Ecological Adaptations in Xerophytes
Xerophytes are group of plants that survive in dry regions. They grow in deserts, dry hilly regions. They adapt themselves to dry and sandy or rocky soils having poor water content and extreme atmospheric conditions. Xerophytes can withstand drought, intense light, extreme temperature and strong wind. Such a habitat is termed as xeric.
Xerophytes are able to live in these environments because they contain special features that help them prevent water loss. Certain xerophytes have waxy covering over their stomata, thus preventing water loss. Some of them have very few stomata, or stomata that open at night. These adaptations limit the loss of water and allow the plant to survive in dry environments.
Some of the xerophytes have the ability to store water. These plants are termed as succulents. This is another adaptation to xeric conditions.
Types of xerophytes
Depending on their ability to withstand drought condition of the soil, following are the different classes of xerophytes:
- Ephemeral annuals (drought escaping): Such xerophytes complete their life cycle before the arrival of dry condition; e.g., Argemone mexicana, Cassia, Solanum xanthocarpum etc.
- Succulents: These are drought resisting xerophytes. Succulents grow in habitats with less or no water but store water whenever available. They are perennial in habit and resist drought by accumulating water in leaves, stems and roots; e.g., Opuntia, Aloe, Cactus etc.
- Perennial non-succulents (drought-resistant): These are drought enduring xerophytes. They grow in habitat with almost no water, they also cannot store water. During drought growth of the plant stops and it takes place only during the brief period of plenty of water supply during rainfall; e.g., Nerium, Calotropis procera, Acacia arabica, etc.
Adaptive features of xerophytes
Xerophytes exhibit some special features for their adaptation to xeric habitat. Following are some of the morphological, anatomical adaptations:
Following are some of the morphological characters of xerophytes.
Root: The roots show the following xerophytic features:
- The root system is well developed.
- The plants usually have a long and stout tap root which branches profusely.
- Presence of root hairs.
- The roots go deep into the soil in search of water.
Shoot: Following are the xerophytic features of shoot.
- The shoot is generally hard and woody.
- It is mostly covered with hairs, wax, and silica etc.
- Stem may also be fleshy and growth remains stunted.
- Some of the plants show modified stems; e.g., in Opuntia, the stem forms leaf-like structure termed as phylloclade. In Asparagus and Ruscus, the leaf-like structure formed by the leaf is termed as cladode.
Leaves: The leaves are modified in such a way that water loss due to transpiration reduces to minimum. Depending on the shape of the leaves and their structure, xerophytes are classified into following four classes:
- Sclerophyllous: Leaves are stiff and hard; e.g., Banksia etc.
- Trichophyllous: Leaves are covered with hairs; e.g., Nerium, Calotropis.
- Microphyllous: Leaves are smaller in size and reduced; e.g. Asparagus, Casuarina, Pinus etc.
- Malacophyllous: Leaves are soft and fleshy; e.g., Begonia, Salicornia etc.
The leaves of xerophytes are generally caducous; e.g., Euphorbia. In some cases, these may even be absent; e.g., Capparis aphylla. In grasses, the leaves roll when the condition becomes dry and hot.
Reproduction: Most of the xerophytes multiply by their perennial organs such as stem. They also reproduce sexually when water becomes available for their growth.
General anatomical characters of xerophytes.
- Presence of thick cutical on leaf and stem epidermis.
- Presence of waxy layer on the epidermis for reflection of light.
- Sunken stomata is present. A thick envelop of hair on epidermis and around sunken stomata. This layer forms an insulating envelope and checks increasing temperature.
- Epidermal cells are radially elongated to absorb necessary amount of light.
- Leaves are small, reduced and may even be absent. If so, palisade and chlorenchyma is present in the cortex of the stem.
- Intercelluar spaces are only a few and also very small.
- Mechanical tissues like collenchyma and sclerenchyma are well developed.
- Presence of water storage tissue.
- Dead tracheids, parenchyma, intercellular spaces, mucilaginous substances are generally present for absorption and retention of sufficient amount of water.
- Vascular tissues are present in large amount.
- In some desert grasses, the leaves roll due to the presence of motor or bulliform cells in the epidermis. These cells are sensitive to changes in turgor and thus, contract when conditions are dry. This results in upward rolling of leaves and cutting of the stomatal contact with external atmosphere. This reduces the rate of transpiration. This feature is termed as hydrochasy.
Following are the anatomical features of different organs of some of the xerophytes:
Anatomy of Casuarina stem:
Transverse section of Casuarina stem shows the following anatomical characters.
- Epidermis: This outer most single layer of cells shows ridges and grooves. A thick cuticle is present in the outer surface of the epidermal cells. Sunken stomata are present in the grooves. These are covered with hairs.
- Cortex: It is divisible into three parts; hypodermis, palisade and parenchyma. Hypodermis is made of a T-shaped group of sclerenchyma in the ridges. A few layers of palisade are present below the sclerenchymatous hypodermis. The remaining part of the cortex consists of parenchyma. A single cortical vascular bundle is present below each of the ridges. This is conjoint, collateral, endarch and open.
- Endodermis: This layer possesses numerous starch grains.
- Pericycle: A sclerenchymatous patch above each vascular bundle represents pericycle.
- Vascular tissues: The vascular bundles are arranged in a ring. The vascular bundles of the inner rings are present below the grooves and they alternate with cortical vascular bundles. These vascular bundles are also conjoint, collateral, endarch and open. The vascular bundles show a small amount of secondary growth.
Xerophytic characters: The transverse section shows the following xerophytic characters :
- Presence of thick cuticle.
- Sclerenchymatous hypodermis.
- Presence of palisade in the cortex.
- Sclerenchymatous cap over the vascular bundles.
- Presence of parenchymatous pith with numerous intercellular spaces.
Anatomy of Nerium leaf:
Transverse section of the leaf shows the following important anatomical characters:
- Epidermis: Upper as well as lower epidermis are multiseriate. These layers possess thick cuticle. Sunken stomata occur in the lower epidermis with dense cluster of hairs.
- Mesophyll: It is differentiated into palisade and spongy parenchyma. Palisade tissues occur near both the epidermis while spongy parenchyma is present in between. Numerous crystals of calcium oxalate are present in the palisade cells. In the midrib, only parenchyma is present instead of palisade and spongy parenchyma.
- Vascular tissues: Vascular bundle present in the midrib is larger than those present in the wings. A parenchymatous bundle sheath surrounds each vascular bundle. Xylem in the vascular bundle directs toward upper epidermis and the phloem towards the lower epidermis.
- Presence of thick cuticle.
- Both upper as well as lower epidermis are multiseriate or multiple.
- Presence of sunken stomata.
- Dense cluster of hairs cover the sunken stomata.
- Numerous crystals of calcium oxalate are present in the palisade cells.
- Well developed vascular tissues.