Ecological Adaptations in Plants : The modifications in the structure of organisms to survive successfully in an environment are called adaptations of organisms. Adaptations help the organisms to exist under the prevailing ecological habitat. Based on the habitats and the corresponding adaptations of plants, they are classified as hydrophytes, xerophytes, mesophytes, epiphytes and halophytes.
Adaptation in aquatic plants (Hydrophytes) : The plants which are living in water or wet places are called hydrophytes. According to their relation to water and air, they are sub-divided into following categories :
(i) Free Floating Hydrophytes :
Example : Eichhornia, Pistia and Wolffia (smallest flowering plant).
(ii) Rooted Floating Hydrophytes :
Example : Nelumbo, Nymphaea, Potomogeton and Marsilea.
(iii) Submerged Floating Hydrophytes :
Examples : Ceratophyllum and Utricularia.
(iv) Rooted - submerged Hydrophytes :
Examples : Hydrilla, Vallisneria and Isoetes.
(v) Amphibious hydrophytes (Rooted emergent hydrophytes) :
Examples: Ranunculus, Typha and Sagittaria.
Morphological adaptations of Hydrophytes :
(1) Roots are totally absent in Wolffia and Salvinia or poorly developed in Hydrilla or well developed in Ranunculus.
(2) The root caps are replaced by root pockets. Example : Eichhornia.
(3) The stem is long, slender, spongy and flexible in submerged forms.
(4) In free floating forms the stem is thick, short stoloniferous and spongy; and in rooted floating forms, it is a rhizome.
(5) Vegetative propagation is through runners, stolon, stem and root cuttings, tubers, dormant apices and offsets.
(6) The leaves are thin, long and ribbon - shaped in Vallisneria or long and linear in Potamogeton or finely dissected in Ceratophyllum.
(7) The floating leaves are large and flat as in Nymphaea and Nelumbo. In Eichhornia and Trapa, petioles become swollen and spongy.
(8) In emergent forms, the leaves show heterophylly (Submerged leaves are dissected and aerial leaves are entire). Example : Ranunculus, Limnophila heterophylla and Sagittaria.
Anatomical adaptations :
(1) Cuticle is either completely absent or if present it is thin and poorly developed.
(2) Single layer of epidermis is present.
(3) Cortex is well developed with aerenchyma.
(4) Vascular tissues are poorly developed. In emergent forms vascular elements are well developed.
(5) Mechanical tissues are generally absent except in some emergent forms. Pith cells are sclerenchyma - tous.