Ecological Adaptations of Xerophytes (with PPT)

What are xerophytes?

Ø  Xerophytes (xerophytic plants) are plants growing in dry habitats (xeric conditions) where the availability of water is very less. Xeric habitat are the places where water is NOT present in adequate quantity.  Xerophytes are the characteristic plants of deserts or semi-deserts areas. Xerophytes can also grow in mesophytic conditions. The present post discusses the Morphological, Anatomical and Physiological Adaptations of Xerophytes.

Ø  Xerophytes can tolerate:

$.  Extreme dry condition

$.  Low humidity

$.  High temperature

$.  High wind-flow

Ø  Three types of xeric habitats occurs on the earth:

(1). Physically dry habitat: the water retaining capacity of the soil very low and climate is dry (Example: a desert).

(2). Physiologically dry: water is present in excess, but not in the absorbable conditions or the plants cannot absorb it (Example: high salt water, high acidic water and high cold water, water as snow).

(3). Physically and physiologically dry: water present as mist, plants cannot absorb water from the atmosphere directly. (Example: mountain slopes)

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Examples of Xerophytes:

(1). Ephemeral Xerophytes: – Short lived xerophytes

Examples: Tribulus terrestris, Trianthema monogyna, Carthamus oxyacantha

(2). Succulent Xerophytes: plants with fleshy and succulent parts, two types:

(a). Succulents with fleshy stem: Opuntia, Echinocactus, Euphorbia royleana

(b). Succulents with fleshy leaves: They are also called as Malacophyllous xerophytes: Example: Aloe, Agave, Peperomia,  Haworthia, Bryophyllum, Kalanchoe

(3). Non-succulent xerophytes: Nerium, Cassuarina, Pinus, Calotropis, Ephedra, Equisetum

Adaptations strategies of xerophytes:

o   To absorb as much of water as they can get from the surroundings.

o   To retain water in their organ for very long time.

o   To reduce the water loss by transpiration to minimum.

o   To prevent high consumption of water.

Classification of Xerophytes:

Ø  Xerophytes are classified on the basis of their drought resisting power.

Ø  They are classified as:

(1).  Drought escaping plants

(2).  Drought enduring plants

(3).  Drought resistant plants

(1). Drought Escaping Plants:

Ø  They are short lived plants (ephemerals) and they complete their life cycle within few weeks.

Ø  They survive in the critical dry periods as seeds or propagules.

Ø  They have hard and resistant fruit walls and seed coats for protecting the embryo from extreme dry conditions.

Ø  These plants germinate suddenly in the favourable conditions.

Ø  They are usually short sized plants in which the flowering and fruiting occur before the next unfavourable season.

Ø  Example: Astragalus, Artemesia, Tribulus and most of the grasses.

(2). Drought Enduring Plants:

Ø  They are small sized plants that have the capacity to endure or tolerate drought conditions.

Ø  These plants usually do not show any xerophytic adaptations.

Ø  Most of the individuals in the population will die in the unfavourable season; the surviving ones contribute the next generation.

(3). Drought Resistant Plants

Ø  They are the true xerophytic plants that can resist the drought conditions.

Ø  They develop adaptations to resist the extreme temperature and drought.

Ø  On the basis of water storing capacity, xerophytes are classified into two groups:

(1). Succulent xerophytes: they can store water in their plant body.

(2). Non-succulent xerophytes: also called true xerophytes.

Xeromorphic vs Xeroplastic Characters

      Plants show TWO types of xerophytic characters (adaptations), they are:

(1). Xeromorphic characters:

Ø  Xeromorphic characters are fixed xerophytic characters.

Ø  These characters appear in the xerophytes irrespective of the environmental conditions.

Ø  Example: Sunken stomata in Cycas; Some cactoid Euphorbias.

(2). Xeroplastic characters:

Ø  Xeroplastic characters are induced by droughts conditions in the plants.

Ø  These characters only appear in plants when they are challenged by xeric conditions.

Ø  These characters are not genetically fixed and thus they are not inherited to the next generation.

Xerophytic Adaptations of Plants

Ø  Xerophytic characters shown by plants can be grouped into the following THREE categories:

(1). Morphological adaptations: external adaptations

(2). Anatomical adaptations: internal adaptations

(2). Physiological and Reproductive adaptations

Morphological Adaptations of Xerophytes:

(a). Roots of xerophytes

Ø  Root system is well developed in true xerophytes.

Ø  They are adapted to reach the area where water is available and to absorb water as much as possible”.

Ø  Roots will be profusely branched and more elaborate than their stem.

Ø  Most of the roots in xerophytes are perennial and they survive for many years.

Ø  Roots grow deep into the soil and they can reach a very high depth in the soil.

Ø  Root surface is provided with dense root hairs for water and mineral absorption.

(b). Stem of xerophytes

Ø  Stem woody and hard in some xerophytic plants.

Ø  Stem usually green and photosynthetic.

Ø  Stem is covered with thick cuticle, wax and silica (Example: Equisetum).

Ø  In many plants, the stem is covered with dense hairs (Example: Calotropis).

Ø  Stem modified into thorns in Ulex.

Ø  Succulent and bulbous xerophytes can store water in their stem. Example: Cactus and some species of Euphorbia.

Ø  Stem may be modified into phylloclades, cladophylls or cladodes.

Ø  Phylloclades: Stem modified into flattened leaf-like organs (Muehlenbeckia).

Ø  Cladode: Many axillary branches become modified into small needle like green structures which look exactly like leaves (Asparagus).

Ø  Cladophyll: branches developed in the axil of scale leaves, become metamorphosed to leaf-like structures (Ruscus).

(c). Leaves of xerophytes

Ø  Leaves usually absent in xerophytes.

Ø  If leaves are present, usually they are caducous (fall off easily).

Ø  Most of the cases the leaves are modified into spines or scales (Casuarina).

Ø  Leaf may modify into phyllode in some plants.

Ø  Phyllode: leaf petiole or rachis modified (flattened) into leaf like organ Example: Acacia.

Anatomical Adaptations of Xerophytes:

(a). Roots

Ø  Root hairs are well developed in xerophytes.

Ø  Roots with well-developed root cap.

Ø  In Asparagus, the roots become fleshy and store plenty of water.

Ø  In Calotropis, root cells are with very rigid cell wall.

 (b). Stem

Ø  In succulent xerophytes, the stem possesses water storing regions.

Ø  Epidermis is well developed and with thick walled compactly packed cells.

Ø  Cuticle is very thick and well developed over the epidermis.

Ø  Hypodermis is several layered; often hypodermis will be sclerenchymatous (Casuarina).

Ø  Stomata are present on the stem for gaseous exchange and transpiration.

Ø  Stomata are sunken type and usually situated in pits provided with hairs (Casuarina).

Ø  Vascular tissue is well developed with prominent xylem and phloem components.

Ø  In most of the xerophytes, the bark will be well developed and thick.

Ø  Many oil and resin canals are present in bark.

Ø  Most of the cases, the stem will be photosynthetic and contains chlorenchymatous cells in the outer cortex.

Ø  In the stem of Casuarina, the chlorenchymatous cells are radially elongated and palisade like tissue in appearance.

 (c). Leaf

Ø  Epidermis of the leaf is thick and may be multilayered.

Ø  Thick cuticle present over the outer tangential wall of the epidermal cells.

Ø  In some plants, the epidermal cells can store water.

Ø  In some monocots, some epidermal cells are larger than rest of the cells.

Ø  These cells are called bulliform cells.

Ø  Bulliform cells are motor cells and they assist in leaf rolling to reduce transpiration.

Ø  Hypodermis usually present.

Ø  In Pinus, the hypodermis sclerenchymatous.

Ø  Mesophyll is well developed in xerophytic leaves.

Ø  Many layered palisade tissue present.

Ø  Spongy tissue is less developed in xerophytes with less intercellular spaces.

Ø  Leaves of Aloe have water storing region in the mesophyll.

Ø  Stomata are reduced in numbers and are situated only on the lower sides of the leaves (hypostomatic leaves).

Ø  Stomata are sunken type and usually situated in pits with hairs (Nerium).

Ø  Vascular tissue is well developed with plenty of xylem elements.

Ø  Mechanical tissue well developed in the leaves of xerophytes.

Ø  Transfusion tissue, if present, will be well developed for the lateral conduction.

(3). Physiological adaptations of xerophytes:

Ø  Structural or morphological adaptations of xerophytes are well suited to survive in drought conditions.

Ø  Xerophytic plants are reported to contain pentosan polysaccharides which are reported to offer resistance against drought conditions.

Ø  Many xerophytes show CAM (Crassulacean Acid Metabolism) cycle.

Ø  In CAM plants, the stomata will be closed at day time.

Ø  Stomata open during the night and they absorb enough carbon dioxide for the photosynthesis.

Ø  Absorbed carbon dioxide is converted into malic acid and store in the vacuoles of the cells.

Ø  The malic acid increases the osmotic concentration of cell sap and this enables the closure of stomata in the day time.

Ø  Some enzymes such as catalase and peroxidase are more active in xerophytes.

Ø  Amylase enzyme in xerophytes is more efficient in the hydrolysis of starch than mesophytes.

Ø  Xerophytes can regulate the rate of transpiration.

Ø  They ensure the reduced rate of transpirational loss of water by thick cuticle, distribution of stomata in the lower side of the leaf, sunken type of stomata, and positioning of stomata in pits with many hairs. 

Ø  Xerophytes possess high osmotic concentration of cell sap.

Ø  Thus cells have high osmotic pressure.

Ø  High osmotic pressure increases the turgidity of the cells.

Ø  Turgidity exerts tension force (turgor pressure) on cell wall.

Ø  Due to this high turgor pressure, the wilting of cells is prevented by the extreme heat.

Ø  High osmotic concentration also ensures the rapid and effective absorption of water.

Ø  Tissue of succulents possesses mucilage to hold large amount of water.

Ø  Loss of high proportion of body mass with rapid recovery when water is available.

Ø  Produce brightly coloured, large and showy flowers for attracting pollination agents.

Ø  Cactoid plants produce large amounts of minute seeds.

Ø  Seeds are with thick seed coat for protection.

Ø  Seed surface also possesses mucilage substances to absorb and hold water when it is available.

Ø  Some plants quickly complete their life cycle before the unfavourable conditions.

Ø  Efficient pollination mechanism by moths, bats and birds.

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