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 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 rainfall.
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.
These are also called drought escapers 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.
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 system.• 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 )
a) A succulent xerophyte: Phylloclade – opuntia
b) Non succulent: Perennial – Capparis
c) Cladode of Asparagus
d) Phyllode – Acacia
• Stems are mostly hard and woody. They may be aerial or underground
• 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).
• 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). Anatomical adaptations
• Presence of multilayered epidermis with heavy cuticle to prevent water loss due to transpiration.
• Hypodermis is well developed with sclerenchymatous tissues.
• Sunken 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 sheath.
• Mesophyll is well differentiated into palisade and spongy parenchyma.
• In succulents the stem possesses a water storage region.
Nerium Leaf –
A succulent leaf of
• Most of the physiological processes are designed to reduce transpiration.
• Life cycle is completed within a short period (Ephemerals).