12th Class Biology Organism And Environments Environment


Category : 12th Class

The environment is the aggregate of all those things and set of conditions which directly or indirectly influence not only the life of organisms but also the communities at a particular place. Any external force or influence, which surrounds and affects the life of a plant in any way, becomes a factor of its environment. These factors are called environmental factors and may be living (biotic) as well as non-living (abiotic). The abiotic factors affect the structure, life history, physiology and behaviour of organisms. The biotic factors mostly influence growth and reproduction. The environmental conditions which influence the life and development of plants, each part of the environment is called ecological factors. Ecological factors are grouped into four main classes (ecological factors) which are as follows :

Climatic factors : The study of climatic factor is known as climatology. The chief climatic factors are :

(1) Water : Rainfall is the chief source of soil moisture. Water exchange between earth surface and atmosphere is called hydrological cycle. Humidity of the air is expressed in terms of relative humidity. It is measured by hygrometer (Psychrometer). Epiphytes and cryoptogamic plants grow in those regions where relative humidity is high.

(2) Light : Light (solar radiations) is a very important ecological factor as it is the source of energy to the whole biosphere. It enters the biosphere through the process of photosynthesis performed by green plants and other autotrophs. Here organic food is manufactured from inorganic raw materials. Solar energy is changed into chemical energy of food. The radiant energy of sunlight carries out all important functions, without this life except few bacteria would disappear. On this basis of relative light requirements and the effect of light on the overall vegetative development, plants are classified ecologically into following categories :

(i) Heliophytes are popularly called sun plants because they grow in open in full sunlight. They possess a number of characteristics like.

(ii) Sciophytes are shade plants which grow in areas having moderate to low intensity light, as below the shade of other plants. Optimum growth occurs with light of 10-30% of full sunlight.

The plants grow in total darkness are called etiolated (Long, thin, weak and yellow in colours).

(3) Temperature : Temperature influences every reaction and activity of organisms. Temperature shows daily as well as annual variations. The phenomenon of change of temperature between day and night and in different seasons of the year is called thermoperiodicity. It is of two types, diurnal (or daily) and seasonal (or annual). Thermoperiodicity determines periodical phenomena like seed germination, stem growth, flower formation, fruiting, dispersal, maturation of gonads, breeding, egg laying, etc. Low night temperature is suitable for seed germination in many plants, e.g., Rumex, Asclepias. Cooler nights also help in increasing size of tuber in Potato and fruit setting in Tomato. Organisms adapted to live at relatively constant temperature during the whole year are known as stenothermal, e.g., many palms, corals, snakes and some fishes. The organisms which can tolerate large changes of temperature are called eurythermal, e.g., Artemesia, Cyclops, Toad, Wall Lizard.

On the basis of temperature the plants are classified as below :

(i) Megatherms or Climate or Tropical : The vegetation growing in the condition in which high temperature prevails throughout the year \[(30-40{}^\circ C).\] The dominant vegetation is tropical rain forest.

(ii) Mesotherms : Climate-subtropical, the high and low temperature alternates. The dominant vegetation is tropical decidous forest type. Those plants in which leaf fall takes place once in a year are called decidous plants e.g., Ficus religiosa (Sacred tree).

(iii) Microtherms : The vegetation growing in the low temperature \[(10-20{}^\circ C)\] condition. (The temperature remains low throughout the year). The vegetation is mixed coniferous forests type (Taiga).

(iv) Hekistotherms : The vegetation growing in the very low temperature \[(0-10{}^\circ C)\] conditions. The dominant vegetation is Alpine vegetation (Tundra).

The plants growing at very low temperature are called cryophytes or psychrophytes.

(4) Wind : High wind velocity causes soil erosion, breakage and up rooting of trees. Most of the pollutants are dispersed through the medium of air. Wind do harm is blossom trees because it prevents working of insects.

Wind modifies the humidity. Dry winds cause dwarfing of plants. Wind helps in pollination, dispersal of fruits and seeds and prevents frost damage. If the areas subjected to strong winds the leaves of plants become small and rolled and these plants develop an overall shape that offer resistance to wind.

Sometimes shrubs and trees are planted to protect the field against wind. Such structures are known as wind breaks or shelter belts. These plants (Trees) are planted at 90° to the wind velocity.

(5) Atmospheric humidity : Moisture in form of invisible vapours in atmosphere is called humidity.

Atmospheric humidity is generally expressed in terms of relative humidity (R.H.) which is defined as, "amount of vapours in atmosphere as the percentage of total amount which the air or atmosphere can hold at the existing temperature." Rate of transpiration is inversely related to relative humidity. Area having high humidity have dense vegetation due to lower rate of transpiration. The areas with low relative humidity are dry. They produce sparse vegetation due to high rate of transpiration.

R.H. is influenced by environmental conditions like solar radiation, temperature, wind, etc. If temperature is high, R.H. is low and if temperature is low, R.H. is high.

(6) Atmosphere

Gaseous cover over the earth surface is called atmosphere. Earth's atmosphere is about upto height of 300 km, out of which 95% of gases is upto height of 20 km and rest 5% in next 280km.

Earth's atmosphere is having a mixture of gases out of which important ones are :




78.08% (by volume)



20.84% (by volume)



0.93% (by volume)



0.03% (300 ppm)


Earth's atmosphere is divided into four different zones from below to above :

(i) Troposphere : First 20 km of earth's atmosphere is called troposphere in which different physical phenomena, i.e., lightening, thundering and cloud formation occur.

(ii) Stratosphere : About 30 km zone over troposphere is called stratosphere.

Temperature increases in this zone (upto 90°C) due to ozone formation. In this zone under the influence of ultra-violet rays, this ozone layer is formed, which prevents entry of harmful UV-rays to reach the earth surface.

(iii) Mesosphere : Next 40 km zone above stratosphere is called mesosphere. Last limit of mesosphere is called mesopause.

(iv) Ionosphere : Uppermost zone of atmosphere is ionosphere and in this zone all the gases are in ionized form.

(7) Fire

Fire may be man caused or natural. It is basically of three types :

(i) Surface fire (ii) Ground fire (iii) Crown fire

Such plants which grow well in fire burnt condition are called pyrophytes or fire loving plants.

Topographic or Geographic factor : Topographic factors are concerned with the physical geography of the earth in an area. The chief topographic factors are as follows :

Micro climate refers to local combinations of factors such as wind, rate of evaporation, humidity, temperature which differ from regional climate.

(1) Altitude : Height of mountain chains. 300 feet rise in altitude decreases temperature by 1°F. There is also increase in humidity and wind velocity. Slope winds are local wind movements which are uphill during the daytime and downhill at night. Shade caused by mountains reduces the total sunlight falling on an areas. Sunlight has more of ultraviolet rays. In high mountains situated in tropical areas, all the vegetation zones are found - tropical, subtropical, temperate and alpine. With the increase in altitude climate changed as decrease in temperature, increase in humidity, increase in precipitation and increase in wind velocity.

Generally the vegetation that develops on base of mountain to top is Tropical\[\to \]Temperate\[\to \]Taiga\[\to \]Tundra. Species diversity generally increase as one proceeds from high altitude to low altitude and from high latitude to low latitude.

(2) Steepness of the slopes : Steep slopes cause fast running of water which result in erosion and do not permit the accumulation of humus so the soil becomes denuded. In such soil plants can not grow properly and vegetation changes to xerophytic plants.

(3) Exposure of slopes : Exposure of slope to sun and wind affects very much the kind of plants growing there. Generally the slopes exposed to sun and wind supports vegetation. That’s why green houses and hot beds are always built in a way to face sun or southern slopes which receive greater amount of solar energy.

(4) Direction of mountains chain : Mountains steer or deflect winds into different directions. Outer Himalayas show frequent rains with luxuriant vegetations while the middle and inner Himalayas are dry with poor vegetation. The southern slopes of Himalayas e.g., Kullu valley are directly exposed to sunlight and has luxurient mesophilous vegetation due to mansoon wind. Where as Northern slopes of Himalayas e.g., Lahul valley exposed to weak light and strong dry wind, thus they have xerophilous vegetation.

Edaphic factor : The study of soil is called edaphology or pedology. The soil can be defined as “the upper crust of earth surface in which plants roots are anchored.” The term soil is derived from the Greek word solum.

(1) Soil formation : It is derived from rocks by weathering which is of three types :




(i) Chemical weathering : It is caused by oxidation, hydrolysis or carbonation.

(ii) Mechanical weathering : It is caused by living organisms, e.g., lichens, grazing animals or earthworm.

(iii) Physical weathering : It is caused by water, wind, gravity, glaciers, etc.

Weathering results into conversion of rocks to small fragments. Humus accumulated and now this can be called as soil. The development of soil is called pedogenesis. Soil is of two types :

(a) Residual soil : If the soil remain at the same place where it is formed.

(b) Transported soil : This soil brought from their place of origin to other place by some agents. It may be :

Alluvial soil                :       Carried by running water (rivers).

Colluvial soil              :       Carried by gravity.

Eolian soil                   :       Carried by wind.

Glacial soil                  :       Carried by glacier.

The soils of planes of India is mainly alluvial. In India the principal residual soil types are :

(a) Reddish soil of Vindhyas and South.

(b) Black soils of South West India.

(c) Calcareous soil : With \[20%\text{ }CaC{{O}_{3}}.\]

(d) Laterite soil : Oxides of iron and aluminium.

(e) Peat soil : With high percentage of humus 90%.

(f) Black soil : Predominantly with clay and humus (very fertile because most of minerals are present in it).

(2) Soil profile : A fully formed soil shows different layers called horizons. The sequence and nature of these layers is called soil profile (Cross section of soil) which consist of following horizons.



(i) Horizon ‘O’ : It is uppermost horizon made of organic matter. It has both fresh or nondecomposed as well as partially decomposed matter. It consist of following two sub-layers :

(a) \[{{O}_{1}}\]region (Aoo) : It is uppermost layer which consists of freshy added organic matter such as dead leaves, branches, flowers and fruits.

(b) \[{{O}_{2}}\] region (Ao) : It is present below \[{{O}_{1}}\] region. It consists of organic matter which is in different stages of decomposition.

(ii) Horizon ‘A’ : It is rich in mineral elements. A large amount of completely decomposed organic matter is present in this region.

(iii) Horizon ‘B’ : It is dark in colour due to accumulation of leached substances like clay, iron and aluminium from horizon. So it is called as zone of accumulation or zone of illuviation.

Horizon ‘O’, A and B are together called as top soil.

(iv) Horizon ‘C’ : It consists of partially weathered parental rock material. It is called as sub soil.

(v) Horizon ‘R’ : It is the lowermost layer of soil which consist of bed rocks (unweathered).

(3) Composition of soil : The garden soil is made up of :

(i) Mineral matter (40%) : They are derived from rocks (by disintegration). The soil, derived from lime stone, is called chalky soil.

Sandy soils have more coarser particles and lower water holding capacity and better aeration. Sand is most porous. Clayey soils have fine particles which have high water holding capacity and very poor aeration. Clay is least porous (water logged). Loam (50% sand + 25% clay + 25% slit) are best for plant growth.

The best apparatus used to analyse the soil is sieving.

(ii) Organic matter : Humus is total organic matter in the soils. It is rich in N P K. The humus is formed from decay and decomposition of dead plant and animal matter. It is in colloidal state and increase water holding capacity of the soil. The formation of humus is called humification which is caused by microbial activity.

The three distinct layers of humus in soil of forests are :

(a) Litter : All dead fresh organic matter fallen (undecomposed) recently to the ground is called litter.

(b) Duff : The layer, where decomposition is just started, is called as duff as duff layer. Partially decomposed litter is called duff.

(c) Leaf mold or Real humus : When the litter is modified into dark, finely divided, amorphous organic matter by the activities of micro-organisms living in soil is called humus. Humus is maximum in peat soil (90%).

(iii) Soil solution : The soil solution is the primary source of inorganic nutrients for plants. Soil solution helps in exchange of ions. pH of fertile soil is 6 to 7. pH below 5 inhibits bacterial activity. The plants prefer to grow in acidic soil are called oxylophytes e.g., Drosera. The plants prefer to grow in alkaline soil are called halophytes.

The soil rich in nutrients is called eutrophic and soil with less amount of minerals is called as oligotrophic.

(iv) Soil air : \[20-25%\] air or \[{{O}_{2}}\] is necessary for proper growth of plants. The well aerated soil support the plant growth well because :

(a) Root respiration increases.

(b) The capillary potential of the soil increases.

(c) The accumulation of \[C{{O}_{2}}\]could not take place.

(d) The root growth increases.

(e) Poor soil aeration supresses root hair development and may reduce the rates of absorption of water and minerals.

(v) Soil micro-organims : Soil contains a number of organisms. They are classified into four groups – microflora, macroflora, microfauna and macrofauna.

(a) Microflora : It consists of microscopic nonphagotrophic organisms.

(b) Macroflora : It consists of those fungi which form fructifications, e.g., mushrooms, tubers, truffles. The fungi are otherwise saprohytic in nature.

(c) Microfauna : It consists of microscopic phagotrophic microorganisms like protozoans (e.g., Amoeba, Arcella), rotifers, nematodes (e.g., Rhabditis).

(d) Macrofauna : They are animals residing in the soil. The common ones are insects (ants, beetles, mites, termites), spiders, millipedes, earthworms, snails and burrowing vertebrates.

Biotic factor : Living organisms living together influence each other's life and these living organisms constitute biotic factors. In these biotic interactions both the organisms may be benefitted or one is benefitted or one is benefitted while another is harmed or both are harmed. These biotic interactions are of two main types :

(1) Positive interactions : Where both the organisms or one organism is benefitted. The main types of positive interactions are :

Mutualism or Symbiosis : Here both the organisms in association are mutually benefitted and further this association is obligatory, i.e., necessary for existence of both organisms. The term symbiosis or mutualism was given by DeBary.

Important examples of symbiosis are :

(i) Dispersal of fruits and seeds by living organisms.

(ii) Pollination by insects, animals, etc.

(iii) Symbiotic nitrogen fixation : In the root nodules of legumes and some non-legumes, Rhizobium bacteria (symbiotic) are present which obtain food and shelter from parent plant and in turn fix atmospheric nitrogen.

(iv) Mycorrhiza or Mycorrhizal association : Association between roots of higher plants and fungal hyphae is called mycorrhiza. It is of two types :

(a) Ectotrophic or Ectophytic mycorrhiza : In this association fungal hyphae are on surface of roots, e.g., in Pinus.

(b) Endotrophic or Endophytic mycorrhiza : Here fungal hyphae are inside the root tissue, e.g., in orchids.

(v) Lichens : These are composite plants in which algal partner and fungal partner are mutually associated.

(vi) Myrmecophilly : Association between ants and higher plants is called myrmecophilly, e.g., jamun, litchi, mango, etc., where ants live. The ants get their food from the plants and in turn act as body-guards.

(vii) Zoochlorellae : Unicellular green alga Chlorella vulgaris lives in gastrodermal cavity of Hydra. The alga gives food and oxygen to Hydra and Hydra in turn gives shelter and nitrogenous substances to alga.

Commensalism : It is the relationship between two living individuals of different species in which one is benefitted while the other is neither harmed nor benefitted except to negligible extent. e.g., epizoic algae, epiphytes and parasitic vascular plants. Jackals follow a lion or tiger while arotic fox follows a seal for obtaining food from pieces or bits left by the predators, e.g. :

(i) Epiphytes or Aerophytes : These are vascular plants which are not rooted in soil and grow upon other plants for support (but not food), e.g., some orchids like Vanda, Dendrobium, etc., members of family Bromeliaceae (bromeliads), Dischidia (here a pitcher like structure is present for accumulation of water), Asplenium (bird's nest epiphyte), etc.

Three types of roots are present in epiphytes :

(a) Clinging roots for attachment.

(b) Absorptive roots for absorption of nutrients and minerals from organic matter accumulated in crevices of bark.

(c) Aerial roots possess a special tissue having thickenings called velamen, which help in absorption of moisture from atmosphere.

(ii) Lianas : These are vascular plants rooted in soil and get support of other plants or objects for their erectness.

Most common lianas in Indian tropical forests is Bauhinia vauhilii.

(iii) Rhizosphere, Rhizoplane, Phyllosphere and Phylloplane : Soil zone around the roots in which a large number of microbes are present due to secretion of sugars, etc., by roots is called rhizosphere. The root proper surface is called rhizoplane. Similarly air zone around leaves having good population of microbes due to secretion of volatile substance by leaves is called phyllosphere and leaf proper surface is called phylloplane.

Some workers consider these as example of mutualism.

Protoco-operation : It is interaction between two living organism of different species in which both are mutually benefitted but they can live without each other. e.g., tick bird ox pecker and Rhinoceros.

(2) Negative interactions : Here one or both organism in association are harmed. Important examples of these interactions are :

Parasitism : A parasite is an organism which lives in constant association with host and gets its food directly or indirectly without killing the host. This phenomenon is called parasitism. In plants four major types of parasites are present.

(i) Total stem parasite, e.g., Cuscuta.

(ii) Total root parasite, e.g., Rafflesia and Orobanche.

(iii) Partial stem parasite, e.g., Viscum and Loranthus.

(iv) Partial root parasites, e.g., Santalum and Thesium.

Predation : A predator is an organism which gets its food from the host after killing it. It does not live in constant association with host. This phenomenon is called predation.

Important examples of predation are :

(i) Grazing and Browsing

(ii) Carnivorous or Insectivorous plants : These plants grow in marshy conditions where there is lack of nitrogen, so in order to fulfil their nitrogen requirement, they catch small insects by some special adaptations in them.

(iii) Predaceous fungi : In soil there are present some fungi like Dactylella, Dactylaria, Arthobotrys and Zoophagus, etc., which are called predaceous fungi.

Competition : It is a type of cold war in which both the organisms in association are harmed. Competition is for basic necessities of life. Competition may be interspecific or intraspecific but intraspecific competition is more severe because organisms of same species have similar basic requirements.

Amensalism : This is a type of association in which one organism in association is harmed and second is not affected.

Amensalism is of two types :

(i) Antibiosis : Some micro-organisms secrete certain chemical substance which kill or inhibit other micro-organisms. These substances are called antibiotics and phenomenon is called antibiosis.

(ii) Allelopathy : Some higher plants also secrete certain poisonous substances which inhibit the growth of other plants. This phenomenon is called allelopathy, e.g., roots of carrot grass or congress grass (Parthenium argentatum), which is most troublesome terrestrial weed in India secrete trans-cinnamic acid which checks the growth of other plants.

Man as biotic factor : Man is always most important biotic factor. He changes the environment by his activities regularly, e.g., by excessive cutting of trees, fire, domestication of plants and animals, by causing different types of pollution, etc.

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