1. Hydrophilous plants:
Hydrophytes (in water); Helophytes (in marsh).
2. Xerophilous plants:
Oxylophytes (on acid soil); Psychro- phytes (on cold soils); Halophytes (on saline soils) ; Lithophytes (on rocks); Psammophytes (on sand and gravel); Chersophytes (on waste lands); Eremophytes (deserts and steppe);-Psilophytes (on savannah); Scleropbyllous formations (bush and forests) and Coniferous formations (forests).
3. Mesophlous plants:
Mesophytes. Thus, on the basis of water requirements following three broad ecological groups of plants can be recognised: (a) Hydrophytes (plants living in water and require large quantities of water); (b) Xerophytes (terrestrial plants which can tolerate extremely dry conditions and pass long periods without water) and (c) Mesophytes (terrestrial plants requiring moderate amounts of water).
Likewise, there exists a broad classification of animals into following three main ecological groups, depending on the requirement of water:
1. Hydrocoles: Aquatic animals which live in and require large quantities of water.
2. Xerocoles: Terrestrial animals which can tolerate extremely dry conditions and pass long periods without water.
3. Mesocoles: Terrestrial animals requiring moderate amounts of water.
Both the plants and animals living in water or dry habitats exhibit a number of special features or adaptations in their morphology, anatomy and physiology which enable them to live under such extreme ecological conditions. These ecological adaptations of plants and animals can be discussed as follows:
Ecological Adaptations in Hydrophytes:
Types of Hydrophytes:
Aquatic plants or hydrophytes grow on extremely wet soil and where water is available to plants in abundance (viz., about ponds and along river banks). According to their distribution in a fresh-water body, following five categories of hydrophytes have been recognised:
Some hydrophytes like Wolffia, Lemna, Pistia, Spirodella, Azolla, Salvinia and Eichhornia float freely on water surface. They remain in contact with water and air, but not soil.
B. Rooted with floating leaves:
In certain hydrophytes such as Nymphaea, Nelumbo, Trapa, Marsilea, and Aponogeton, the roots are fixed in mud but leaves have long petioles which keep them floating on the water surface. Rest of the plant body is submerged in water.
C. Submerged floating:
Hydrophytes like Ceratophyllum, Utri- cularia; Najas, etc. remain in contact only with water. They are not rooted in the mud and being completely submerged in water.
D. Rooted submerged:
Some hydrophytes such as Hydrilla, Potamogeton, ValUsneria, Chara and Isoetes remain completely submerged in water, but rooted in the soil.
E. Rooted emergent:
These hydrophytes grow in shallow waters and require excess of water but their shoots are partly or completely exposed to air. The root system is completely under water, fixed in soil. In some of these hydrophytes such as Ranunculus, Sagittaria, Limnophila, and Monochoria (shoots are partly in water and partly exposed out of water in air.
Some other hydrophytes of this category such as Cyperus, Rumex, Scirpus, Typha, etc., are called marsh plants and thick shoots are completely exposed to air, like the land plants.
Morpholog cal adaptations:
In most hydrophytes plant body is greatly reduced and the stem is modified to an all, relatively less branched rhizome or slender and much principles of Ecology branched system which does not offer resistance to the water currents. Plants like Lemna, Spirodela and Wolffia show extreme education in the size of plant body. Different parts of various lydrophytes exhibit following specialized features:
Due to availability of water in plenty, roots, the principal organs of water absorption, in such plants become of less significance. Roots may be either entirely absent as in Wolffia, Salvinia and Ceratophvllum or are poorly developed as in Hydrilla.
However, the emergent forms like Ranunculus and Typha contain well-developed and branched roots with distinct root caps. Root hairs and root caps are usually absent. However, in free-floating plants the root-hairs and root-caps develop prominently. In Eichhornia, root caps are replaced by “root pockets”.
Roots of hydrophytes are generally fibrous, adventitious, short, unbranched or poorly branched. In Lemna roots act simply as balancing and anchoring organs. Jussiaea repens possesses floating aerial roots besides normal roots.
In free-floating forms, stem is slender, floating horizontally on water surface as in Azolla, or thick, short, stoloni- ferous and spongy as in Eichhornia. In submerged hydrophytes such as Hydrilla and Potamogeton, it is long, slender, spongy and flexible. In hydrophytes which are rooted with floating leaves, stem is a rhizome which may be either small as in Nymphaea or well-developed as in Nelumho.
The leaves are generally thin, small and translucent in submerged aquatic plants. They are either long and ribbon-shaped as in Vallisneria or long and linear as in Potamogeton., or finely dissected as in Ceratophyllum. Floating leaves of plants as in Nymphaea and Nelumbo are large, flat and with their entire upper surfaces coated with wax and with petioles which are long, flexible and often covered with mucilage.
In Trapa and Eichhornia petioles are swollen and spongy. The leaves of Trapa and Utricularia are very much branched. Emergent hydrophytes such as Ranunculus, Sagittaria and Limnophylla heterophylla show heterophylly (i.e., occurrence of several kinds of leaves on the same plant) with submerged, floating and aerial leaves.
4. Flowers and Reproduction:
The flowers are not common in submerged plants. They are usually produced on the surface of water or above it but in many cases the fruit ripens under water (Pistia, Eichhornia and Vallisneria). Pollination and dispersal are performed by the help of water or aquatic insects and birds. Vegetative reproduction by runners, stolons, stem and root tubers, dormant apices, offsets, etc., is most extensive and common.
Anatomically, the hydrophytes show excessive development of parenchyma and elaborate system of air spaces (aerenchyma), and usually poor development of vascular and mechanical tissues.
The cuticle is absent or poorly developed. Stomata are completely absent in submerged leaves (e.g, Analiaris and Potamogeton); in floating forms as Vi7nphaea, they are confined only to the upper surface of leaf. The chlorophyll is found in all the tissues throughout the plant body. Even the roots of some plants as Azolla contain chlorophyll. Mucilage cells and muscle age canals are present which secrete mucilage to protect the plant body from decay under water.
The reserve food is in the form of starch grains which occur abundantly in pith and cortex. Oystoliths (sclereids) of various shapes are also common in leaves and other tissues. These provide some mechanical support to the tissues.
Physiologically, the aquatic plants exhibit a low compensation point and low osmotic concentration of cell sap. In submerged plants, the nutrients are absorbed through the plant surface. The gases are also exchanged from the water through epidermis.
The gases produced during photosynthesis and respirations are partly retained in the air chambers of aerenchyma to be utilized when required. There is no transpiration from the submerged hydrophytes, however, emergent plants and free-floating hydrophytes have excessive rate of transpiration.
Ecological Adaptations in Hydrocoles:
Aquatic animals usually have streamlined and flexible bodies which are provided with structures like flagella antennae or fins which help in movement. They show following two kinds of adaptations for aquatic environment:
Primary aquatic adaptations of hydrocoles: Such adaptations occur only in those animals such as fish which had never lived on land and their ancestors were also aquatic. They live permanently in water and have evolved from more primitive aquatic forms. They exhibit following adaptations:
1. The compression of head, body and tail into an elongated stream-lined form. There is no protuberance over the spindle- shaped body which would retard passage of the animal through water.
2. Development of paired and unpaired fins for swimming occurs. A fish bears two pairs of paired fins (pectoral fins and pelvic fins) and the unpaired median fins (one or two dorsal fins, an anal fin and a caudal fin). The pectoral fins and dorsal fin act as stabilizers or balancers and the caudal fin provides a forward push to the body.
3. Body muscles offish are arranged in the form of bundles separated by myocommata. Their contraction produces lateral undulations of “e flexible body and helps in locomotion.
4. They respire by gills which are specialized breathing organs to use gases solved in water.
5. Many teleost fishes have air-bladder which is filled with gas or air and acts like a necessary respiratory as well as hydrostatic organ. By controlling contents in the swim-bladder, fishes are able to maintain themselves at a particular depth.
6. Fishes possess lateral-line system which contains neuromast organs. The neuromast organs function as rheorecepotors which help in echo locating the objects in the aquatic medium.
7. The integument of aquatic animals is either rich in mucous glands or is protected with scales.
Secondary aquatic adaptations of hydrocoles:
Such adaptations occur in those animals which are lung breathers and whose ancestors were terrestrial but due to stress of circumstances such as inhospitable lands, where food was scarce or competitions were severe, they were forced to return again to the water.
Secondary aquatic adaptat lions occur in diverse groups of animal kingdom such as amphibians-frog, mud puppy (Necturus), Amphiuma and Siren ; reptiles—turtles, tortoise, snakes (Natrix. Hydrophis), crocodiles, gavials, alligators; birds-albatrosses, petrals, penguins, ducks; and mammals—whales, dolphins, porpoises some of them are amphibious and they spend part of their life on land and part in water. All of these animals exhibit following secondary aquatic adaptations:
1. Body shape is stream-lined, neck constric ion disappears and tail enlarges (e.g.., Getacca, Sirenia, Pinnipedia and Ichthyosa- uria). 2. Water-borne animals become larger in size because in these creatures energy, which in terrestrial forms is exhausted in gravitational forces, is turned into growth (e.g., whales, Hippopotamus). 3. The cranium is shortened; dorsoventrally flattened and more wider, while facial portion becomes an elongated and slender snout or rostrum (e.g., Porpoises and Ichthyosaurs). 4. The neck becomes short and immobile. In whales cervical vertebrae are coalesced to form a solid compressed mass of bone. 5. The bones become light sand spongy. 6. External ear (pinna) which may hinder locomotion tend to disappear in aquatic mammals. 7. Nostrils and eye: s also shifts on the apex of face. 8. Limbs become fleshy and -like expansions or paddles for swimming. In whales even the traces of limbs disappeared. 9. In aquatic mammals, the power of mastication of food is lost except in sea- cows and walruses. Therefore, the coronoid process of mandible is reduced. The dentition is homodont in whales but in sperm whales teeth are present only on one jaw. 10. In aquatic mammals, skin becomes smooth and naked, since the scales, hair, any other exoskeletal structures and skin glands like sweat and oil glands are lost. For the retention of body heat and for the reduction of specific gravity of beady, a thick fatty layer called blubber is formed below skin.
Ecologicial Adaptations in Mesophytes:
Mesophyes are those terrestrial plants which grow in moderately moist habitats and need well-aerated soils. Broad-leaved trees growing in wet depressions, along lakes and rivers, are mesophytes. They stand between hydrophytes and xerophytes and lack specific adaptations of them. Some of the significant morpho-anatomical features of mesophytes are as follows:
1. Root system is well developed. Roots are fairly branched and contain root caps and root- hairs. 2. Stems are generally aerial, solid and fairly branched. 3. Leaves are generally large, broad, and thin and varied in shapes. They are green and lack hairy or waxy coatings. 4. In all aerial parts, cuticle is moderately developed. 5. Epidermis is well developed and has no chloroplasts. 6. Stomata generally present on both the surfaces of leaves. 7. Mesophyll in leaves is differentiated into palisade and spongy parenchyma, with many intercellular spaces. 8. Well developed vascular and mechanical tissues are well differentiated. 9. Mesophytes may exhibit temporary wilting during noon hours.
Ecological Adaptations in Mesocoles:
Water economy is one of the major problems confronting terrestrial and semi terrestrial (amphibious) animals. The amount of water necessary to maintain life varies from species to species. It also varies with the stage in the life cycle of a single species. Many mechanisms have evolved which enable animals to maintain the critical level of water balance, thereby permitting some species to live in deserts, some in water and others in habitat intermediate in moisture content.
Adaptations of animals to meet the problem of dryness or desiccation are behavioural, anatomical and physiological. Two of the main effects of desiccation are of loss of body water and the drying out of the respiratory epithelium.
The body wall of some animals, such as land planarians, earthworms and amphibians, is an especially sensitive epithelium, which also functions as a respiratory membrane; if this membrane is selected to drying, the cells are permanently injured.
Therefore, to survive, these animals must always remain in moist, humid conditions to maintain a moist situation, many animals secrete a mucus coating and in unusually dry situations they may resort to behavioural adaptations such as burrowing, coiling or forming aggregates.
Arthropods have an external covering over the respiratory surfaces, and some of the predominantly terrestrial forms of marine Crustacea have an actual reduction in the respiratory surfaces (Gray, 1957).
Insects and myriapods have a particularly well- developed covering over the respiratory surface as well as a tracheal system Vertebrates have stratified skin with several cellular layers in addition to a well-protected respiratory surface.
Many of the structural modifications that prevent water loss are devices for effectively closing off the animals from the external environment. This is well illustrated particularly by the operculum of gastropod molluscs.
The serpulid polychaetes have one or more of the till filaments enlarged at one end, which are utilized to close the mouth of the lube when the animal withdraws into it during periods of stress. In insects a wax layer (in epicuticle) makes the integument relatively impermeable. The integument of terrestrial Crustacea is less permeable to water than that of predominantly littoral Crustacea.
Some animals have the ability to regulate the rate of loss of body water; an example is the amphibians, that arc able to slow the rate of water loss when they are first exposed to dry air (Thor- son, 1955, 1956). In the crab Carcinus maenas, the rate of water loss through transpiration is reduced by increased calcification of the exoskelcton. In intertidal-zone animals, the loss of water is accompanied by osmotic adjustments and regulation.
Some animals obtain needed water from food as partial replacement of water lost through excretion. The excretory process has .also evolved differently for the terrestrial animals. Aquatic species tend to excrete more non-protein nitrogen than do terrestrial species.
With the approach to terrestrialism, there also appears to be a general suppression of nitrogen metabolism rather than conservation of water, and the species shifts from ammono- telic to uricoletic excretion so that little or no water is excreted with uric acid. In the more terrestrial species the faecal pellets tend to be drier (see Vernberg and Vernberg, 1970).
Ecological Adaptations in Xerophytes:
Xerophytes are the plants which grow in dry habitats. Daubenmire (1950) has defined xerophytes as “plants which grow on substrata that usually become depleted of growth water to a depth of at least 2 decimeters during a normal season”. Thus, all plants of arid zone which are not confined to the margins of streams or lakes have been considered as xerophytes.
Likewise, in regions of heavy rain fall, xerophytes are represented by shallow- rooted plants of sandy soils, by plants of dry ridge tops, and by algae, mosses and lichens which grow on tree barks or rock surfaces, etc.
Types and adaptations of xerophytes:
Xerophytes are classified into following three categories:
They are short-lived annual plants of arid zones which complete their life-cycle within a very small period when sufficient moisture is available. They are also called ‘drought evaders” or ‘drought escapers”. Since these plants avoid dry seasons and thus escape dryness in external and internal environments.
In this case, the seed germination is soon followed by flowering without undergoing a phase of vegetative growth. The dry phase of the year is passed in the form of seed, e.g., Cassia tora, Argemone Mexicana, Solcnum xanthocarpum, Tephrosia purpurea, etc.
These xerophytes suffer dryness in external environment only. Their succulent, fleshy organs (stems, leaves, roots) serve as water-storage organs which accumulate large amounts of water during brief rainy seasons. In these xerophytes the succulence, i.e., development of water-storing parenchymatous tissues may occur in roots (Ceiba parvifolia, Asparagus), or leaves
(Aloe, Begonia, Peperomia, Salsola, Agave, Bryophylhmt, Yucca, Tredescantia, etc.,) or stems (Opuntia and other cacti, Euphorbia splendens). The rest of the plant body other than the succulent organs is generally very much reduced.
The roots system is shallow and roots arc fibrous and help in the absorption of traces of moisture from the soil surface (as dew and small amounts of rain). Among cacti and Euphorbias, the leaves are absent or modified to spines. In Opuntia, stein becomes fleshy, green, leaf-like (phylloclade) covered with spines, The cuticle is thick and vascular system is usually ill-developed.
These succulents have an advantageous physiological adaptation: they can utilize carbon dioxide during night through the CAM pathway (crassulacean acid metabolism). This adaptation is in response to the fact that the stomata remain closed during the day to avoid water loss and gaseous exchange can occur only during night when stomata are open.
3. True xerophytes or drought-resistants:
The plants which have the ability to maintain growth under critical dry conditions and high temperature are true xerophytes. Thus, these plants suffer from dryness both in their internal as well as external environments. The plants are woody trees, shrubs or herbs. The common examples are Acacia nelotica, Calotropis procera, Zizyphus jujuba, Capparis aphylla, Saccharum, Nerium, Prosopis, Casuarina, Alhagi, Salvaclora, etc.
To avoid dessication under drought conditions, they exhibit following morphophyiological adaptations: 1. They have very extensive root system; for example, in Alfalfa, Prosoj is and Calotropis, roots may be more than 125 feet long. 2. To minimize the rate of transpiration there are : (i) no leaves (Capparis) or small-sized leaves (Acacia neloticay, (m) dying back of leaves as in many grasses ; (iii) rolling and folding of leaf blades to protect stomata (Poa, Anmwphila, Agropyron and other grasses); (iv) delicate leaves which are shed under conditions of less supply of water; (v) heavy cuticular and epidermal layers ; (vi) sclerenchymatous hypodermis; (vii) waxy coating on leaves ; (viii) sunken type stomata on leaves; and (ix) rigid stems. 3. Leaf surfaces are generally shining and glazed to reflect light and heat. 4. The osmotic pressure of the cell sap is generally high. 5. The plasmodesmata are absent. 6. Plants contain excess of sulphahydryl molecules and a variety of sclereids in leaf tissues.
Halophytes are special type of xerophilous plants, which grow on physiologically dry soils such as saline soils with high concentration of such salts as sodium chloride, magnesium chloride and magnesium sulphate. Worming (1909) have classified halophytes into following four categories : 1. Lithophilous, 2. Psammophilous, 3. Pelophilous, and 4. Helophilous types, according to the substratum of rock and stones, of sand, of mud and of swamp, respectively. Helophilous halophytes include two types of vegetation: (i) salt-swamp and salt desert and (ii) littoral swamp-forest (mangroves).
In India, mangroves are common on seashores of Bombay and Kerala and in Andaman and Nicobar islands. They grow on water-logged soil, where habitat is characterized by sandy, swampy and saline soils, high precipitation, high atmospheric humidity and almost no fluctuation in temperature throughout the year. Common examples of mangrove plants are Sonneratia and Rhizophora mucronata.
Xeromorphic characters of halophytes are following: 1. Most of them are succulents; their leaves are evergreen, thin, small and leathery with water storage tissues, thick cuticle and well-developed palisade tissues. 2. Most halophytes produce special type of branched, negatively geotropic roots that come out of the mud surface to encourage the entry of oxygen gas. These roots are called pneumatophores and they possess breathing pores for gaseous exchange. 3. Some halophytes like Rhizophora produce stilt roots which have well-developed cork, lacunar primary cortex with sclereids and secretory cells and well developed mechanical and vascular tissues. 4. Their osmotic pressure values are very high (150 atms. in Salicornia). 5. Some halophytes as Rhizophora show vivipary, i.e., germination of seeds before they are shed from parent plant. 6. Psammophilous halophytes are Suaeda Fruticose and Salsola foetida.
Ecological Adaptations in Xerocoles (Desert Animals):
Like plants, desert animal are also adapted to conserve moisture and escape heat of shimmering sun. They fall into following two categories:
Drought evader xerocoles, like ephemeral plants, adopt an annual life style or go into aestivation or some other stage of dormancy. For eight or nine months, perhaps even several years, the eggs of insects and other invertebrates and insect pupae lie dormant.
When the rains arrive and plants nourish, the deserts swarm with insects—crickets, grasshoppers, ants, oees, wasps, butterflies, moths, beetles, etc. Young bees emerge from underground cells at the very time when the particular flowers on which they feed are in flower.
Some amphibians like spade- foot toads (Scaphiopus) an estimate for eight or nine months in an underground cell lined with a gelatinous substance that reduces evaporative losses through the skin.
They appear on ground when the rain fall saturates the earth, move to the nearest puddle, mate and lay eggs. Young tadpoles hatch in 1? to 2 days, rapidly nature (15 to 45 days) and metamorphose into functioning adults capable of their own retreats in which to aestivate until the next rainy season.
Likewise, birds nest during the rainy season, when food ii most abundant for the young. If extreme drought develops during the breeding season, some birds do not reproduce. Keast (1959) has reported that among some desert birds the endocrine control of reproduction depends on rainfall rather than day length.
A few birds such as swifts, poorwills and humming birds become torpid when food is scarce. Small rodents such as kangaroo or pocket mice, aestivate during periods of most severe drought.
Drought resister xerocoles are active all the year round and have evolved ways of circumventing aridity and heat through morpho-physiological adaptations or by modifying their feeding and activity patterns (i.e., behavioural adaptations). Some animals simply avoid the heat by adopting nocturnal habits and remaining underground or in the shade during the day.
Some desert rodents that are active by day periodically seek burrows and passively lose heat through conduction by pressing their bodies against the burrow walls. Some birds such as poor will (Phaiamoptilus nuttallii) and humming birds and bats go into a daily torpor.
The large ears of some desert animals such as jackrabbits (Lepus) and kit fox (Vulpes velox) may serves to reduce the need of water evaporation to regulate body heat. The ears may function as efficient radiators to the cooler desert sky, which on clear days may have a radiation temperature of 25C below that of the animal.
By seeking shade, where the ground temperatures are relatively low and solar radiation is screened out, and sitting in depressions, where radiation from the hot ground surface is obstructed, the Lepus could radiate 5kcal a day through its two large ears (400 cm2). Such a radiation heat loss minimizes the heat loss through water evaporation (sweat glands) (Schmidt-Nielsen, 1964).
The kangaroo rat, which seals its burrow by day and thus keeps its chamber moist, can live throughout year without drinking water. Similar adaptation occurs in the jerboas and gerbils of Africa and the Middle East and the marsupial kangaroo mice and pitchi of Australia.
These animals feed on dry seeds and dry plant material even when succulent green plants are available. The kangaroo rat obtains water from its own metabolic processes and from hygroscopic water in its food. To conserve water the animal remains in its burrow by day, it possesses no sweat glands, its urine is highly concentrated and its faeces is also dry.
Large desert animals such as the camel can use water effectively for evaporative cooling through the skin and respiratory system because their low surface-area-to-bouy-size ratio and lower internal heat production result in slower accumulation of heat.
The camel not only excretes highly concentrated urine but can with stand dehydration upto 25 per cent of body weight, and it lose water from body tissues rather than from blood (Schmidt-Nielsen, 1959). Body temperature of camel is labile, dropping to 33.8°C over night and rising to 40.6°C by day, at which point the animal begins to sweat.
The camel accumulates its fat in the hump rather than over the body. This speeds heat flow away from the body and its thick coat prevents the flow of heat inward toward the body.
More extreme adaptation to aridity occurs in some African antelope such as Oryx. Many African ungulates migrate to escape the heat and dryness, but the Oryx does not migrate. Its water requirements’ are low because it stores heat in its body during the day. This causes a substantial rise in body temperature (hyperthermia) (Taylor, 1969).
The Oryx further reduces daytime evaporative loss by suppressing heating. It pants only at very high temperatures. It reduces its metabolic rate and conserves water by decreasing the rate of internal production of calories, thus reducing the amount of evaporative cooling. These are daytime adaptations.
By night the Oryx decreases its non-sweating cutaneous evaporation by 60 per cent and its metabolic rate by 60 per cent. Its respiratory rates are proportional to its respiratory efficiency and inversely proportional to body temperature. A cool animal breathes more slowly than a warm one, thus using a greater portion of inspired oxygen.
With a lowered night-time body temperature, the saturation level for water vapour in the exhaled air is lower. The animal normally does not drink water; it exists on metabolic water and by feeding on grasses and shrubs, many of them are succulent.
Some desert birds, like marine birds, utilize a salt gland to help in the maintenance of water balance. Some desert birds like black-throated sparrow (Amphispiza hilincala) of North America, budgerigar and zebra finch of Australia, and grey-backed finch lark of Africa feed on dry seeds and when have some access to green vegetation, they do not drink water.
These birds can survive indefinitely without drinking by reducing the water content of its excreta from about 81 percent to 57 per cent (Smyth and Bartholomew, 1966; Dawson and Bartholomew, 1968: and Serventy, 1971).