The below mentioned article provides an essay on hydrophyte.
Hydrophyte embraces all the different and varying plant forms that adapted to live under direct influence of free water or in waterlogged soil, e.g. a small number of ferns and angiosperm. Gymnosperms have not migrated into aquatic habitats.
Hydrophytes, also called hydrophytic plants, are classified according to forms:
Submergent, Floating and Emergent. Submergent hydrophytes grow and remain entirely submerged below the surface of water throughout life. Floating hydrophytes have leaves or shoots that float on top of water surface. Emergent hydrophytes have roots anchored to hydric soil, but a portion of shoot lies above water surface.
The different form of hydrophyte exhibits the following characteristics that help them to survive in water and waterlogged soil:
i. Increase in leaf surface that helps plants for floatation.
ii. The presence of air chambers that assist in buoyancy of the plant.
iii. Stomata, if present, nonfunctional because the plants need not regulate transpiration due to abundance of water in their environment.
iv. Reduction of xylem because the hydrophytes need not conserve water.
v. Reduction in mechanical and protective tissue as water pressure supports them.
vi. Hydrophytes have thin cuticle or lack it. Because the functions of cuticle like protection from strong sunlight, conservation of water and deterrence of insects are not required by hydrophytes. They are protected from excess sunlight by surrounding water; no question arises for water conservation as they have abundant free water in their environment; the hydrophytic angiosperms suffer little predation by aquatic animals.
Due to the thinness of cuticle the leaves absorb nutrients from the water directly. But the major bulk of minerals are absorbed by roots and transported to the leaves. As there is no transpiration pull like land plants the transportation of minerals occur through root pressure. Hydathodes are very common in the leaf tips of many submerged hydrophytes. Transportation of water occurs as a bleeding stream ending in hydathodes. The opening of hydathodes may be large.
vii. Stomata when present may be many in number and occur on upper surface of floating leaves (epistomatous).
viii. Roots are small and reduced in all respects as water can diffuse directly into leaves.
ix. Formation of special roots and these are negatively geotropic and designed to function in gas exchange. Such roots are termed as pneumatophore and present in halophytes.
x. Hydric soils have diluted oxygen and minerals and the hydrophytes adapted to absorb nutrients directly from water.
xi. The piliferous layer scarcely has (or no) root hairs. Root hairs are entirely absent from Ceratophyllum.
xii. The piliferous layer is not cutinized/suberized and so absorptive in function. Moreover the turgid piliferous zone provides mechanical strength, as mechanical cells are absent in hydrophytic roots.
xiii. The cortex of root is wide and provided with air cavities.
xiv. Xylem of root and stem is poorly developed and has thin walls. Vessels are absent in many hydrophytes. In the root of Potamogeton (Fig. 29.9A) the vascular tract in the centre is occupied by a large xylem cavity. The reduction in xylem is due to the fact that in hydrophytes no elaborate system of water absorption is required as all the epidermises of plant body are capable of water and mineral absorption.
xv. The stem of submerged hydrophytes is very weak and flexible. It moves according to the direction of flow of water currents.
xvi. The epidermis in the stem of submerged hydrophytes is uniseriate. It is not cuticularized and in some cases the cells have cellulose walls only. As a result the epidermal cells can absorb mineral salts and gases dissolved in water directly.
xvii. The cortex is wide and has large air chambers. The air chambers may be arranged symmetrically e.g. Myriophyllum (Fig. 29.9B) or scattered irregularly all over the cortex, e.g. Elodea, Hydrilla, Hippuris etc.
xviii. The cells of cortex have low osmotic pressure. The cells contain chloroplastids that help in photosynthesis.
xix. The vascular tissues in stem of submerged hydrophytes are not well developed. Endodermis and pericycle are indistinct. The vascular tissues are mostly composed of phloem.
xx. Though the endodermis is not distinct many hydrophytic angiosperms have the innermost layer of cortex that functions as endodermis. This endodermoid layer is present in submerged roots, stems and leaves. It is assumed that this layer is involved in channelizing the flow of water through reduced xylem.
In hydrophytes water is pushed up by root pressure produced as the endodermal cells pump ions into the xylem. This is due to the absence of transpiration pull like land plants. If there had been no endodermis like layer, the pressurized water could fill up the air chamber thus affecting buoyancy of the plant. So with the endodermoid layer the flow of water is effectively channelized to the leaves.
xxi. Plants growing in running or tidal water have very linear leaf, e.g. Posidonia, Zostera etc. The leaves of Ceratophyllum are segmented. The leaves of Ranunculus, Myriophyllum are finely dissected. The submerged leaves of monocotyledons are often band-like and undivided, e.g. Potamogeton, Najadaceae etc. Such shapes of leaves offer little resistance to water that move around them.
xxii. The leaf epidermises of submerged hydrophytes lack cuticle and contain abundant chloroplasts. As a result the epidermal cells have absorptive function and help in photosynthesis. The epidermises lack stoma and differ in no way from the ground tissue.
xxiii. The mesophyll cells of submerged leaves are not differentiated into palisade and spongy tissue. They contain abundant chloroplasts and well- developed air spaces. The submerged species of Potamogeton (Fig. 29.10B & C) has a single layer of mesophyll cells between the two epidermises. Mesophyll cell is absent in Elodea (Fig. 29.10A).
The term ‘epidermis’ is singular noun. The plural is epidermi. The plural epidermi was rarely used by anatomists and so became obsolete.
In this Title the plural ‘epidermises’ is used following the suggestion of Mauseth:
xxiv. Hydrophytes have strongly developed air lacunae in their roots, stems and leaves. They are referred to as air chambers. Though they are typically called air chambers, but they are not filled with air. The gases, generated by the plant, fill the chambers. The air chambers assist in buoyancy of the plant.
Beside this, the air chamber and intercellular spaces function as an aeration system. This system allows the diffusion of oxygen to the non- chlorophyllous part of the plant. Oxygen is generated in the upper chlorenchyma and then it is diffused down to the roots. The roots always remain in contact with water and so they need internal supply of oxygen.
The soils, where the roots are anchored, must periodically dry slightly to maintain soil oxygen content. It never occurs, so the root requires internal supply of oxygen. The air chambers also facilitates in regulating photosynthetic gases carbon dioxide and oxygen. Moreover the air chambers along with the turgid tissues compensate the deficiencies of mechanical elements, which are very much reduced in hydrophytes.
xxv. Fibres are mechanical element and provide elastic flexibility. Hydrophytes exposed to wave action or tidal fluctuations have fibre bands on leaf margins, which are liable to tear in wave actions.
xxvi. In submerged hydrophytes there are localized groups of cells on the epidermises that are concerned with absorption of water and minerals. This multicellular absorbing structure is termed as hydropoten. Hydropoten is observed in Sagittaria (Fig. 29.9C), Echinodorus, Ceratophyllum, Myriophyllum etc.
xxvii. Laticifers are present in many hydrophytes in their rhizomes, petioles, pedicel and peduncle etc. Ex. Sagittaria (Fig. 29.11A), Limnophyton etc.
A. Sagittaria guayanensis. B. Cross-section of a portion of floating leaf of Nymphaea alba.
xxviii. Periderm is formed in many submerged dicotyledons, e.g. Lycopus, Jussiaea (Fig. 29.10D), Onagraceae, Lythraceae etc. Here phellogen, the cork cambium, instead of producing dense phellem on the peripheral side, gives rise to aerenchyma with abundant intercellular spaces.
The aerenchyma consists of thin radial elongated cells. The radial lamellae of cells separate the wide air ducts. Periderm with dense phellem in land plants and periderm with peripheral aerenchyma in hydrophytes both have same function, i.e. to insulate the inner tissues from external environmental influence.
xxix. The leaves of floating hydrophytes, e.g. Nymphaeaceae (Fig. 29.11B), Hydrocharis etc. exhibit a remarkable combination of xeromorphic and hydromorphic features. The upper surface of leaf is exposed to strong sunlight.
So the upper portions of leaf have xeromorphic adaptations like:
(1) Upper epidermis with cuticle,
(2) Sunken stomata,
(3) Dense and many layered upper palisade tissue, and
(4) Presence of idioblast-sclereids.
In contrast the lower portions of leaf exhibit hydromorphic adaptations like:
(1) Lower epidermis with thin cuticle,
(2) Absence of stoma from lower epidermis,
(3) Large intercellular spaces, with
(4) Thin walled parenchyma.
Species of Ranunculus, Polygonum amphibium etc. are amphibious plants. These plants are able to live both in air and under water. The hydromorphic and xeromorphic adaptations that are mentioned above with reference to floating leaves are also encountered in these amphibious plants.
xxx. The petiole of Nymphaea (Fig. 29.12A & B) exhibits the following anatomical characteristics:
(1) The uniseriate epidermis contains chloroplastid and consists of thin walled compactly set more or less round cells with cuticle on their outer walls. Numerous hairs occur on epidermis and they are multicellular and not branched.
(2) Beside epidermis all the tissues present in the petiole compose the ground tissue. The peripheral two or more layers of ground tissue are composed of collenchyma. Rest of cells of the ground tissue are parenchymatous, thin walled and enclose large air spaces. Trichoblasts (= trichosclereids) frequently occur in the spaces. Trichoblasts are branched, are of different shapes and attached to the cells bordering the air spaces. Calcium oxalate crystals deposit on trichoblasts.
(3) The vascular tissue xylem is poorly developed and represented by a cavity only in a vascular bundle. Phloem is well developed and associated with xylem thus composing collateral vascular bundle. The bundles are closed, i.e. cambium is absent between xylem and phloem.
The bundles are of different sizes. The smaller bundles have a single patch of phloem whereas two patches of phloem occur in large vascular bundles. The bundles are scattered over the ground tissue. They occur on the partition wall between air spaces.
(4) The petiole of Nymphaeaceae exhibits hydromorphic adaptations due to presence of (a) numerous large air spaces that assist the petiole to remain erect, (b) reduced development of xylem, (c) well development of phloem and (d) thin cuticle on the outer wall of epidermal cells.
Collenchyma in hypodermal layer, turgid aerenchyma and large air spaces supplement the mechanical cells. The occurrence of scattered vascular bundle in the petiole is regarded as anomaly, because it is a dicot where circular arrangement of vascular bundle is the norm in stem and petiole.
Plants inhabiting in acid bogs develop anatomical features that help them to survive because in water nitrogenous salts are absent and concentration of minerals is low. Such adaptations are meant for trapping animals and are observed in insectivorous plants, e.g. Lentibulariaceae and Droseraceae.
In Dionaea (Droseraceae) the upper portion of leaf blade has quadrangular shape. Specialize hinge or motor cells occur along the midrib. The leaf margin projects as long teeth close together. The edge of blade is green and the rest of the surface is covered with reddish dots. The dots are digestive glandular hairs that are revealed under microscope.
The two halves of the blade are bent upwards and in cross- sectional view appear as V-form. On each half of blade there are three long hairs also called trigger hairs. The hairs are very sensitive and hinged at the base. The hairs require two or three tactile stimuli to cause immediate folding of leaf. When a prey stimulates the trigger hairs the leaf folds vigorously.
The marginal teeth cross one another and so the prey cannot escape. The two halves squeeze together tightly. Reddish glandular hairs secrete digestive enzymes that act upon the proteids of the prey. Absorption follows and when the process is complete the leaf opens again.