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.
The plants which are living in water or wet places are called hydrophytes. According to their relation to water and air, they are subdivided into following categories:
i) Free floating hydrophytes,
ii) Rooted- floating hydrophytes,
iii) Submerged floating hydrophytes,
iv) Rooted -submerged hydrophytes,
v) Amphibious hydrophytes.
i. Free floating hydrophytes:
These plants float freely on the surface of water. They remain in contact with water and air, but not with soil. Examples: Eichhornia, Pistia and Wolffia (smallest flowering plant).
ii. Rooted floating hydrophytes:
In these plants, the roots are fixed in mud, but their leaves and flowers are floating on the surface of water. These plants are in contact with soil, water and air.
Examples: Nelumbo, Nymphaea, Potomogeton and Marsilea. Lotus seeds show highest longevity in plant kingdom.
iii. Submerged floating hydrophytes:
These plants are completely submerged in water and not in contact with soil and air. Examples: Ceratophyllum and Utricularia. iv. Rooted- submerged hydrophytes: These plants are completely submerged in water and rooted in soil and not in contact with air.
Examples: Hydrilla, Vallisneria and Isoetes.
v. Amphibious hydrophytes (Rooted emergent hydrophytes):
These plants are adapted to both aquatic and terrestrial modes of life. They grow in shallow water.
Examples: Ranunculus, Typha and Sagittaria.
The plants which can grow in moist damp and shady places are called hygrophytes.
Examples: Habenaria (Orchid), Mosses (Bryophytes), etc. Morphological adaptations of Hydrophytes:
• Roots are totally absent in Wolffia and Salvinia or poorly developed in Hydrilla or well developed in Ranunculus.
• The root caps are replaced by root pockets. Example: Eichhornia
• The stem is long, slender, spongy and flexible in submerged forms.
• In free floating forms the stem is thick, short stoloniferous and spongy; and in rooted f loating forms, it is a rhizome.
• Vegetative propagation is through runners, stolon, stem and root cuttings , tubers, dormant apices and offsets.
• The leaves are thin, long and ribbon shaped in Vallisneria or long and linear in Potamogeton or finely dissected in Ceratophyllum
• The floating leaves are large and flat as in Nymphaea and Nelumbo. In Eichhornia and Trapa petioles become swollen and spongy.
• In emergent forms, the leaves show heterophylly (Submerged leaves are dissected and aerial leaves are entire). Example: Ranunculus, Limnophila heterophylla and Sagittaria
• Cuticle is either completely absent or if present it is thin and poorly developed
• Single layer of epidermis is present
• Cortex is well developed with aerenchyma
• Vascular tissues are poorly developed. In emergent forms vascular elements are well developed.
• Mechanical tissues are generally absent except in some emergent forms. Pith cells are sclerenchymatous.
Physiological adaptations of Hydrophytes:
• Hydrophytes have the ability to withstand anaerobic conditions .
• They possess special aerating organs.