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
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, ii) Rooted- floating
hydrophytes, iii) Submerged floating hydrophytes, iv) Rooted -submerged
hydrophytes, v) Amphibious hydrophytes.
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
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 showing highest longevity in plant kingdom.
Submerged floating hydrophytes: These plants are completely submerged in
water and not in contact with soil and air. Examples: Ceratophyllum and
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.
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.
Hygrophytes: The plants which can grow in moist damp and shady places are called hygrophytes. Examples: Habenaria (Orchid), Mosses (Bryophytes), etc.
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 sub-merged
In free floating forms the stem is thick, short stoloniferous and
spongy; and in rooted floating 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
In emergent forms, the leaves show heterophylly (Submerged
leaves are dissected and aerial leaves are entire).
Example: Ranunculus, Limnophila heterophylla and
Cuticle is either completely absent or if present it is thin and
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.
Hydrophytes have the ability to withstand anaerobic conditions .
They possess special aerating organs.
The plants which are living in dry or xeric condition are known as
Xerophytes. Xerophytic habitat can be of two different types. They are:
a. Physical dryness: In these habitats, soil has a little
amount of water due to the inability of the soil to hold water because of low
b. Physiological dryness: In these habitats, water is sufficiently
present but plants are unable to absorb it because of the absence of capillary
spaces. Example: Plants in salty and acidic soil.
Based on adaptive characters xerophytes are classified into three
categories. They are Ephemerals, Succulents and Non succulent plants.
i. Ephemerals: These are also called droughtescapers or
drought evaders. These plants complete their life cycle within a short
period (single season).
These are not
true xerophytes. Examples: Argemone,
Mollugo, Tribulus and Tephrosia.
ii. Succulents: These are also called drought enduring plants.
These plants store water in their plant parts during the dry period.
These plants develop certain adaptive characters to resist extreme drought
conditions. Examples: Opuntia, Aloe, Bryophyllum and Begonia.
iii. Non succulents: These are also called drought resistant
plants ( true xerophytes). They face both external and internal dryness.
They have many adaptations to resist dry conditions. Examples: Casuarina,
Nerium, Zizyphus and Acacia.
Root system is well developed and is greater than that of shoot
Root hairs and root caps are also well developed.
In Xerophytic plants with the leaves
and stem are covered with hairs are called trichophyllous plants .
Example: Cucurbits (Melothria and Mukia )
• Stems are mostly hard and woody. They may be aerial or
• The stems and leaves are covered with wax
coating or covered with dense hairs.
• In some xerophytes all the internodes in the stem are modified
into a fleshy leaf structure called phylloclades (Opuntia) .
• In some of the others single or occasionally two internodes
modified into fleshy green structure called cladode (Asparagus).
In some the petiole is modified into a fleshy leaf like structure
called phyllode (Acacia melanoxylon).
a) A succulent xerophyte:
Phylloclade – opuntia
b) Non succulent: Perennial
c) Cladode of Asparagus
d) Phyllode – Acacia
Leaves are generally leathery and shiny to reflect light and heat.
In some plants like Euphorbia, Acacia, Ziziphus and
Capparis, the stipules are modified into spines.
The entire leaves are modified into spines (Opuntia ) or
reduced to scales (Asparagus).
Presence of multilayered epidermis with heavy cuticle to prevent
water loss due to transpiration.
Hypodermis is well developed with sclerenchymatous tissues.
Sunken shaped stomata are present only in the lower epidermis with
hairs in the sunken pits.
Scotoactive type of stomata found in succulent plants .
Vascular bundles are well developed with several layered bundle
Mesophyll is well differentiated into palisade and spongy
In succulents the stem possesses a water storage region.
Most of the physiological processes are designed to reduce
Life cycle is completed within a short period (Ephemerals).
The plants which are living in moderate conditions (neither too
wet nor too dry) are known as mesophytes. These are common land plants.
Example: Maize and Hibiscus.
Root system is well developed with root caps and root hairs
Stems are generally aerial, stout and highly branched.
Leaves are generally large, broad, thin with different shapes.
Cuticle in aerial parts are moderately developed.
Epidermis is well developed and stomata are generally present on
both the epidermis.
Mesophyll is well differentiated into palisade and spongy
Vascular and mechanical tissues are fairly developed and well differentiated.
All physiological processes are normal.
Temporary wilting takes place at room temperature when there is
Tropophytes are plants which behave as
xerophytes at summer and behave as mesophytes (or) hydrophytes during rainy
Epiphytes are plants which grow perched on other plants
(Supporting plants). They use the supporting plants only as shelter and not for
water or food supply. These epiphytes are commonly seen in tropical rain
forests. Examples: Orchids, Lianas, Hanging Mosses and Money plant.
Root system is extensively developed. These roots may be of two
types. They are Clinging roots and Aerial roots.
Clinging roots fix the epiphytes firmly on the surface
of the supporting objects.
Aerial roots are green coloured roots which may hang downwardly and
absorb moisture from the atmosphere with the help of a spongy tissue called velamen.
Stem of some epiphytes are succulent and develop pseudo bulb or
Generally the leaves are lesser in number and may be fleshy and
Myrmecophily is a common occurrence in the epiphytic vegetation to
prevent the predators.
The fruits and seeds are very small and usually dispersed by wind,
insects and birds.
Multilayered epidermis is present. Inner to the velamen tissue,
the peculiar exodermis layer is present.
Presence of thick cuticle and sunken stomata greatly reduces
Succulent epiphytes contain well developed parenchymatous cells to
Special absorption processes of water by velamen tissue .
There are special type of Halophytic plants which grow on
soils with high concentration of salts. Examples: Rhizophora, Sonneratia
Halophytes are usually found near the sea-shores and Estuaries.
The soils are physically wet but physiologically dry. As plants cannot use salt
water directly they require filtration of salt using physiological processes.
This vegetation is also known as mangrove forest and the plants are
The temperate halophytes are herbaceous but the tropical
halophytes are mostly bushy
In addition to the normal roots, many stilt roots are developed
A special type of negatively geotropic roots called pneumatophores
with pneumathodes to get sufficient aeration are also present.
They are called breathing roots. Example: Avicennia
Presence of thick cuticle on the aerial parts of the plant body
Leaves are thick, entire, succulent and glossy. Some species are aphyllous
Vivipary mode of seed germination is found in halophytes
Epidermal cells of stem is heavy cutinized, almost squarish and
are filled with oil and tannins.
‘Star’ shaped sclereids and ‘H’ shaped heavy thickened spicules
that provide mechanical strength to cortex are present in the stem.
The leaves may be dorsiventral or isobilateral with salt
High osmotic pressure exists in some plants .
Seeds germinate in the fruits of mother plant itself (Vivipary).