Adaptations in hydrophytes can be discussed under three headings: mor- phological, anatomical and physiological.

A. Morphological adaptations

Hydrophytes show various kinds of morphological adaptations in their roots, stems and leaves

Roots

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Roots of hydrophytes are not of much importance, because most hydro- phytes are partly or wholly immersed in water.

(i)Roots arc totally absent in plants like Utricularia Ceratophyllum, Myriophyllum, and Salvinia.

(II)Poorly developed roots are found in submerged plants like Ilydrilla, Vallisneria.

(iii)Root pockets in place of root caps arc found in floating hydrophytes like Pistia and

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Eichhornea that project the root tip

(iv)Root hairs are poorly developed in most hydrophytes.

(v)Some plants like Jessie have two types of roots; one type being normal but the other spongy type and negatively geotropic.

Stem

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(i)In submerged hydrophytes the stem is slender, spongy, flexible and long as in Ilydrilla, Potamogeton .N

(ii) In some floating hydrophytes like Azolla, Pistia or Eichhornia, it is horizontal, spongy and floating.

(iii) In rooted hydrophytes like Sagittaria, Cyperus, Scirpus, Potamogeton, the stem is a rhi­zome or stolon.

Petioles

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Some hydrophytes show special features in the petioles.

(i) Petioles in submerged plants, with free floating leaves like Nymphaea and Nelumbium, are long, slender and spongy.

(ii) In the free floating hydrophyte Eichhornia, the peti- ole is swollen and helps in’ floating

Leaves

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Hydrophytes show a number of variations in the structure of their leaf lamina.

(i) In submerged hydiophytes like Utricularia, Myriophyllum and Ceratophyllum, the leaves are finely dis- sected and in plants like Vallisneria, they are long and narrow. In both types of adaptations the intention is to offer little resistance to water currents.

(ii) In free floating hydro- phytes, the leaves are smooth, shining and coated with wax. Presence of wax not only prevents water clogging, but also protects from physical and chemical injuries.

(iii) In floating but rooted hydrophytes like Nelumbium and Nymphaea the petioles are long and the lamina are peltate with their lower surfaces in direct contact with water and the upper surfaces exposed to air.

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(iv) One important feature that is usually shown by amphibious hydrophytes is heterophylly (leaf dimorphism), i.e., the presence of two types of leaves. In plants like Sagittaria, Ranun­culus and Limnophyla heterophylla, the submerged leaves are ribbon shaped or dissected and the leaves above the surface of water are broad.

B. Anatomical adaptations

In general, hydrophytes show the following trends in anatomical features:

(i) Reduction in protecting structures.

(ii) Reduction in mcchanical tissue.

(iii) Reduction in conducting tissue.

(iv) Increase in aeration.

As the above features are seen in most organs of a plant, the anatomical adaptations are better discussed on the basis of such features rather than on the basis of organs.

Reduction in protecting structures

(i)Absence of cuticle in the sub- merged portions.

(ii)Use of epidermis as an absorbing or photosynthesising (when epidermis has chloroplasts) organ rather than a protecting organ.

(iii)Poorly developed hypodermis

Reduction of mechanical tissue

(i) Total absence or poor development of scleren-chyma in the submerged portions.

(ii) Presence of special type of sclereids called asterosclereids in some hydrophytes that provide mechanical support in the absence of sclerenchyma.

(iii) Presence of sclerenchyma in little or moderate quantities in the aerial portions

Reduction of conducting (Vascular) tissue

(i) Vascular bundles are reduced to few or even one and located at the centre.

(ii) Xylem cells are very few as there is hardly any need of conduction.

(iii) Phloem is usually ill developed but in some cases it is well developed.

(iv) Secondary vascular tissue is never developed. Increase In aeration

(i) Stomata arc totally absent or ves- tigial in submerged parts. ,

(ii) Stomata are confined to upper surface leaves of rooted but floating hydrophytes.

(iii) In amphibious plants, stomata are scattered on the aerial portions.

(iv) Roots, stems and leaves of most hydro- phytes have air chambers. These cham- bers store gases like C02 and 0″ and help in respiration and photosynthesis. Besides, the air chambers help in buoyancy and provide mechanical support.

C. Physiological adaptations.

Besides their adaptations in the morphological and anatomical characters, hydrophytes also show physiological adaptations.

(i) Osmotic concentrations of cell sap are low.

(ii) No transpiration fr6m submerged plants.

(iii) Photosynthetic and respiratory gases are retained in air chambers for future use.