Plant adaptations: Hydrophytes (Morphological, anatomical and physiological adaptations)

Plant Adaptations:- Any feature of a plant or its part, which enables it to exist under its habitat conditions, is called plant adaptation.

Hydrophytes:- Plants which grow in water partially or completely are known as Hydrophytes.

Types of 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.

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.

Morphological adaptations:-

i. Root system is poorly developed.

ii. Roots of floating hydrophytes show very poor development of root hairs, absence of true root caps, with root pockets to protect their tips from injuries. (Eg.- Eichhornia)

iii. Rooted hydrophytes like Hydrilla, Vallisnera, Elodia derive their nourishment through their body surfaces. More plants partly depend on their roots for the absorption of minerals from the soil. Roots are totally absent in Ceratophyllum, Salvinia, Azolla, Utricularia etc.,

iv. In Jussiaea repens two types of roots develop. Some of them are normal, while others are negatively geotropic, floating roots, spongy in nature and keep the plants afloat.

v. In free floating hydrophytes, the stem is thick and short, floating on the surface of water. Eg.- Eichhornia.

vi. In Nymphaea and Nelumbium the stem is a rhizome. These rhizomes live for many years and produce leaves every year.

vii. In rooted plants with floating leaves, the leaves are large, flat and entire Eg.- Nymphaea, Victoria regia. Their upper surface is coated with wax. The wax coating protects the leaves from mechanical and physical injuries and also prevents clogging of stomata by water.

viii. In floating plants of Eichhornia, Trapa etc., the petioles become characteristically swollen and become spongy, providing buoyancy.

ix. Plants such as Limnophylla heterophylla, Sagittaria, Ranunculus, Salvinia, Azolla etc show heterophylly, with submerged dissected leaves offering little resistance against the water currents, and absorbing dissolved carbon-di-oxide from water. The aerial leaves show typical mesophytic features. It acts as foliage leaf.

x. Pollination (Eg. Vallisneria) and dispersal of fruits and seeds are accomplished by the agency of water.

Anatomical adaptations:-

i. The root and shoot systems show common features such as cuticle which is very thin or absent.

ii. Epidermis is usually a single layer of thin walled cells, not protective in function.

iii. Cortex is well developed. It has numerous air chambers. It helps in buoyancy and rapid gaseous exchange.

iv. Mechanical tissues are generally absent.

v. In the vascular tissue, xylem vessels are less common. Only tracheids are present in submerged forms.

vi. In amphibious form, the xylem and phloem are well developed Eg. Limnophylla heterophylla or vascular bundles may be aggregated towards the centre. Eg. Jussiaea

vii. Epidermal cells of phloem leaves contain chloroplasts and they can function as photosynthetic tissue, especially where the leaves and stems are very thin. Eg. Hydrilla

viii. Stomata are totally absent in submerged, but in floating leaves, stomata are confined only to the upper surface. In amphibious plants stomata may be scattered on all the aerial parts.

ix. In submerged leaves, air chambers are filled with respiratory and other gases and moisture.

x. In Water Lilly (Nymphaea) and some other plants, special type of star shaped lignified cells called asterosclereids are developed. It gives mechanical support to the plants.

Physiological adaptations:-

i. Hydrophytes show the low osmotic concentration of cell sap. Osmotic concentration of cell sap equal or slightly higher than that of the habitat water.

ii. Some hydrophytes possess hydathodes (water stomata) in floating hydrophytes which reduce the excessive amounts of water present in the plant body of the hydrophyte.

iii. The gases are exchanged from water through surface cells. Some CO2 evolved during respiration is stored in the air spaces which is consumed during photosynthesis and O2 evolved photosynthesis is used in respiration.

iv. The mucilaginous secretion of mucilage cells and mucilage canals around the body of the hydrophytes provides protection against friction, desiccation, decaying and microbial injuries.

v. Flowering and fruiting are comparatively poor. They are adapted more for vegetative propagation.