Though plants are capable of adapting themselves to a variety of environmental conditions, they usually prepare certain habitats and form definite ecological groups. Eg. Plants growing in water are entirely different from those seen in deserts. Such ecological groups of plants are classified as xerophytes, hydrophytes, mesophytes, halophytes, epiphytes, parasites, and insectivorous plants.
Characteristics of Xerophytes
The plants which grow in dry habitats (xeric conditions) are called xerophytes. Characteristics of xerophytes will depend on the xeric habitats. The xeric conditions may be of the following types.
- Physically dry: Here the water-retaining capacity of the soil is very low and the climate is dry. Eg Deserts, rock surfaces, etc.
- Physiologically dry: Places where water is present in excess amounts but cannot be absorbed by the plants easily. Such habitats may be too salty, acidic, or too cold.
- Physically and physiologically dry: Slopes of mountains have such habitat.
Xerophytes develop special structural and physiological characteristics to help them adapt to dry environment conditions of deserts, the Mediterranean, semi-arid conditions, etc. Adaptations of xerophytes have the following objectives.
- Leaves in some extreme xerophytes absorb maximum water from the surroundings.
- Retain water in their organs for a very long time.
- Reduce transpiration to a bare minimum.
- Check the high consumption of water.
Thus, according to their drought-resistant capacity, xerophytes are classified into three categories.
Drought Escaping Plants (Ephemerals)
Ephemerals are short-lived during critical dry periods. They survive in the form of seeds and fruits which have hard seed coats and resistant pericarp. During favorable conditions, the seeds germinate into small-sized plants which complete their life cycle within a short period. They are called ephemerals.
Ephemerals complete their life cycle and the seeds are formed before the dry condition approaches and remain unaffected by extreme conditions. Eg, Members of the family Gramineae and Solanaceae.
Drought-Enduring Plants
These are small-sized plants that can endure or tolerate drought or dry conditions.
Drought-Resistant Plants
These plants develop certain adaptive features through which they can resist extreme dry conditions. They develop fleshy or succulent parts. In succulent plants, some organs become swollen and fleshy due to the active accumulation of water in them.
In other words, the bulk of the plant body is composed of water-storing tissue. Water stored in these tissues is consumed during the period of extreme drought. Eg. Aloe, Agave, etc. Here, leaves are highly modified to form water-storing organs.
Morphological Adaptations Of Xerophytes
Root Adaptations
Xerophytes have well-developed root systems which may be profusely branched. It is extensive and more elaborate than the shoot system. The roots of perennial xerophytes grow very deep and reach the layers where water is available in plenty. Root hairs are densely developed near the growing tips of the roots. They enable the roots to absorb sufficient quantities of water.
Stem Adaptations
In several plants, the stem is succulent in nature. They develop fleshy parts. Stem and leaves usually contain thin-walled layers of cells that expand due to water absorption during the brief rainy season. Plants like Opuntia, Euphorbia tirucalli, cactus, etc are important fleshy xerophytes.
Their leaves are converted into spines or are reduced in size to decrease transpiration rate. Photosynthetic function is now taken over by the thick green stem. The stem also has a strong cuticle over the epidermis.
In plants like Asparagus, many axillary branches become modified into small needle-like green structures that look exactly like leaves. They are called cladodes, which are definite in growth with no differentiation into nodes and internodes.
When there is a differentiation into nodes and internodes with an indefinite growth, it is called phylloclades. The main function of leaves is taken up by these phylloclades and cladodes which are modified stems or branches.
In Casuarina, the stem shows ridges and furrows. The leaves are highly reduced and scale-like. In plants like Calotropis, the stem contains milky juice or latex which prevents the escape of water to the outside and the plant body is completely covered with hairs.
Adaptations of Leaves
- In xerophytes, if leaves are present, they fall easily in the dry season. Eg. Muehlenbeckia, Euphorbia tirucalli, etc.
- But in the majority of plants, they are reduced to scales as in Asparagus, Casuarina, Ruscus, etc.
- Some evergreen xerophytes have needle-like leaves. Eg Pinus.
Leaf succulence
The leaves swell remarkably and become fleshy owing to the storage of excess amounts of water and latex in them. Plants with succulent leaves generally develop very reduced stems. Eg. Aloe and agave.
In the majority of xerophytes, leaves are generally much reduced. Sometimes they may be reduced to spines. E.g.. Opuntia. In plants like Acacia melanoxylon, the leaflets of the compound leaves are reduced while the main rachis of the leaf expands to form leaf-like structures called phyllodes.
In Parkinsonia, the main rachis and stipules are converted into spines. From the spine like midrachis or primary rachis a secondary rachis develops. They are ribbon-like green structures called phyllodes, on which highly reduced leaflets are arranged.
In some xerophytes, especially those growing in the region exposed to strong wind, the undersurface of leaves is covered with thick hairs which protect the stomatal guard cell and also check transpiration.
Xerophytes that have hairy coverings on their leaves and stems are known as trichophyllus and this phenomenon is called trichophylly. Eg Calotropis, Nerium etc.
Leaves of some xerophytic grasses can roll or fold. In this case, stomata are scattered on the upper surface and leaves roll upwardly. The stomata are effectively shut away from the outside atmosphere. This is the effective modification in these plants for reducing water loss by transpiration. Eg, Sun dew grass.
Anatomical Adaptations Of Xerophytes
Cuticle
Heavy cutinization, lignification, and wax deposition on the surface of the epidermis. The shining smooth surface of the cuticle reflects the rays of light and does not allow them to go deep into plant tissues. Thus it checks the loss of water.
Epidermis
Cells are small and compactly arranged. In Nerium, multiple epidermis are present. Wax, tannin, resin, etc are deposited on the surface of the epidermis. It forms a screen against light intensity.
Certain grasses with rolling leaves have specialized cells called bulliform or motor cells. They are found usually on the upper surface of the leaves. The motor cells facilitate the rolling of leaves.
During dry conditions they become flaccid and in moist conditions these cells regain their normal turgidity which causes the unrolling of the leaf margins. Eg. Grass, sugarcane, bamboo, etc.
Hairs
Epidermal hairs are a common occurrence in xerophytes. They may be simple or compound unicellular or multicellular. These hairs protect stomata and prevent excessive water loss. Eg. Nerium.
In some plants, the surface of the stem develops characteristic ridges and furrows and inside these furrows and pits, stomata are present and hairs protect these stomata from the direct strokes of strong wind. E.g., Casuarina.
The reduction of stomata in xerophytes is of utmost importance for the reduction of transpiration. It is possible only if the stomatal number where the unit area is reduced or the stomata are elaborately modified in their structure. They are generally of the sunken type. Eg Nerium. In some cases, they may be found in furrows or pits. Eg Casuarina.
In dorsiventral leaves, stomata are on the lower surface. However, in rolling leaves, they are scattered mostly on the upper surface. This is a very important device for lowering the rate of transpiration in xerophytes.
Hypodermis
In xerophytes, just below the epidermis one or several layers of thick-walled compact groups of cells are developed that form the hypodermis. These cells may be filled with tannin and mucilage and they usually reduce water loss.
Ground Tissue
In succulent stems and leaves, ground tissues are filled with thin-walled parenchymatous tissue to store excess water, mucilage, latex, etc. This makes the stem swollen and fleshy.
Conducting Tissue
Xerophytes aim to get as much water as possible from soil, hence they develop a very extensive root system. Xylem elements are richly developed to avoid internal resistance to the conduction of water.
Physiological Adaptations
The scarcity of water in soil coupled with the high rate of transpiration makes the xerophytes exhibit physiological specialization. The exposed regions of plants always develop thick cuticles to avoid cuticular transpiration.
The air space of mesophyll is very much reduced to avoid great diffusion and increased transpiration. Sunken stomata and protection of stomata by hairs also regulate transpiration.
The concentration of cell sap is always higher in xerophytes. This increases the absorption of water from the soil. The xerophytic stems are usually fleshy and contain hydrophilic substances which bind water.
Xerophytes do not wilt usually during the dry season because protoplasm retains water in them. The transpiration rate is usually higher. This helps reduce the temperature of leaves and provides a cooling effect.
This is because the escaping water molecules take the temperature from their surroundings. This increase in transpiration also gives great suction force as a result of which conduction and absorption of water increase. Many anatomical characteristics like hairs, sunken stomata, and rolling up of leaves tend to maintain high humidity conditions at the leaf surface.
Conclusion
Thus it can be seen that a peculiar habitat to which xerophyte plants are exposed, has a profound influence on their morphological, anatomical, and physiological characteristics of plants. However, it is found that there is no specific adaptation that is common to all plants of the xeric habitat.
References
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell biology genetics molecular biology evolution and ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
