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Consortium for Educational Communication Module on Morphological and anatomical responses of plants to water By Shauket Ahmed Pala Research Scholar, Department of Botany, University of Kashmir Consortium for Educational Communication Text Availability of water is a major selective force in the evolution of the plants ability to respond to moisture stress. Mosses, algae, fungi and lichens have no protective mechanism against water loss because their internal water status tends to match atmospheric moisture conditions. At the condition of water scarcity, their cells shrink without disturbing the fine protoplasmic structures and the vital processes are suppressed. With the improvement of moisture condition, they imbibe water and cells resume normal functioning. Such organisms are called poikilohydric as they restrict their growth to moist periods. Other plants such as ferns and seed plants are able to maintain stable water balance within limits independent of fluctuations of atmospheric moisture levels and are called homohydric. The ability is supported by vacuoles that store water within the cell, a protective cuticle slowing down evaporation, stomata to regulate transpiration, a combination of osmotic pressure and turgor pressure of water within the cells and the extensive root system. All features of specialization that help plants to grow under different habitats are referred to as ecological adaptations. The term ‘adaptation’ can be defined as ‘the structural and functional characteristics of the living organisms which develop over a period of time and enable them to survive and reproduce in a particular environment or habitat’. It is quite evident that water is an important ecological factor in the life of plants. Tissues of growing plants may contain from 60 to 95 percent water by weight. Water is the prime factor responsible for the distribution of vegetation and its structure. Warming (1909) classified the plants on the basis of their water requirement into three ecological groups. They are; a) Hydrophytes, b) Xerophytes and c) Mesophytes A. Hydrophytes Hydrophytes are plants that grow in regions where there is plenty of water supply such as lakes, ponds, rivers, streams and marshes or wet soils. The organisms found in aquatic habitats experience a variety of physical factors. These include the availability of oxygen and light, pressure fluctuations, resistance to motion, salt concentration, etc. To adjust to the prevailing conditions, aquatic plants have various types of adaptations. According to the way in which they develop in water, they are divided into the following five categories: Consortium for Educational Communication (a) Free-floating hydrophytes: These plants float freely on the surface of water but are not rooted in the mud. These remain in contact with water and air, but not soil. Very small such hydrophytes float on the surface of water by their flattened bodies and surface tension of water. The larger plants possess special air storing organs for balancing them over the surface of water. Examples of free floating hydrophytes are: Eichhornia, Pistia, Wolffia, Lemna, Salvinia, Azolla, etc. (b) Rooted hydrophytes with floating leaves: These hydrophytes are rooted in the mud but their leaves and flowering shoots float on or above the water surface. Such plants usually grow in shallow water and posses elongated and flexible petioles or stems to bring the lamina on the surface of water. Victoria regia, Nymphaea, Trapa, Nelumbo nucifera and Marsilea are some of the examples of rooted hydrophytes with floating leaves. (c) Submerged floating hydrophytes: This group of submerged hydrophytes include those plants which are not rooted in mud and remain completely suspended in water body. They neither reach the surface nor are in contact with the bottom. Examples include Najas, Utricularia and Ceratophyllum (d) Submerged rooted hydrophytes: These plants are completely immersed in water and rooted in the mud. However, the root system is poorly developed and helps in the absorption of some nutrients from the bottom. Examples of such plants are Hydrilla, Vallisneria, Potamogeton pectinatus, Isoetes, etc. (e) Rooted emergent (amphbious) hydrophytes: This group includes those plants which grow in shallow water with their shoots extending above the surface of water. These plants are rooted in the mud. Their basal parts including roots, rhizomatous stem, basal part of stem and some leaves may be submerged in water but the upper part of stem, branches, some leaves and flowering shoots are exposed to air. These plants show both hydrophytic and mesophytic characters. The aerial parts of these amphibious plants show mesophytic characters, while the submerged parts develop hydrophytic characters. Rununculus aquaticus, Limnophylla heterophylla, Typha, Sagittaria, Myriophyllum heterophylum are some of the examples of such plants. I Morhological adaptations 1. Roots. Consortium for Educational Communication Due to availability of water in plenty, roots (the principal organs of water absorption) in such plants become of secondary importance and less significant. Their overall development is usually very poor and insignificant in most of the hydrophytes. (i) Roots may be entirely absent as in Wolffia, Utricularia, Salvinia and Ceratophyllum or poorly developed as in Hydrilla. In Salvinia submerged leaves compensate for roots. However, in emergent forms, as in Rununculus which grow in mud, roots are well developed with distinct root caps. (ii) Root hairs and root caps are generally absent in floating forms. In Eichhornia and Lemna, the root caps are replaced by fingerglove like root-pockets which help in balancing. (iii) Roots, if present, are generally fibrous, adventitious, reduced in length, unbranched, or poorly branched. (iv) In Jussiaea repens two types of roots develop. Some of them are normal, while others are negatively geotropic floating roots, spongy in nature and keep the plants afloat. 2. Stems. (i) In submerged forms as Hydrilla and Potamogeton stem is long, slender, spongy and flexible. In free floating forms it may be slender, floating horizontally on water surface as in Azolla or thick and short, stoloniferous and spongy as Eichhornia. (ii) In forms which are rooted with floating leaves like Nymphaea and Nelumbo with their upper surface coated with wax, the stem is a rhizome. These rhizomes live for many years and produce leaves every year. (iii) In Neptunia oleracea, the stems are swollen and store gases to function as floats. 3. Leaves. (i) In submerged hydrophytes leaves are thin, and are either long and ribbon shapped as in Vallisneria, or long and linear as in Potamogeton, finely dissected as in Ceratophyllum and Utricularia. (ii) Leaves of floating forms are large, flat and entire as in Nymphaea and Nelumbo with their upper surfaces coated with wax. The wax coating protects the leaves from mechanical and physical injuries and also prevents clogging of stomata by water. Consortium for Educational Communication Petioles of floating leaves are long, spongy and flexible and often covered with mucilage. In Eichhornia and Trapa petioles become swollen and spongy. (iii) In some amphibious plants Ranunculus and Sagittaria the leaves show heterophylly, i.e., the plants develop two different types of leaves. The submerged leaves are usually dissected or ribbon shaped offering little resistance against the water currents, and absorbing dissolved carbon-dioxide from water, and aerial leaves are entire, rounded or lobed and show typical mesophytic features. 4. Vegetative reproduction is common. Vegetative propagation by runners, stolons, stem, root tubers, offsets etc., is most common and extensive method of reproduction. 5. Flowers and seeds are less common in submerged forms. Pollination (e.g. Vallisneria) and di s p e r s a l o f f r u i t s a n d seeds are accomplished by the agency of water. Seeds and fruits of many hydrophytes are light in weight and they easily float on the surface of water. 6. All the submerged parts are surrounded by mucilage which functions as lubricant, prevents desiccation, and protects plants from epiphytes and attack of pathogens II Anatomical adaptations: The hydrophytes show following anatomical features: 1. Cuticle is completely absent in submerged parts of the plants but may be present as a thin film on surface of parts exposed to atmosphere 2. The epidermis is usually single layer of thin walled cells, not protective in function. Epidermal cells of leaves contain chloroplasts and they can function as photosynthetic tissue, especially where the leaves and stem are very thin e.g. Hydrilla. The epidermal cells of submerged parts are mostly covered on outer side by mucilage to protect the plant body from decay under water. 3. Wax coating is present on the upper surface of floating leaves. 4. The aerial parts of some hydrophytes bear hydathodes to lose excess water that enters into the plant due to endosmosis. Consortium for Educational Communication 5. Stomata are totally absent in submerged forms and exchange of gases takes place through diffusion. In floating leaves stomata are confined only to the upper surface (epistomatous leaves) e.g Nelumbo. Non-functional stomata are seen in Potamageton. In amphibious plants stomata may be scattered on all the aerial parts. Cortex is well developed, thin-walled and parenchymatous, major portion of which is occupied by well developed prominent air cavities- the ‘aerenchyma’ which offers resistance to bending stress, increases buoyancy and allows a rapid gaseous exchange. 6. Mechanical tissues, i.e. sclerenchyma and collenchyma, are absent or poorly developed in floating and submerged parts. They are, however, present in aerial and terrestrial parts. Branched or star shaped sclereids and idioblasts are found in the aerenchyma of Nymphaea, Eichhornia and related plants. I t g i v e s mechanical support to the plants. 7. In the vascular tissue xylem is poorly developed in hydrophytes, especially in submerged forms like Potamogeton, as the water absorption takes place all over the surface of the plant body. In xylem vessels are less common but tracheids are generally present. In amphibious plants, the xylem and phloem are well developed and are usually aggregated towards the centre. Generally secondary growth in thickness does not occur in aquatic stem and roots. 8. Mesophyll is undifferentiated in submerged leaves as in Potamogeton, but is well differentiated into palisade and spongy parenchyma with air cavities as in floating leaves (e.g. Nymphaea) and emergent leaves (Typha). B. Mesophytes These are very extensive on the land surface. They are such land plants which grow in habitat where water is not scarce or not abundant and need well aerated soils. They prefer soil and air of moderate humidity and avoid soil with standing water or containing a great abundance of salts. In some respects they stand in between the hydrophytes and xerophytes. They generally lack special structural and physiological adaptations found in hydrophytes and xerophytes. The general morpho-anatomical features of mesophytes are as follows. 1. Root system is well developed. Roots are generally fairly branched, with root caps and root hairs. 2. Stems are generally aerial, solid and freely branched. Consortium for Educational Communication 3. Leaves are generally large, broad, thin and varied in shapes; generally oriented horizontally, green, without hair or waxy coating. 4. Cuticle in aerial parts is moderately developed. 5. Epidermis well developed, without any hair or waxy coatings and cells without chloroplasts. 5. Stomata are generally present on both surfaces of leaves. 6. Mesophyll in leaves is differentiated into palisade and spongy parenchyma, with many intercellular spaces. 7. Vascular and mechanical tissues fairly developed and well differentiated. C. Xerophytes Xerophytes are those plants which grow in xeric habitats i.e., under deficient supply of water and extremely dry conditions. Under such habitats the conditions are favourable for excessive loss of water from aerial parts of plants (transpiration) and the water is not available for absorption. Xeric habitats are characterized by: 1. High temperature of atmosphere and soil 2. Deficiency of water in the soil or presence of water deep in the soil 3. High intensity of light and wind velocity. The plants facing such adverse conditions develop certain morphological, anatomical and physiological adaptations. Adaptations in xerophytes are of two types: (i) Xeromorphic adaptations are those which are inherited whether the xerophyte grows in xeric conditions or not. For example, a Cactus has the same feature, whether it is in a desert or in a normal land. (ii) Xeroplastic adaptations are the ones that are induced temporarily but disappear when the conditions are favourable. On the basis of morphology, physiology and life cycle patterns, xerophytes are generally classified into following three categories. Consortium for Educational Communication 1. Ephemeral annuals They are also called ‘drought evaders’ or ‘drought escapers’. They are mostly found in arid zones. They are annuals, which complete their life cycles within a very short period of 6-8 weeks and thus escape dryness. Generally they germinate and grow when the habitat remains moist during the short rainy season. They flower and set seeds before the habitat becomes dry. They pass the period of drought by means of their seeds. Examples of such plants are Solanum xanthocarpum, Argemone mexicana, Cassia tora etc. 2. Succulents They are also called 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 season. Examples include Agave, Aloe, Euphorbia, Opuntia, Asparagus etc. In some plants, stem becomes succulent which are called the ‘F l e s h y xerophytes; as in Opuntia and Euphorbia. Those succulent xerophytes in which leaves become fleshy are also known as Malacophyllous xerophytes, such as Aloe, Yucca, Agave, Bryophyllum, Tradescantia etc., 3. Non- succulent perennials They are also called drought resisting xerophytes and are the true xerophytes. They possess a number of morphological, anatomical and physiological characteristics which enable them to withstand critical dry condition. They are the plants that suffer from dryness both in their internal as well as external environments. Examples of such plants are Calotropis procera, Acacia nilotica, Zizyphus jujuba, Capparis aphylla, Casuarina, Nerium, Saccharum etc. I Morphological adaptations 1. Roots (i)In contrast to hydrophytes, roots of xerophytes develop under water deficient conditions. Thus, in order to secure water, which is present in less amount and moreover in deeper layers of soil, roots in xerophytes become the principle organ of primary importance. The plants develop extensive and deeply penetrating root system which in some cases is several times longer than the shoot. Root hairs and root caps are well developed. Consortium for Educational Communication (ii). In some cases the roots become fleshy and succulent to store large quantity of water and mucilage e.g. Asparagus (iii). Some fleshy and succulent xerophytes develop shallow root system to obtain water from light showers as well as dew e.g. Opuntia 2. Stem The stem remains stunted, thick and heavily branched giving a bushy appearance. They also develop thick bark to escape transpiration. In some xerophytes, the stem becomes fleshy and succulent e.g. Euphorbia. Sometimes the stem is modified into green, flat, leaf like structure called phylloclade, covered with spines and performs photosynthesis. 3. Leaves (i) The leaves in xerophytes exhibit comparatively more variation than any other part. In many cases they are either reduced or scale like modified into spines to check transpiration. In some plants leaves are permanently oriented in a vertical position to avoid direct sun rays. Some plants shed their leaves during the dry periods to reduce loss of water through stomata. (ii) The foliage leaves of some xerophytes are thick, fleshy and leathery. They are the main storing organs. They have shining surfaces to reflect light and heat and protect plant from desiccation. (iii). In some grasses such as Ammophila and Poa leaves become folded and rolled in such a manner that the sunken stomata become hidden, and thus rate of transpiration is considerably minimized. 5. In some xerophytes, the leaves and young parts of plant are covered over by dense coating of hair which act as shade against Sun to reduce transpiration e.g. Calotropis 9. In some of xerophytes such as Euphorbia, Acacia nelotica, Zizyphus jujuba and Capparis aphylla, stipules become modified into spines. II Anatomical modifications 1. The epidermis of leaves and other delicate parts is covered with thick cuticle. In addition some xerophytic plants (e.g. Calotropis) possess protective coating of wax and silica. The wax Consortium for Educational Communication is impermeable to water vapours, where as silica protects against solar radiations. Epidermis is multilayered in some xerophytic plants e.g. Nerium 2. The epidermal cells of some xerophytes are coloured due to presence of anthocyanin and betacyanin in their cell sap. The pigments act as light screen and reduce insolation. 3. The stomata are generally confined to lower epidermis of leaves called hypostomatous (except some grasses). They are sunken below the level of other epidermal cells. In some cases, they are located in deep pits which are filled with hairs. In some xerophytes the stomatal pores are plugged with wax or resins during the period of drought. The stomata of mature and old leaves are usually non functional due to greater thickening. 4. Leaves of some xerophytic grasses possess bulliform or motor cells in their epidermis. These cells are thin-walled and greatly enlarged. These cells are generally present at the basses of grooves and furrows in upper epidermis. In moist conditions these cells remain turged and keep the leaf surface flat. But when they lose their turgidity in the period of drought they get flaccid and cause rolling of leaves. 5. In many xerophytes the hypodermis is one to several layers thick consisting of sclerenchymatous tissue. 6. The mesophyll is differentiated into palisade and spongy parenchyma. In some xerophytes palisade is located on both upper and lower surfaces of leaves. The succulent leaves of Aloe and Salsola have prominent water storage regions in their mesophyll. 7. Mechanical tissue system is very well developed in xerophytes. 8. The water conducting system (i.e. xylem) is well developed. It also provides mechanical support. The tracheids are smaller in size where as vessels are larger with lignified thick walls. Annual rings are also prominent.