process of disappearing; Lemna has only one strand of root), Wolffia (entirely rootless, one of the minutest of the flowering plants), Aldrovanda, Eichhornia (water hyacinth), Lim- nanthemum, the water fern, Salvinia, etc.
(b) Suspended Hydrophytes:
Plants such as the brown alga, Sargassum and also Lemna trisulca, etc., are in contact with water alone. They are not on the surface but just below the surface of water. The plants under this group are absolutely free from any necessity of transpiration, yet for some quaint reason or other, they are found in the best-lighted and best-aerated part of the water.
(c) Floating but rooted Hydrophytes:
The plants in this category are anchored in the muddy soil at the bottom of the ponds and lakes. Examples—water-lily, lotus, Potamogeton indicus, P. crisptis, P. mucronatum, Aponogeton crispum. Neptunia oleracea, etc. (Figs. 772 and 773). The hydrophytic fern, Ceratopteris thalictroides (Parkeriaeeae) which is of world-wide distribution throughout tropical and subtropical regions may grow on damp soil, but usually it is a true aquatic, either rooted in the mud or free-floating.
(d) Submerged and rooted hydrophytes:
Plants in this group are completely submerged and are also anchored to the substratum. Extensive development of blooms of floating blue-green algae such as Anabaena, Oscillatoria, etc., (sometimes the growth is so luxuriant that more than 9,000,000 filaments may be found per litre of water!) on the water surface particularly under calm conditions may cut down light penetration to such an extent that the growth of submerged hydrophytes may be severely reduced.
Because of this shade effect compared to light available to land plants (light both in quantity and quality is already altered for deeply submerged plants due to passage of light through several feet of water) many submerged hydrophytes show features which are associated with shade-loving land plants, sciophytes, e.g., long inter- nodes, no palisade parenchyma, presence of chloroplastids in the epidermal cells, etc.
Three genera or blue-green algae, Anabaena, Aphanizomtnon and Microcystis, which produce the most spectacular water blooms bear the specific name of ‘flos aquae’. Examples—Mostly monocotyledons such as Vallisneria, Hydrilla, some Potamogeton species, Najas, etc., and dicotyledons such as Ceratophyllum,
Myriophyllum (Fig. 774), Utricularia (insectivorous bladderwort), etc. Chara and almost all macroscopic algae. Vallisneria has an interesting pollination mechanism. The unisexual staminate flowers break off from the inflorescence and after rising to the water surface float about.
The scape of the pistillate flower elongates and reaches the surface of water where it remains until pollinated by pollens of the staminate flower. After fertilisation the scape shortens drawing and developing young plant under water where it completes its development (Fig. 775). In Myriophyllum, however, inflorescences are always maintained above water and flowers are always (?) wind pollinated.
(e) Rooted amphibious Hydrophytes:
These are so named because the plants in this group grow in shallow water with their underground parts in water or in water- saturated muddy soil, while extending their shoots well above the surface of water. They are also known as marsh plants or helophytes. The characteristic feature of marsh plants is that their lower parts, buried in muddy water are adapted to aquatic life, while the air-exposed upper parts resemble ordinary mesophytes or sometimes may even be adapted to withstand atmospheric drought.
However, the reduced leaf surface and the presence of the cuticle in aerial parts, observed in some marsh plants, may only be xeromorphic in nature, i.e., the persistence of ancestral features—they must have been land plants at the beginning before they started their watery career—in spite of a striking change of habitat. These so-called “marsh xerophytes are really typical hydrophytes, wearing a false xerophytic mask.”
Some varieties of rice, Oryza sativa (rice plants which live with their roots partly submerged in water, i.e., under oxygen-poor conditions, usually have large air-channels in these roots); the typical marshland plants, e.g., Scirpus, the grass Typha; the mangroves Rhizophora, Avicennia, etc. (ecological adaptations not in the least hydrophytic; sometimes striking xeromorphic characteristics); the heterophyllous Sagittaria, Proserpinaca palustris; Enhydra fluctuans (Fig. 776), Polygonum barbatum, Jussiaea repens (may also grow completely submerged), Jussiaea suffruticosa, Commelina sp., etc.
Among other common amphibian hydrophytes from water-logged rice fields of southern part of West Bengal are Cardenthera triflora—usually heterophyllous with leaves above the water level entire, submerged leaves in water are much dissected. The plant remains heterophyllous even when water is drained off from the paddy fields after the harvesting of rice.
When the land becomes completely dry, sometimes homophyllous plants are seen; Hygrophila spinosa, common in rice fields with only one type of leaves; Limnophila gratioloides, a small herbaceous plant, with pink leaves and stems appear in the wet rice fields after rice harvest, etc.
Many of these amphibious hydrophytes grow comfortably in this type of muddy roil because the oxygen concentration is low in such habitats and these plants have developed especially low oxygen requirement for germination, as for example oxygen requirement for the most satisfactory germination of some rice varieties is only about one-sixth of that of wheat.
In many of this type of marshy hydrophytes such as Rhizophora, Ceriops, etc., special root branches grow erect until they project above the poorly aerated muddy soil. These structures called pneumatophores usually have well-developed intercellular system of airspaces continuous with the stomata of the leaf and must be of unquestioned value in supplying oxygen to the plants.
Somewhat loose associations of two hydrophytes are sometimes observed in many watery-habitats of India as for example, Eichhornia-Potamogeton association, Hydrilla- Vallisneria association and Ceratophyllum-Vallisneria association. Similar associations are sometimes observed between moisture-loving, insectivorous Drosera burmani (also Drosera indica) and the marshland hydrophyte, Eriocaulon. In certain localities of North Bengal the presence of Eriocaulon indicates the probable presence of Drosera in the neighbourhood.
The proportion of endemic species is pretty high among the hydrophytes—about 25-30 per cent. The high frequency of endemism is probably related to the edaphic characteristics of the soil and is a possible relic status. Most endemic hydrophytes are naturally to be found in the tropics; numerous ecologically restricted members of the Podostemaceae (perhaps the strangest and most provocative family of angiosperrns) being prominent amongst them.
In some members of the family Podostemaceae, where the vegetative body is reduced to a creeping or floating thallus, closely resembling certain fresh-water and marine algae or may even be strongly reminiscent of hepatics and lichens,—this resemblance of the thalloid types to far distant cryptogams is not easily interpreted—astonishing polymorphism is clearly shown.
The rock-creeping anomalous thallus is most probably a rhizome-like photosynthetic root (the presence of root cap supports this), adhering to the rocks by hairs or haptera. In the species Dicraea dichotoma, the thallus is ribbon-like and distally free-floating while in D. stylosa, it is broad, freely branched, and often floating, reminiscent of Fucus and other sea weeds. The endogenous secondary shoots float freely in water and may bear simple, subulate or linear leaves.
The Asiatic hydrophytic flora includes many endemics and relatively few widespread species. The ancient family of hydrophytic Aponogetonaceae contains species which fill into several endemic groups, which display- morphological affinities. These distinct groups are Australasian group, South-East Asiatic group, lndo-Sri -Lanka group (Aponogeton undulatus, A. echinatus, A. microphyllus, A. crispus and A. rigidifolius), Sri Lanka group, Madagascar group and continental African group.
The wide prevalence of endemic hydrophytes in India and Sri Lanka may be partly due to the tropical rain forest and suitable coastal low lands being isolated- from those of Africa and of continental and oceanic south-east Asia.
The total number of families of vascular hydrophytes which have gone back to water and is represented at present in the aquatic medium perhaps does not exceed 40. This is certainly a small total. Of these families, thirty have less than 10 genera and seventeen are monogeneric. The number of aquatic species probably exceeds 100 in only two families, Podostemaceae and the continental, Haloragaceae.