Depending upon their water requirements, more specially upon the quantity of water available in their habitats, the plants, are divided into three ecological communities. They are
(I) Hydrophytes : Plants adapted to aquatic environment are called hydrophytes.
(2) Xerophytes : Plants adapted to grow in dry lands where water content is low are called xerophytes.
(3) Mesophytes : Plants adapted to grow on land with moderate supply of water are called Mesophytes.
HYDROPHYTES: Hydrophytes (Gk. Hydro - water; phyton - plant) are the plants which grow in aquatic habitats. Water is available to these plants throughout the year. The water may be fresh water (or) saline. The common fresh water bodies are lakes, ponds, pools, ditches, springs, rivers, canals and lagoons. Saline water occurs in Inland seas, oceans and estuaries.
Classification of Hydrophytes :
According to their relation to water and air, the hydrophytes are grouped into the following categories.
(a) Free-floating Hydrophytes : These plants float freely on the surface of water, but are not rooted in the mud. These plants grow in large numbers in ponds and pools. Leaves in some forms are very minute (Wolffia), while in others they are quite large. Examples : Wolffia, Lemna, Azolla, Salvinia, Eichhornia, pistia, Trapa.
(b) Rooted hydrophytes with floating leaves : These submerged aquatic plants are rooted in the muddy substrata of ponds, rivers and lakes. But their leaves and flowering shoots float on the water surface, due to the presence of long petioles and pedicles. They normally grow in shallow regions. Examples: Nymphaea (water lily), Nelumbium (Lotus), Victoria (Gaint water lily).
(c) Submerged floating hydrophytes : These plants grow below water surface and have no contact with air. They are restricted to hallow regions where the plants can get adequate supply of light. These plants remain freely suspended in water. Their stems are long, slender and bear small leaves at the nodes. eg. Hydrilla, Ceratophyllum, UtricuIaria,Najas etc.
(d) Submerged rooted hydrophytes : These plants also grow entirely under water. Such plants are anchored to the bottom of the ponds, lakes and other water resources. The plants bear tuberous stem ( corn-like ) with long leaves, which are narrow and ribbon shaped e.g. Vallisneria, Isoetes.
(e) Rooted emergent hydrophytes : These plants are amphibious, and are adapted to live both in aquatic and terrestrial habitats. The plants grow either in shallow water or on the muddy substratum. The basal part of the plant is normally submerged in water (or) buried in the mud, while the shoots spring well above the surface of water. e.g. Ranunculus, Sagittaria, Scirpus, Marsilea, Cyperus, Neptunia, Typha, Oryza etc.
Hydrophytes show many external and internal modification for a successful living in the water medium. Some of them are described in the following heads:
Morphological features :
1. Root system is generally poorly developed. Roots may be completely absent in some cases. e.g. Wolffia, Ceratophyllum, Salvinia.
2. Well developed root system is seen in emergent forms as in Ranunculus, Eichhornia, Typha etc.
3. Root hairs are generally absent except in some plants growing in the mud.
4. Roots caps are usually absent. In some species, they are replaced by elongated finger - glove like sheaths known as root - pockets . e.g. Lemna, Eichhornia.
5. In some plants, as in Jussiaea repens, two types of roots develop when the plants are growing on the surface of water. Some of them are normal, while others are floating roots, which are negatively geotrophic. They store air and help in buoyancy to plant.
6. Tufts of balancing roots develop in free floating forms like Eichhornia, Pistia etc.
II. Stem :
1. The stem is long, slender, spongy and flexible in submerged forms like Hydrilla and Potamogeton.
2. In some cases, the stem is modified into a rhizome (e.g. Nymphaea, Nelumbo) or a runner (e.g. Azolla, Eichhornia).
3. The shoot system is extremely reduced in lemnids - e.g. Lemna, Spirodela, Wolffia.
III. Leaves :
1. In submerged forms, leaves are thin, small (e.g.Hydrilla), ribbon shaped (e.g Vallisnaria), linear (e.g potamogeton) or finely segmented (e.g Ceratophyllum, Utricularia).
2. In floating plants, leaves are large, flat, peltate and entire (e.g.Nymphaea, Nelumbo). Their upper surfaces are exposed in the air, but the lower ones are in contact with water. The petioles are long, flexible and often covered with mucilage.
3. In free - floating forms, the petioles become swollen and spongy. They make the plant buoyant. e.g. Eichhornia, Trapa.
4. Heterophylly (or) occurence of more than one type of leaves is quite common in partly submerged plants. (e.g. Sagittaria, Ranunculus, Limnophilla, Salvinia, Azolla, Cabamba etc). In this case, the submerged leaves are linear, ribbon - shaped or highly dissected. The floating and aerial leaves are entire, rounded or lobed. The reasons for the occurrence of heterophylly include
(i) change in habitat,
(ii) change in nutritional structure (light and C02),
(iii) absence of transpiration in the submerged leaves. The submerged leaves act as water absorbing organs.
I. Sexual reproduction is less common in submerged plants. Where flowers develop, seeds are rarely formed.
2. Vegetative reproduction is the common method of propagation in hydrophytes. It is accomplished either through fragmentation ( e.g. Elodea), stolons (e.g. Vallisneria), offsets (e.g. Pistia,Eichhornia), tubers (e.g. Sagittaria) by winter buds called turions (e.g.Utricularia,MyriophyIIum, Potamogeton).
1. Cuticle is completely absent in submerged forms, If present it is thin and poorly developed as in emergent forms.
2. Epidermis is usually single layered. It is not a protective layer, but it functions mainly as an absorptive tissue.
3. Stomata are absent, even if present they are non-functional in submerged regions. In floating leaves, stomata are present only on the upper surface,
4. Aerenchyma is well developed in submerged leaves and stems. The air chambers are filled with respiratory and other gases and moisture. The air chambers are separated by cross septa partitions called diaphragms. The aerenchyma provides buoyancy and mechanical support to aquatic plants.
5. Mechanical tissues like sclerenchyma and collenchyma are poorly developed. However in some emergent forms like Typha, sclerenchymatous cells develop in the cortex. Branched sclereids and idioblasts occur in Nymphaea and related plants to provide support.
6. Vascular tissues are poorly developed in submerged plants. Since, all submerged organs are capable of absorbing water, xylem shows greatest reduction. In some aquatic plants, xylem is replaced by a central cavity. e.g. Hydrilla, Potamogeton.
7. Endodermis may (or) may not be clearly defined.
8. Secondary growth is generally absent.
Physiological characters :
1. The osmotic potential of hydrophytes is usually very low. It is equal to (or) slightly higher than that of external water.
2. The submerged parts secrete mucilage to protect the plants from friction, dessication, epiphytes and decaying effect.
3. Many plants possess hydathodes to excrete excess water.
4. The stored gases in aerenchyma are useful for metabolic use and for strength.
5. Flowering and fruiting is reduced, as the seeds show poor rate of germination.
Plants which grow in dry habitats or xeric conditions are called Xerophytes. Places where the available water is not present in adequate quantity are termed as xeric habitats. Xeric habitats are following types.
1. Physically dry : The soil receives very little rainfall. Further it can not hold more water as the soil is sandy (or) rocky in nature. e.g. desert, rock surface, waste land etc.
2. Physiologically dry : Water is present in sufficient quantity in the habitat. But it cannot be absorbed by plants because of unfavourable conditions like the presence of excess salts, acidity (or) low temperature. The areas with very low soil temperature are also called cold deserts. e.g. tundra and alpine regions.
3. Physically and Physiologically dry : The dryness of the habitat is caused by both actual deficiency of water as well as Inability of plants to absorb water due to physiological reasons. Such a habitat is found on the slopes of high mountains.
Classification of Xerophytes:
Xerophytes belong to different categories, basing upon their resisting capacity.
1. Ephemerals (or) drought escaping plants : These xerophytes are short lived annuals. They complete their life cycle a very short period of 6-8 weeks. During critical dry periods, the plants survive in the form of dormant seeds. These plants are not true xerophytes because they do not face dry environment. It is because of this reason, some prefer to drought escaping plants. e.g. Tribulus, Euphorbia, echinatus, Cassia tora.
2. Succulents : The succulents are the perennial plants of dry areas especially sandy soils and sandy beachen, The plants absorb large quantity of water during rainy season and store it in different body parts. Such water storing organs become fleshy (or) succulent. Storage of water is accompanied by the presence of pentosans (or) mucilage. Succulency in these plants in related to deficiency of nitrogen and accumulation of K + ions (Delgado and Medina, 1978) Succulence is generally present in the stem, These plants are called fleshy xerophytes. eg. Cacti, Euphorbias and Opuntia. In some other plants, the leaves become fleshy due to water storage tissue. These plants are known as malacophyllous xerophytes. e.g. Aloe, Begonia, Bryophyllum, Agave, Yucca, Tradescantia etc., In a few desert plants the roots store water. e,g. Ceiba, Pelarogonium Oxalis and Asparagus.
3. Non-succulent perennials : These are said to be true xerophytes (or) euxerophytes, because they are capable of with standing long periods of drought. The plants suffer dryness both in internal as well as external environments. For this, the plants several morphological, anatomical and physiological characteristics. Casuarina, e.g. Nerium, Zizyphus, Saccharam Calotropis, Acacia, Capparis,
Morphological characters :
(1) The non-succulent perennial xerophytes have a well developed root system, which is profusely branched.
(2) Root hairs and root caps are very well developed.
(1) The stem is stunted in growth and the branching gives the plant a bushy appearance.
(2) Plants like Acacia, Zizyphus, Prosopis have very hard and woody stems. They are covered with thick bark.
(3) Stems are covered with wax and silica or with dense hairs.
(4) In some the stems are modified into flat, green leaf-like structures termed as phylloclades. (e.g. Cacti and Cocotoba). In Ruscus, the branches developing in the axils of scale leaves, become metamorphosed into leaf like structure called cladophylls.
(1) In many plants the leaves are reduced to scales or spines. e.g. Ruscus, Asparagus, Casuarina, Muehelenbeckia.
(2) Foliage leaves, where ever present, become very thick and leathery to reduce transpiration. e.g. Calotropis, Salvadora etc.
(3) Certain xerophytic plants shed these leaves during the dry period. e.g. Capparis aphylla.
(4) In many Australian species of Acacia (A. mehnoxylon) the petiole flattens to carryout photosynthesis. This modified petiole is termed as phyllode.
(5) In some plants, the stipules become modified into spines. e.g. Zizyphus, Acacia. The spines protect the plants against browsing.
(6) Leaves of some of the grasses are folded in such a manner that the stomata are completely protected. e.g. Agropyron, Amnophila, Poa.
Anatomical characters :
(1) Leaves and stems of many xerophytic plants have thick cuticle to check transpiration.
(2) Epidermis is well developed, with heavily thickened cell walls. Some of the plants like Nerium have multiple epidermis both on the upper as well as on the lower surfaces.
(3) Bulliform cells are of common occurrence in the leaf epidermis of sugarcane, bamboo, typha and a number of other grasses.
(4) The stomata are mostly located in deep pits (Sunken Stomata). The pits are filled with large number of hairs.
5) The stomata are greatly reduced in number and are mostly restricted to the lower surface of the leaf. However, grass leaves are amphistomatic.
(6) Hypodermis is several layered and sclerenchymatous to check evaporation of water. e.g. Nerium, Calotropis, Ficus.
(7) In leaves, the mesophyll is very compact and intercellular spaces are greatly reduced. Spongy parenchyma cells are comparatively fewer in number. The palisade parenchyma is many layered and is well developed not only in the upper portion of the leaf but also in the lower portion. e.g. Nerium
(8) In some plants like Lactuca, Greggia, the entire mesophyll is made up of palisade.
(9) Mechanical tissues are well developed, including several types of sclereids.
(10) Vascular tissues are also well developed. Vascular bundles are usually surrounded by several layered bundle sheaths.
(1) Maximov gave the popular concept that xerophytes show high rate of transpiration per unit area of leaf surface. Later several workers found it to be incorrect.
(2) The rate of photosynthesis per unit area is rapid. Levitt have shown that the ratio between starch and sugar is also lower in these plants.
(3) Osmotic pressure is quite high. This helps the true xerophytes to withstand considerable dehydration.
(4) Xerophvtes show a high rate synthesis of wall materials (Cellulose) and fats (suberin, cutin). The enzymes amylase, catalase and peroxtdase are more active.
(5) The protoplasm is less viscous and more permeable.
The land plants which grow in normal conditions of temperature and moisture are called mesophytes. Such habitats are called mesic habitats. In mesic habitats the water supply is neither in excess nor in deficit. The soils are well aerated and receive moderate to heavy rainfall. The soils are rich in humus. Mesophytic conditions are best suitable for the cultivation of different crops.
Mesophytes show the following ecological adaptations :
(1) The root system is well developed. Roots are generally fairly branched. Root caps and root hairs are also present. Root/shoot ratio is unity (or) more than one.
(2) The stems are usually aerial, solid, stout and branched.
(3) The leaves are thin and broad with various shapes and sizes. They are oriented horizontally and are without waxy (or) hairy coating.
(4) The epidermis is single layered and the cells usually lack chloroplasts.
(5) The cuticle is in the form of a thin layer. Cuticular transpiration is high.
(6) Stomata are generally present on both surfaces of the leaf'. The stomata are mostly in level with the epidermis and are not sunk in cavities.
(7) Mesophyll is differentiated into palisade and spongy tissues is dicots. However in monocots the mesophyll is homogeneous.
(8) Vasuclar tissues and mechanical tissues are well developed. Collenchyma and sclerenchyma are the two main mechanical tissues.
(9) The osmotic potential of the cells is usually low. Mid day wilting (Noon time) is quite common in summer.
Halophytes are a special type of xerophilous plants which inhabit saline soils. The soils contain higher concentration of salts like NaCI, Mg Cl2 and Mg S04. The presence of excess salts in water interferes with the absorption of water by these plants. Consequently the saline soils are termed physiologically dry soils.
Classification of Halophytes:
Halophytic communities may be divided into five groups depending upon the nature of the substratum.
1. Lithophilous halophytes : They grow on rocks and stones near the ocean.
2.Psammophilous halophytes : They grow on sandy shores.
3.Pelophilous halophytes : They grow in places where saline soil is loam with a predominance of clay.
4.Helophilous halophytes : They grow in swamp (or) marsh regions. These are again two types.
(a) 'Salt-swamp and salt-desert : They grow in calm waters and muddy places as in lagoons, inlets and estuaries etc.
(b) Littoral-swamp forest (or) Mangrove vegetation: They are extensive in all tropical seas, especially on flat, muddy shores. Soil is flooded with sea water either permanently (or) at high tide. These littoral swamp forests form a characteristic vegetation known as mangrooves in tropical and sub tropical regions.