Plant Adaptations: Introduction and Ecological Classification of Plants

The living organisms react with their environments and they bear full impression of the environments in which they grow.

In order to withstand adverse conditions of the environment and utilize to their maximum benefit the nutrients and other conditions prevailing therein, the organisms develop certain morphological, anatomical, physiological and reproductive features.

Any feature of an organism or its part which enables it to exist under conditions of its habitat is called adaptation. Every organism develops certain adaptations and so does the population or a community. The completion of life cycle of an organism or stabilization of a community results through a series of adaptations which have survival value.

Adaptations of survival value comprise such features as prevent destruction of vital vegetative tissues and help in large production and efficient dissemination of reproductive bodies. Warming (1895) had realized for the first time the influence of controlling or limiting factors upon the vegetation in ecology. He classified plants into several ecological groups on the basis of their requirements of water and also on the basis of nature of substratum on which they grow.

Warming classified plants on the basis of nature of substratum (soil) into the following groups.

(1) Plants of acidic soil (Oxylophytes)

(2) Plants of saline soil (Halophytes)

(3) Plants growing on the sand (Psammophytes)

(4) Plants growing on the surface of rocks (Lithophytes)

(5) Plants growing in the crevices of rocks (Chasmophytes).

Epiphytes are not included in the above classification because of the fact that they do not have permanent connection with the soil. Warming’s second classification (1909) of the plants is based on their water relations. The supply of water to the plants and regulation of transpiration are the factors that evoke great differences in plant forms and plant life.

On the basis of their water requirement and nature of soils, the plants have been classified as follows:

1. Hydrophytes:

Plants growing in or near water.

2. Xerophytes:

Plants adapted to survive under the condition of very poor supply of available water in the habitats.

Xerophilous plants are further classified on the basis of their habitats as follows:

(i) Oxylophytes (on acid soils)

(ii) Halophytes (on saline soils)

(iii) Lithophytes (on rocks)

(iv) Psammophytes (on sand and gravels)

(v) Chersophytes (on waste land)

(vi) Eremophytes (on deserts and steppes)

(vii) Psychrophytes (on cold soils)

(viii) Psilophytes (savannah)

(ix) Sclerophytes (Forest and bushland)

3. Mesophytes:

Plants growing in an environment which is neither very dry nor very wet. The detailed description of only some important ecological groups is given here.

Hydrophytes:

(Greek, Hudor = water and Phyton = Plant; water plant):

Plants which grow in wet places or in water either partly or wholly submerged are called hydrophytes or aquatic plants. Examples are Utricularia. Vallisneria, Hydrilla Chara Nitella Lotus, Ceratophyllum, Trapa, Pistia, Eichhornia (water hyacinth), Wolffia, Lemna, etc Aquatic environment provides a matrix for plant growth in which temperature fluctuation is at minimum and the nutrients occur mostly in dissolved state but light and oxygen become deficient with the increase m depth of water bodies. Zonation of aquatic vegetation with increasing depth is a device for maximum utilization of light energy.

The aquatic environment is subject to water movements ranging from small vertical circulation to strong currents. Streams have a unidirectional movement and m seas the movement is reversible. The currents of water often abrade the inhabiting flora and varied modifications are encountered to withstand this abrasive action. Since water makes up a large proportion of the bodies of plants and animals (70 to 90% water in protoplasm), it affects all life processes directly.

In plants, the rate and magnitude of the photosynthesis, respiration absorption of nutrients, growth and other metabolic processes are influenced by the amount of available water. Low relative humidity increases water loss through transpiration and affects plant growth. Conversely, plants in the regions with high moisture show reduced transpiration.

Some aquatic groups of higher plants probably originated from mesophytes. In the course of evolution several changes m the physiology, morphology and behaviour, all related to the aquatic mode of life, took place and by these evolutionary changes the mesophytic plants have become adapted to aquatic mode of life.

Classification of Hydrophytes:

According to their relation to water and air, the hydrophytes are grouped into the following categories:

(a) Submerged hydrophytes

(b) Floating hydrophytes

(c) Amphibious hydrophytes.

(a) Submerged hydrophytes:

Plants which grow below the water surface and are not in contact with atmosphere are called submerged hydrophytes. Such plants may be free-floating (Fig 8.1) or rooted (Fig. 8.2). Example Vallisneria, Hydrilla, Potamogeton, Najas. Ceratophyllum Mynophyllum, Utricularia, Chara, Nitella and a number of aquatic microbes.

(b) Floating hydrophytes:

Plants that float on the surface or slightly below the surface of water are called floating hydrophytes. These plants are in contact with both water and air. They may or may not be rooted in the soil. On this ground, the floating plants have been divided into two groups.

(i) Free floating hydrophytes:

These plants float freely on the surface of water but are not rooted in the mud. Examples-wolffia arhiza and Wolffia microscopica (a rootless minutes duck weed). Trapa bispinosa, Lymnanthemum. Eichhornia crassipes (water hyacmth, verna—Jalkumbhi), Salvinia (a fern), Azolla (a water fern) (Fig. 8.6).

(ii) Floating but rooted hydrophytes:

Some submerged plants are rooted in muddy substrata of Ponds Rivers and lakes but their leaves and flowering shoots float on or above the surface of water. They are grouped as floating but rooted hydrophytes. Nelumbium speciosum (Lotus), Victoria regia (water lily), Ceratopteris thalictroides (a hydrophytic fern of family Parkariaceae), etc. (Fig. 8.5).

(c) Amphibious hydrophytes:

These plants are adapted to both aquatic and terrestrial modes of life. Amphibious plants grow either in shallow water or on the muddy substratum^ Amphibious plants which grow in saline marshy places are termed as ‘halophytes. Roots and some parts of stems and leaves in these plants may be submerged in water or buried m mud but some foliage, branches and flowering shoots spring well above the surface of water or they may spread over the land (Fig. 8.3).

The aerial parts of these amphibious plants show mesophytic or sometimes xerophytic features, while the submerged parts develop true hydrophytic characters. Some varieties of rice plants, (Oryza sativa), Marsilea, Sagittaria. Alisma, Jussiaea. Neptuma, Commelina, Polygonum, Ranunculus aquatilis, Phragmites. Enhydra fluctuans, etc. are familiar examples of this group of hydrophytes. In some amphibious plants the shoots are completely exposed to air as m land plants but the roots are buried in water lodged soil or mud. They are called marsh plants. The common examples of marsh plants are Cyperus, Typha, Scirpus, Rumex, etc. (Fig. 8.4).

These are listed below:

1. Temperature of water

2. Osmotic concentration of water

3. Toxicity of water

The osmotic concentration and toxicity are dependent upon the amount and nature of chemical substances dissolved in water. The physiology of aquatic plants is greatly affected by the change in osmotic concentration of water. The aquatic plants are subjected to less extremes of temperature because water is bad conductor of heat (i.e., it takes long time m its heating and cooling). Hydrophytes are less affected as the transpiration from the plant tissue is completely out of question.

Hydrophytic Adaptations:

As the aquatic environment is uniform throughout, the hydrophytes develop very few adaptive features.

Important features of these plants are described in the following heads:

A. Morphological:

(i) Roots:

Root systems in hydrophytes are poorly developed which may or may not be branched in submerged hydrophytes. Roots are meaningless as body which is in direct contact with water acts as absorptive surface and absorbs water and minerals. This may probably be the reason why roots in hydrophytes are reduced or absent. Roots of floating hydrophytes show very poor development of root hairs.

Roots in floating plants do not possess true root caps but very often they develop root pockets or root sheaths which protect their tips from injuries (Fig. 8.5). Exact functions of these root pockets, however, are not fully understood. Some rooted hydrophytes like Hydrilla (Fig. 8.6), Valhsnena sptrahs, Elodta canadensis, though they derive their nourishments from water by their body surfaces, are partly dependent on their roots for minerals from the soil.

Roots are totally absent in some plants, e g., Ceratophyllum, Salvinia, Azolla, Utricularia, etc. In Jussiaea repens two types of roots develop when the plants grow on the surface of water, some of them are floating roots which are negatively geotropic having spongy structures (Fig. 8.7). The floating roots keep the plants afloat.

(ii) Stem:

In aquatic plants, stem is very delicate and green or yellow in colour (Fig. 8.2 A, B). In some cases it may be modified into rhizome or runner, etc. (Fig. 8.2 D, 8.4).

(iii) Leaves:

(a) In floating plants leaves are generally peltate, long, circular, light or dark green in colour, thin and very smooth. Their upper surfaces are exposed in the air but lower Les are generally in touch with water. In lotus plant petioles of leaves show indefinite power of growth and they keep the laminae of leaves always on the surface of water.

(b) Heterophylly:

Some aquatic plants develop two different types of leaves in them. This phenomenon is termed as heterophylly. Examples are Sagittaria sagittaefolia, Ranunculus aquatilis, Limnophila heterophylla, Salvinia, Azolla etc. In this phenomenon, generally the submerged leaves are linear ribbon shaped or highly dissected and the leaves that are found floating on or above the surface of water are broad circular or slightly lobed (Fig. 8.3 A, B, C). The occurrence heterophylly is associated probably with the following characteristic physiological behaviours of these aquatic plants.

1. Quantitative reduction in transpiration.

2. The broad leaves on the surface overshadow the submerged dissected leaves of the same plant and thus they reduce the intensity of light falling on the submerged leaves. The submerged leaves require light of very low intensity.

3. Plants show very little response to drought because the necessity of excess water during drought period is compensated by submerged leaves which act as water absorbing organs.

4. Variation in the life-forms and habitats.

5. Broad leaves found on the surface of water transpire actively and regulate the hydrostatic pressure in the plant body.

(c) Leaves of free floating hydrophytes are smooth, shining and frequently coated with wax. The wax coating protects the leaves from chemical and physical injuries and also prevents the water clogging of stomata.

(d) In floating plants of water hyacinth, Trapa etc., the petioles become characteristically swollen and develop spongyness which provides buoyancy to these plants (Fig. 8.6).

(e) Leaves in submerged hydrophytes are generally small and narrow. In some case, e.g., Myriophyllum, Utricularia, Ceratophyllum, etc., they may be finely dissected (Fig. 8.2). The mall slender terete segments of dissected leaves offer little resistance against the water currents. In this way plants are subjected to little mechanical stress and strain of water.

(f) In the Amphibious plants, the leaves that are exposed to air show typical mesophytic features. They are more tough than the leaves of other groups of hydrophytes.

(g) Pollination and dispersal of fruits and seeds are accomplished by the agency of water. Seeds and fruits are light in weight and thus they can easily float on the surface of water.

(h) Vegetative reproduction is common method of propagation in hydrophytes. It is accomplished either through fragmentation of ordinary shoots or by winter buds. In algae, reproduction is accomplished by zoospores and other specialized motile or non-motile spores.

B. Anatomical Modifications:

The anatomical modifications in hydrophytes aim mainly at:

1. Reduction in protecting structures,

2. Increase in the aeration,

3. Reduction of supporting or mechanical tissues, and

4. Reduction of vascular tissues.

Various anatomical adaptations of hydrophytes are listed below:

1. Reduction in protecting structures:

(a) Cuticle is totally absent in the submerged parts of the plants. It may be present in the form of very fine film on the surfaces of parts which exposed to atmosphere.

(b) Epidermis in hydrophytes is not a protecting layer but it absorbs water, minerals and gases directly from the aquatic environment. Extremely thin cellulose walls of epidermal cells facilitate the absorption process.

(c) Epidermal cells contain chloroplasts, thus they can function as photosynthetic tissue, especially where the leaves and stems are very thin, e.g. Hydrilla (Fig. 8.8).

(d) Hypodermis in hydrophytes is poorly developed. Its cells are extremely thin walled.

2. Increase in the aeration:

(a) Stomata are totally absent in submerged parts of the plants (Fig. 8.9, 8.10 C & D). In some exceptional cases, vestigial and functionless stomata have been noticed. In these cases exchange of gases takes place directly through cell walls. In the floating leaves, stomata develop in very limited number and are confined only to the upper surface (Fig. 8.10 A). In amphibious plants stomata may be scattered on all the aerial parts and they develop comparatively in larger number per unit area than those on the floating leaves (Fig. 8.11).

(b) Air chambers:

Aerenchyma in submerged leaves and stem is very much developed. Air chambers are filled with respiratory gases and moisture. These cavities are separated from one another by one or two cells thick chlorenchymatous partitions. The different types of air chambers are shown in Figs. 8.8, 8.9 A, and 8.10. CO₂ present in the air chambers is used in the photosynthesis and the O₂ produced in the process of photosynthesis and also that already present in the air chambers is used in respiration.

The air chambers also develop finely perforated cross septa which are called diaphragms (Fig. 8.12). The diaphragms afford better aeration and perhaps check floating. The Aerenchyma provides buoyancy and mechanical support to aquatic plants. Air chambers are abundantly found in the fruits of hydrophytes rendering them buoyant and thus facilitating their dispersal by water.

Development of air chambers in the plants is governed by habitat. This point is clear from the anatomy of Jussiaea suffructicosa. In this case, air chambers develop normally if plants are growing in water but they seldom develop if the plants are growing on the land.

3. Reduction of supporting or mechanical tissues:

(a) Mechanical tissues are absent or poorly developed in the floating and submerged parts of plants because buoyant nature of water saves them from physical injuries. The thick walled sclerenchymatous tissue is totally absent m submerged and floating hydrophytes. They may, however, develop in the cortex of amphibious plants particularly in the aerial or terrestrial parts (Figs. 8.13, 8.14 B, D). Generally elongated and loosely arranged spongy cells are found in the plant body. These thin-walled cells, when turgid, provide mechanical support to the plants (Figs. 8.15, 8.16 and 8.17).

(a) The reduction of absorbing tissue (roots act chiefly as anchors and root hairs are lacking).

(b) In water lily and some other plants, special type of star shaped lignified cells, called asterosclereids, develop which give mechanical support to the plants (Fig. 8.10 A, B).

4. Reduction of vascular tissues:

Conducting tissue is very poorly developed. As the absorption of water and nutrients takes place through the entire surface of submerged parts, there is little need of vascular tissues in these plants. In the vascular tissues, xylem shows greatest reduction. In some cases, it consists of only a few tracheids while in some, xylem elements are not at all developed (Fig. 8.18). Some aquatic plants, however, show a lacuna in the centre in the place of xylem. Such spaces resemble typical air chambers (Fig. 8.8).

Phloem tissue is also poorly defined in most of the aquatic plants but in some cases it may develop fairly well. Sieve tubes of aquatic plants are smaller than those of mesophytes. Phloem parenchyma is extensively developed. Endodermis may or may not be clearly defined. The Vascular bundles are generally aggregated towards the centre. Secondary growth in thickness does not take place in the aquatic stem and roots.

Distinctive features of different groups of hydrophytes are summarized in the following chart.

Physiological adaptations in hydrophytes:

The aquatic plants exhibit a low compensation point and low osmotic concentration of cell sap. Osmotic concentration of cell sap is equal or slightly higher than that of water. Nutrients are absorbed by the submerged plants through the general plant surface. The gases are exchange from the water through the surface cells.

The gases produced during photosynthesis and respiration are partly retained in the air chambers of aerenchyma to be utilized as and when required. There is no transpiration from the submerged hydrophytes. However emergent plants and free floating hydrophytes have excessive rate of transpiration. Mucilage cells and mucilage canals secrete mucilage to protect the plant body from decay under water.

Xerophytes:

Plants which grow in dry habitats or xeric conditions are called xerophytes. Places where available water is not present adequate quantity are termed xeric habitats.

Xeric habitats may be of following types:

1. Habitats physically dry (where water retaining capacity of the soil is very low and the climate is dry, e.g., desert, rock surface, waste land, etc.).

2. Habitats physiologically dry (places where water is present in excess amount but it is not such as can be absorbed by the plants easily. Such habitats may be either too salty or too acidic, too hot or too cold).

3. Habitats dry physically as well as physiologically, e.g., slopes of mountains.

Xerophytes are characteristic plants of desert and semi-desert regions, yet they can grow in mesophytic conditions where available water is in sufficient quantity. These plants can withstand extreme dry conditions, low humidity and high temperature.

When growing under un-favourable conditions, these plants develop special structural and physiological characteristics which aim mainly at the following objectives:

(i) To absorb as much water as they can get from the surroundings;

(ii) To retain water in their organs for very long time;

(iii) To reduce the transpiration rate to minimum; and

(iv) To check high consumption of water

Xerophytes are categorized into several groups according to their drought resisting power. These groups are as follows:

1. Drought escaping plants:

These xerophytes are short-lived. During critical dry periods they survive m the form of seeds and fruits which have hard and resistant seed-coats and pericarps respectively. At the advent of favourable conditions (which are of very short duration), the seeds germinate into new small sized plants which complete their life cycles within a few weeks time. The seeds become mature before the dry condition approaches.

In this way, plants remain unaffected by extreme conditions. These are called ephemerals or drought evaders or drought escapers. These plants are very common in the semiarid zones where rainy season is of short duration. Examples—(Papilionatae), some inconspicuous compositae (e.g., Artemesid) and members of families Zygophyllaceae, Boraginaceae, some grasses etc.

2. Drought enduring plants:

These are small sized plants which have capacity to endure or tolerate drought.

3. Drought resistant plants:

These plants develop certain adaptive features in them through which they can resist extreme droughts. Xerophytes grow on a variety of habitats. Some grow on rocky soils (Lithophytes) some in deserts, some on the sand and gravels (Psammophytes) and some may grow on the waste lands (Eremophytes). Some plants of xeric habitat have water storing fishy organs, while some do not develop such structures.

On this ground xerophytes can be divided into two groups which are as follows:

(1) Succulent xerophytes.

(2) Non-succulents, also called true xerophytes.

Succulent xerophytes are those plants in which some organs become swollen and fleshy due to active accumulation of water in them or in other words, the bulk of the plant body is composed of water storing tissues. Water stored in these tissues is consumed during the period of extreme drought when the soil becomes depleted of available water.

Xerophytic Adaptations:

Plants growing in the dry habitats develop certain structural devices in them. These structure modifications in xerophytic plants may be of two types.

(i) Xeromorphic characters:

Xerophytic characters that are genetically fixed and inherited are referred to as xeromorphic. They will appear in the xerophytes irrespective of conditions whether they are growing in deserts or in humid regions. Halophytic mangroves and many other evergreen trees, although growing in moist conditions always develop xeromorphic characters.

(ii) Xeroplastic characters:

These features are induced by drought and are always associated with dry conditions. They are never inherited. These characters may disappear from plants if all the favourable conditions are made available to them.

Important xerophytic features are summarized under the following heads:

(1) Morphological (external) adaptations;

(2) Anatomical (internal) adaptations;

(3) Physiological adaptations.

1. External Morphology of Xerophytes:

(A) Roots:

Xerophytes have well developed root systems which may be profusely branched. It is extensive and more elaborate than shoot system. Many desert plants develop superficial root system where the supply of water is restricted to surface layer of the earth. The roots of perennial xerophytes grow very deep in the earth and reach the layers where water is available in plenty. Root hairs are densely developed near the growing tips of the rootlets. These enable the roots to absorb sufficient quantity of water.

(B) Stem:

Some of the important characteristics of xerophytic stems are listed below:

(i) Stems of some xerophytes become very hard and woody. It may be either aerial or subterranean.

(ii) They are covered with thick coating of wax and silica as in Equisetum. Some may be covered with dense hairs as is Calotropis.

(iii) In some xerophytes, stems may be modified into thorns, e.g., Duranta, Ulex, etc. (Fig. 8.19).

(iv) In stem succulents, main stem itself becomes bulbous and fleshy and it seems as if leaves in these plants are arising directly from the top of the roots. Example—Kleinia articulata.

(v) Stems in some extreme xerophytes are modified into leaf-like flattened, green and fleshy structures which are termed as phylloclades. Many cacti (Fig. 8.23A, B) and cocoloba (Muehlenbeckia) (Fig. 8.20 A) are familiar examples for this. In Ruscus plants, the branches developing in the axils of scaly leaves become metamorphosed into leaf-like structures, the phylloclades or cladophylls (Fig. 8.20 B).

In Asparagus plant (Fig. 8.20 C) also a number of axillary branches become modified into small needle-like green structures which look exactly like leaves. They are called cladodes. A number of species of Euphorbia also develop succulence and become green. In these plants, leaves are greatly reduced, so the main function of leaves, the photosynthesis, is taken up by these green phylloclades or cladodes which are modified stems.

(C) Leaves:

(i) In some xerophytes the leaves, if present, are greatly caducous, i.e., they fall early in the season, but in the majority of the plants leaves are generally reduced to scales, as in Casuarina (Fig. 8.21), Ruscus (Fig. 8.20 B), Asparagus (Fig. 8.20 C), etc.

(ii) Some evergreen xerophytes have needle-shaped leaves, e.g., Pinus (Fig. 8.22 A, B).

(iii) In leaf succulents, the leaves swell remarkably and become very fleshy owing to storage of excess amount of water and latex in them. Plants with succulent leaves generally develop very reduced stems. Examples of leaf succulents are Sedum acre, Aloe spinossissima (Gheekwar) (Fig. 8.23 C), Mesembryanthemum, Kleinia ficoides and several members of family Chaenopodiaceae.

(iv) In majority of xerophytes, leaves are generally much reduced and are provided with thick cuticle and dense coating of wax or silica. Sometimes they may be reduced to spines, as for example, in Ulex, Opuntia, Euphorbia splendens (Fig. 8.23 A, B), Capparis (Fig. 8.24 B) and Acacia (Fig. 8.24D).

(v) Generally, the leaves of xerophytic species possess reduced leaf blades or pinnae and have very dense network of veins. In Australian species of Acacia (Babool) the pinnae are shed from the rachis and the green petiole swells and becomes flattened taking the shape of leaf. This modified petiole is termed as phyllode (Fig. 8.25). The phyllode greatly reduces the water loss, stores excess amount of water and performs photosynthesis.

(vi) Trichophylly. In some xerophytes especially those growing well exposed to strong wind, the under surfaces of the leaves are covered with thick hairs which protect the stomatal guard cells and also check the transpiration. Those xerophytes which have hairy covering on the leaves and stems are known as trichophyllous plants. Zizyphus (Fig. 8.24 C), Nerium, Calotropis procera (Fig. 8.24 A) are important examples.

(vii) Rolling of leaves. Leaves in some extreme xerophytic grasses have capacity for rolling or folding. In these cases stomata are scattered only on the upper or ventral surface and as the leaves roll upwardly, stomata are effectively shut away from the outside atmosphere. This is effective modification in these plants for reducing the water loss. Sun-dune grass is an important example for this (Fig. 8.27).

(D) Flowers, fruits and seeds. Flowers usually develop in the favourable conditions. Fruits and seeds are protected by very hard shells or coatings.

2. Anatomical Modifications in the Xeropliytes:

A number of modifications develop internally in the xeric plants and all aim principally at water economy.

The following are the anatomical peculiarities met within xerophytes:

(i) Heavy:

Heavy cutinisation, lignification’s and wax deposition on the surface of epidermis (Fig. 8.26) and even in the hypodermis are very common in xerophytes. Some plants secrete wax in small quantity but some are regular source of commercial wax. Shining smooth surface of cuticle reflects the rays of light and does not allow them to go deep into the plant tissues. Thus, it checks the heavy loss of water.

(ii) Epidermis:

Cells are small and compact. It is single layered, but multiple epidermis is not uncommon. In Nerium leaf, epidermis is two or three layered (Fig. 8.26). In stems, the epidermal cells are radially elongated. Wax, tannin, resin, cellulose, etc., deposited on the surface of epidermis form screen against high intensity of light. This further reduces the evaporation of water from the surface of plant body. Certain grasses with rolling leaves have specialized epidermis (Figs. 8.27, 8.28).

In these, some of the epidermal cells that are found in the depressions become more enlarged than those found in the ridges. These enlarged cells are thin walled and are called bulliform cells or motor cells or hinge cells. These are found usually on the upper surface of leaves between two parallel running vascular bundles.

The highly specialized motor cells facilitate the rolling of leaves by becoming flaccid during dry periods. In moist conditions these cells regain their normal turgidity which causes unrolling of the leaf margins. Bulliform cells are of common occurrence in the leaf epidermis of sugarcane, bamboo, Typha and a number of other grasses.

(iii) Hairs:

Hairs are epidermal in origin. They may be simple or compound, uni- or multicellular. Compound hairs are branched at the nodes. These hairs protect the stomata and prevent excessive water loss. In some plants, surfaces of stems and leaves develop characteristic ridges and furrows or pits. The furrows and pits in these plants are the common sites of stomata. Hairs found in these depressions protect the stomata from the direct strokes of strong wind (Figs. 8.29, 8.30).

(iv) Stomata:

In xerophytes, reduction of transpiration is of utmost importance. It is possible only if the stomatal number per unit area is reduced or if the stomata are elaborately modified in their structures. In xerophytes, number of stomata per unit area of leaf is greater than in mesophytes. They are generally of sunken type. In some cases, they may be found in the furrows or pits.

Subsidiary cells of sunken stoma may be of such shapes and arrangement that they form an outer chamber that is connected by narrow opening or the stoma. Such type of specialized stomata are very common in conifers, Cycas, Equisetum, etc. (Fig. 8.31). Walls of the guard cells and subsidiary cells are heavily cutinized and lignified in many xeric plants.

These devices have little value in directly reducing transpiration when stomata are open. When the plants are wilting and stomata are closed then only lignified or cuticularized walls of guard cells have protecting properties and under such circumstances only cuticular transpiration is possible which is of little significance.

In dorsiventral leaves stomata are generally found on the lower surface, but m rolling leaves they are scattered mostly on the upper surface. In the rolled leaves, stomata are protected from the direct contact of outside wind. This is very important rather secured device for lowering the rate of transpiration in xerophytic grasses.

(v) Hypodermis:

In xerophytes, just below the epidermis, one or several layers of thick walled compactly grouped cells may develop that form the hypodermis. The cells may be much like those of epidermis and may either be derived from epidermis or from the cortex (m case of stem) or from the mesophyll (in case of leaf). The hypodermal cells may sometimes be filled with tannin and mucilage.

(vi) Ground tissue:

(a) In the stem, a great part of body is formed of sclerenchyma. In those cases, where the leaves are either greatly reduced or they fall in the early season, the photosynthetic activity is taken up by outer chlorenchymatous cortex (Fig. 8.32). The chlorenchymatous tissue is connected with the outside atmosphere through stomata. The gaseous exchange takes place in regular manner in the green part of stem.

(b) In succulent stems and leaves, ground tissues are filled with thin walled parenchymatous cells which store excess quantity of water, mucilage, latex, etc. This makes the stems swollen and fleshy (Figs. 8.33, 8.34).

(c) In the leaves, mesophyll is very compact and the intercellular spaces are greatly reduced. Palisade tissue develops in several layers. There are some xerophytes in which mesophyll is surrounded by thick hypodermal sheath of sclerenchyma from all the sides except from below. This sheath forms a diaphragm against intense light. Such xerophytes in which sclerenchyma is extensively developed are called sclerophyllous plants. In succulent leaves, spongy parenchyma develops extensively which stores water (Figs. 8.33, 8.34). In Pinus, the spongy cells of mesophylls are star shaped (Fig. 8.36).

(d) Intercellular spaces are greatly reduced. Cells in the body are generally very small, thick walled and compactly grouped. They may be spherical, rounded or cuboid m shape. Such cells are very common in xerophytes. (Fig. 8.35).

(vii) Conducting tissues: Conducting tissues, i.e., xylem and phloem, develop very well in the xerophytic body.

Physiological Adaptations:

It was long assumed that the structural adaptations in the body of xerophytes were useful in reducing the transpiration but now a number of experiments related with the physiology of these plants reveal some facts which are contrary to the early assumptions. Works of Maximov support that except succulents, true xerophytes show very high rate of transpiration. Under similar conditions, the rate of transpiration per unit area in xerophytes is much higher than that in mesophyte. Stomatal frequency per unit area of leaf surface in xerophytes is also greater than that in the mesophytic leaf.

(1) Succulents are well known to contain polysaccharides, pentosans and a number of acids by virtue of which they are able to resist drought. The structural modifications in these succulent xerophytes are directly governed by their physiology. How does the succulence develop? Metabolic reaction which induces development of succulence is the conversion of polysaccharides into pentosans. Pentosans have water binding property. These pentosans together with nitrogenous compounds of the cytoplasm cause accumulation of excess amount of water in the cells and consequently the succulence develops.

(2) Another experimented fact in the physiology of succulent plants is that their stomata open during night hours and remain closed during the day. This unusual feature is associated with metabolic activity of these plants. In dark, these plants respire and produce acids. The heavy accumulation of acids in the guard cells increases osmotic concentration which, m turn, causes inward flow of water in the guard cells. When guard cells become turgid the stomata open. In the sunlight, acids dissociate to produce carbon dioxide which is used up in the photosynthesis and as a result of this osmotic concentration of cell sap decreases which ultimately causes closure of stomata.

(3) In xerophytes, the chemical compounds of cell sap are actively converted into wall forming compounds that are finally incorporated into the cell walls. Conversions of polysaccharides into anhydrous forms as cellulose, formation of suberin, etc., are some examples.

(4) Some enzymes, such as catalases, peroxidases, are more active in xerophytes than in mesophytes. In xerophytes, amylase enzyme hydrolyses the starch very actively.

(5) The capacity of xerophytes to survive during period of drought lies not only in the structural features but also in the resistance of the hardened protoplasm to heat and desiccation.

(6) Regulation of transpiration. Presence of the cuticle, polished surface, compact cells and sunken stomata protected by stomatal hairs regulate the transpiration.

(7) High osmotic pressure of cell sap. The xerophytes have very high osmotic pressure which increases the turgidity. The turgidity of cell sap exerts tension force on the cell walls. In this way, wilting of cell is prevented. High osmotic pressure of cell sap also affects the absorption of water.

Mesophytes:

Mesophytes are common land plants which grow in situations that are neither too wet nor too dry. These plants can neither grow in water or water-logged soils nor can they survive in dry places. In other words, mesophytes are the plants of those regions where climates and soils are favourable. Vegetations of forests, meadows and cultivated fields belong to this category. The simplest mesophytic community comprises the grasses and herbs, richer communities have herbs and bushes, and the richest ones have trees (rainforests in tropics).

Mesophytes can be classified into two main community groups:

(1) Communities of grasses and herbs.

(2) Communities of woody plants.

1. Communities of Grasses and Herbs:

These include annual or perennial grasses and herbs. The grasslands occur in area of approximately 25 to 75 cm rainfall per annum. They occur over large interior areas in many countries of the world such as U.S.A., Canada, Australia, Southern Russia, Africa, and India. The different types of grasslands and herb communities are listed below.

(i) Arctic and alpine mat-grasslands and mat-herbage:

Such communities are restricted to Polar Regions and mountain tops. The plants are small sized soft shrubs, and the under-shrubs are totally absent. Mosses may be intermingled but lichens do not appear.

This group is subdivided into two:

(a) Mat grassland (Gramineae)

(b) Mat herbage (Dicotyledonous herbs such as Saxifraga, Delphinium, Potentilla, Ranunculus, etc.).

(ii) Meadow:

This forms a connecting link between mesophytes and hydrophytes as they grow in soils where moisture is 60—83%. Plants are tall perennial herbs with long stems. Soil is invisible due to overcrowding of plants. Plants are mostly rhizomatous. The leaves show mesophytic features, i.e., they are thin, broad, flat and glabrous. Members of families Gramineae, Ranunculaceae, Papilionatae and Compositae are found in abundance.

(iii) Pasture on cultivated land:

Vegetation is shorter and more open in pasture than in meadow. It is disturbed very often by grazing. The vegetation usually includes grasses, dicot herbs and some mosses.

2. Communities of Woody Plants (Bushland and Forests):

These are classified and described in the following ways:

(i) Mesophytic bushlands:

Such a mesophytic community occurs where temperature and other conditions are not favourable for the growth of forest but they are too much favourable format herbage vegetation. In many places, xerophytic and mesophytic bushlands merge with each other. Salix, Arabis, Lathynis, Vicea, etc., are the important plants of bushlands.

(ii) Deciduous forests:

These forests are found in the areas where rainfall is high enough (about 75 150 cm per year) and evenly distributed and the temperature is moderate. Such forests are characterised by trees which become leafless for certain periods of the year. The foliage persists for about five to eight months. This phenomenon of repeated foliation and defoliation of trees is prominent in temperate and cold regions (where there is long winter) and in tropics as well where the summer is of long duration. Leaves are dorsiventral and they exhibit many shapes and structures.

The trees are profusely branched. Mycorrhizae are present on the roots. Epiphytic mosses and lichens grow in abundance on the surface of the trees. The majority of the plants are pollinated by wind. The soil is very rich in microflora. The deciduous forests are named after dominant trees of those particular communities, as for example, Quercus-Oak forest, Betula-Birch forest, Fagus-Beach forest and so on.

Tropophytes (changing plants), an interesting group of tropical plants can be included in this group of mesophytes. Generally in tropical regions, the climate remains, more or less, uniform throughout the year but in some tropics there is alternation between damp and dry cold climates. Plants growing in the tropics of disuniform climate develop some structural modifications through which they can endure the regular cycle of favourable and unfavourable seasons in one way or the other. In other words, tropophytes behave as mesophyte during rainy season and as xerophytes during dry cold season. The shedding of leaves may occur in the beginning of winter season or in the summer.

Important adaptive features of these plants are:

(a) Better protection of winter buds

(b) Thick bark covering on the stem

(c) Formation of underground stem which protects the perennating buds from extreme drought.

(iii) Evergreen forests:

These forests are found in the tropical and subtropical regions extending into the cold temperate zones of southern hemisphere. Plants in these forests are evergreen (i.e., they retain their leaves for more than one year until new foliage appears). Very few species in these forests may show leaffall.

Evergreen forests are of three types:

(a) Antarctic forests:

These forests cover mountains of New Zealand and a number of other countries in the world where annual temperature ranges from 5°C to 70°C and the rainfall is abundant throughout the year. Important plants found in these forests are conifers, Myrtaceae, Hymenophyllaceae. Mosses and Liverworts may also be present.

(b) Subtropical forests:

These forests are found in the regions of fairly high rainfall but where temperature differences between winter and summer are less marked. Winter generally goes without rains. The plants are about 30 metres in height. These forests include Oaks. Magnolias, Tamarindus and mosses. Subtropical forests occur in eastern part of U.S.A., South Brazil, South Africa, East Australia, Southern China, and Japan.

(c) Tropical Rain forests or Tropical Evergreen forests:

Tropical rain forests are found in low lying regions near the equator with annual rainfall of 180 cm or more. This type of forest is most dense and is undisturbed by biotic agencies and is therefore, called “primeval forest”. The rain forests represent the climax vegetation of the whole world.

Climate of such forest is characterized by:

(1) High humidity (air saturated with 95% humidity)

(2) High temperature

(3) Daily rains

(4) No distinct dry season

(5) Soil very rich in humus, black in colour, and porous.

The plants show luxuriant growth and they are found in several storeys. Humboldt very appropriately commented “forest is piled upon forest”, i.e., highest trees form top layer about 40—50 metres up, beneath which is the storey of short trees, then storey of low palms and trees, ferns, then, storey of scattered herbs and shrubs (4 to 5 metres in height). On the surface of ground may be found Selaginella, mosses etc. (Fig. 8.38).

Roots of the plants may be found covered with saprophytes and parasites, e.g., Rafflesia, Balanophora. Monotrapa etc. Epiphytes and lianas are very common in these forests. Very dense growth of shrubs and climbers makes the forests impenetrable. Plants usually acquire tree forms. Most of them produce root buttresses for the support of their huge trunks. They show cauliflory in which the buds are protected by stipules, leaf sheaths and petioles, etc. Flowers are of various colours and they develop high over the heads.

Plants do not show periodicity for foliation and flowering. Each species has its own flowering and foliation time. The leaves exhibit almost all shapes and are usually directed upwardly to drain off excess water. The leaf surface is cutinised and impregnated with silica which protects them from violent rains. Leaves may be provided with channelled nerves and dripped tips (i.e., they have long and narrow apices). Trees develop thick barks.

Plants belonging to families Leguminoceae, Lauraceae, Myrtaceae, Moraceae, etc. are very commonly found in tropical rain forests.

Vegetational succession in the tropical rain forest takes place in the following sequence:

The pioneer colonisers are deciduous plants that are replaced gradually by semi-deciduous vegetation that persists for very short period of time after which semi-evergreen plants make their appearance. The semi-evergreen vegetation becomes intermingled with some evergreen plants which finally become dominant. In this way the climax forests develop. This sequence is possible only if the biotic factors are not allowed to affect the vegetation to a major degree.

Tropical rain forests are found in central and southern America, central Africa, Pacific Islands, and Malaya and in many other equatorial countries of the world. In India, these forests are found in south-eastern Himalayas, tracts of Assam, and western slopes of Nilgiri. The tropical rain forests are of great economic values to the human beings. These yield timbers of high quality.