Environmental Biology Prof.(Dr.) Punam Jeswal Head M.Sc semester IV Botany Department

XEROPHYTES

A xerophyte (Gr. Xeros, dry; phyton, plant) is a plant which is able to survive in an environment with low availability of water or moisture, such as desert, ice or snow covered region. Xerophytes defined as "plants of dry habitats". Xerophytes are plants adopted in order to survive or grow in dry habitats (xeric condition) where the availability of water is very less. Daubenmire (1959) defined xerophytes as "plants which grow on substrate that usually become depleted of water to a depth of at least two decimeters during normal growth season". Xerophytes are plants of relatively dry habitats - dry in soil and most often also climatically. Xerophytes plants live in condition of water scarcity i.e. in xeric condition (habitat). Xeric habitats are of two types - 1) Physically dry habitat :- Water retaining capacity of the soil very low and climate is dry. Water cannot retained like desert or rock surface. 2) Physiologically dry :- Water is present in excess, but not in absorbable conditions or the plants cannot absorb it. e.g. high salt water, high acidic water and high cold water, water in snow form. Adaptation Strategies of Xerophytes :- Xerophytes adopt various features according to their climate, geography and requirements. Xerophytes plants aim at the following adaptation strategies :- 1) Absorb more water from the surrounding. 2) Retain water in their organs. 3) Reduce the water loss by transpiration to minimise. 4) Reduce utilisation of water ( Prevent high consumption of water)

Types Of Xerophytes

On the basis of morphology, physiology and life cycle patterns, xerophytes are classified into three categories :- 1) Ephemeral Annuals ( Drought Evaders or Drought Escapers) * They are also called as 'drought evaders' or 'drought escapers' common in arid zones. * They are annuals, which complete their life cycles within a very short period. * They actually avoid and not withstand dry seasons, and thus escape dryness in external and internal environments. * They survive critical dry periods as seeds or propagules. With the onset of rainfall, the plant seeds germinate, quickly grow to maturity, flower, and set seed, i.e., the entire life cycle is completed before the soil dries out again. * They are usually shorts sized plants in which the flowering and fruiting occur before the next unfavourable season. * The principal morphological adaptation of ephemeral are their small size and large shoot in relation to root system. * Most of these plants are small, roundish, dense shrubs represented by species of Papilionaceae, some inconspicuous Compositae, and some grasses * Examples - Solanum xanthocarpum, Argemone mexicana, Cassia tora etc.

2) Succulents Xerophytes ( Drought Enduring Xerophytes) -

* These plants suffer from dryness in their external environment only. Their succulent, fleshy organs (stems, leaves, roots) serve as water storage organs which accumulate large amount of water during the brief raining reason. * In some xerophytic plants, stem becomes succulent which are called the 'fleshy Xerophytes' as in Opuntia and Euphorbia. * Those succulent xerophytes in which leaves become fleshy; are also known as malacohyllous xerophytes', such as Aloe, Bryophyllum, Agava, Yucca etc.

3) Non- Succulent Perennial ( Drought Resisting Plants) * These are drought resistant plants and called as the true xerophytes. * They possess a number of morphological, anatomical and physiological characteristics, which enable them to withstand critical dry condition. Thus they suffer from dryness both in their internal as well as external environments. * They exhibit rapid root growth and form an extensive root systems to maximize water supply from the soil. The leaf size is greatly reduced and in many xerophytic grasses, leaves roll on to reduce the leaf surface for restricting loss of water due to transpiration. Some of the examples are Calotropis procera, Acacia nelotica Casuarina, Nerium, Saccharum, Pinus and Salvadora.

ECOLOGICAL ADAPTATIONS IN XEROPHYTES

The environmental adaptation or chief characteristics features exhibits by xerophytes to escape drought or to endure recurrent drought can be either morphological , anatomical or physiological . These features are more or less common to all xerophytes which enables them to survive under the prevailing xeric conditions

Morphological Features (Adaptation) Xerophytes exhibits a number of special features in their morphological organs. Root :- Xerophytes develop under water deficient condition and in order to secure water, the root system in xerophytes are well developed and show following characteristics :- * Root system is well developed. Profusely branched . It is very extensive and more elaborate then shoot system in true xerophytes. * Roots of perennial xerophytes reach greater depth to absorb water but some xerophytes have shallow root system especially when water is available in the surface layers * Roots are long, tap roots, with extensive branching spread over wide areas. * Root hairs and root caps are very well developed. Stems :- * Stems are mostly stunted, woody , dry, hard, ridged and covered with thick bark. * In some xerophytes as Saccharum, stem becomes underground, whereas in Opuntia it becomes fleshy, green, leaf-like covered with spines. In Euphorbia also it becomes fleshy and green. * Stems are covered with dense hairs ( Calotropis), coated with wax (Opuntia) or silica (Equisetum). * Stem in some xerophytes are modified into thorns ( Ulex). * Succulent and bulbous xerophytes can store water in their stem. e.g. Cactus and some species of Euphorbia. * In succulent xerophytes the stems are modified into structures like phylloclades, cladophylls or cladodes. a) Phylloclades - Stem modified into flattened leaf like organs (Opuntia) b) Cladophylls - Branches developed in the axil of scale leaves, become metamorphosed to leaf-like structure ( Ruscus) c) Cladophyll - Many axillary branches become modified into small needle like green structures which look exactly like leaves (Asparagus).

Fig :- Stem and Leaf Modified Structures ( Phylloclade, Cladode, Cladophyll and phyllode). Leaves :- Usually leaves of xerophytes are reduced or modified to various kinds of structure to minimise transpiration. * Leaves are usually absent in many xerophytes or they fall very early (caducous). * In majority of the xerophytes the leaves are modified into spines or scales as in Casuarina, Asparagus . Lamina may be long narrow or needle-like as in Pinus or divided into many leaflets as in Acacia. * Foliage leaves, wherever present, may become thick, fleshy and succulent, or tough and leathery in texture. * Leaf may modified into phyllode (Leaf petiole or rachis modified or flattened into leaf like organ) in some plant e.g. Acacia. * leaves of some xerophytes posses reduced leaf blades or pinnate and have very ` dense network of veins. * In some xerophytes especially those growing in high wind blowing area, the under surface of the leaves are covered with thick hairs which protect the stomatal guard cells and check the transpiration. These xerophytes are known as trichophyllous. * Leaf surfaces are shiny glazed to reflect light and heat, e.g. Calotropis. * In some monocots as Ammophila, Poa, and Agropyron, leaves become folded and rolled in such a manner that the sunken stomata become hidden and thus the rate of transpiration is considerably minimised. * In some of them as Euphorbia, Acacia nelotica and Capparis anphylla, stipules become modified into spines. * In some xerophytes, leaves are stiff and hard (sclerophyllous) e.g. Banksia.

A) b)

Fig :- A) Non-succulent Perenials Xerophytes B) Succulent Xerophytes.