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

Xerophytes:- any plant adapted to life in a dry or physiologically dry habitat (salt marsh, saline soil, or acid bog) by means of mechanisms to prevent water loss or to store available water.

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.

Types of xerophytes:-

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.

Morphological adaptations:-

i. The root system is very well developed with root hairs and root caps. Eg. Calotropis.

ii. The roots are fasciculated as in Asparagus.

iii. Stems are stunted, woody, dry, hard, ridged, and covered with thick bark, may be underground, Eg. iv. Stem is covered with thick coating of wax and silica in Equisetum or dense hairs as in Calotropis.

v. Stems may be modified into a thorn e.g. Ulex or cladodes Eg. Asparagus.

vi. Leaves are very much reduced, small scale-like, appearing only for a brief period (Caducous) sometimes modified into spines or scales as in Casuarina, Ruscus, Asparagus.

vii. Lamina may be narrow or needle like as in Pinus or divided into many leaflets as in Acacia or succulents as in Aloe.

viii. In Euphorbia and Zizyphus jujuba stipules become modified into spines.

ix. Xerophytes like Calotropis have hairy covering on the leaves and stems to check transpiration.

Anatomical adaptations:-

i. Root hairs and root caps are well developed in Opuntia.

ii. Roots may become fleshy to store water as in Asparagus.

iii. In succulent xerophytes, stems possess a water storage region (thin walled parenchyma cells).

iv. Stems of non-succulent xerophytes show a very thick cuticle, well developed epidermis with thickened cell wall, several layered and sclerenchymatous hypodermis Eg. Casuarina.

v. The stems have sunken stomata and well developed vascular and mechanical tissues.

vi. Leaves show well developed cuticle, succulent leaves in Aloe, multilayered epidermis in Nerium, sclerenchymatous and several layered hypodermis in Pinus, bulliform cells in Sugarcane.

vii. Mesophyll is well differentiated and vascular tissues and mechanical tissues are well developed.

Physiological adaptations:-

i. The stomata of these plants open during night hours and remain closed during the day. This unusual feature is associated with metabolic activities of these plants.

ii. In xerophytes, the chemical compounds of cell sap are converted into wall forming compounds (eg) Cellulose, Suberin etc.

iii. Some enzymes, such as catalases, perioxidases are more active in xerophytes than in mesophytes.

iv. The capacity of xerophytes to survive in long period of drought is due to the resistance of the hardened protoplasm to heat and desiccation.

v. The Xerophytes have very high osmotic pressure, which increases the turgidity of the cell sap.