Current Affairs 12th Class

(3) Qualitative pollutant : The pollutant is a harmful substance, chemical or factor that does not occur in nature but is added by human beings, e.g., insecticide, herbicide. (4) Quantitative pollutant : It is a normal component of nature that becomes pollutant when its concentration becomes higher than the normal, e.g., nitrogen oxides, carbon monoxide. (5) Degradable pollutant : The pollutant degrades after some time either automatically (e.g., heat) or through the agency of microorganisms (= biodegradable, e.g., sewage, livestock wastes, market garbage). Biodegradable pollutants are easily disposed off or degraded through natural processes or waste treatment plants. They can be turned into a resource, e.g., compost, manure. (6) Non-Degradable pollutant : It is a pollutant which does not breakdown or get converted to harmless state by natural processes of dissipation, dispersal, oxidation or biological decomposition, e.g., DDT, glass, plastic. It is also known as conservative or persistent pollutant. Types of pollution (1) Natural pollution : It is pollution caused by natural sources, e.g., volcanic eruptions, release of methane by paddy fields and cattle, release of carbon monoxide by plants and animals, emission of natural gas, ozone, nitrogen oxides, soil erosion, dust storms, cosmic rays, ultra-violet rays, etc. (2) Anthropogenic or Man-made pollution : It is pollution caused by human activities like noise, automobiles, industries, smoking, pesticides etc. Anthropogenic pollution is often quite small in quantity as compared to natural pollution. Example, 0.05% of atmospheric pollution is man-made while 99.95% is natural pollution.

• On the basis of source of pollution, it can be :

(1) Air pollution : Air or atmospheric pollution is the addition of materials or chemicals into atmosphere in such concentration that they begin to exert adverse effect on human beings, other desirable species, human assets and resources. Total amount of air pollutants is estimated to be \[1\times {{10}^{12}}\] tonnes, out of which pollutants added by human activities are only \[5\times {{10}^{8}}\] tonnes or 0.05%. (i) Air Pollutants : The common air pollutants are : Dust, Smoke, Carbon monoxide \[(CO),\] Ammonia \[(N{{H}_{3}}),\] Sulphur dioxide\[(S{{O}_{2}}),\] Hydrogen sulphide \[({{H}_{2}}S),\] Nitrogen dioxide \[(N{{O}_{2}}),\] Hydrogen cyanide, Hydrogen fluorides, Chlorines, Phosgenes, Arsines, Aldehydes, Ozone, Ionising and radiations. \[C{{O}_{2}}\] is not a normal air pollutant. There is 0.03% \[C{{O}_{2}}\] in the air its higher percentage is the cause of green house effect. Types of air pollutants : It is of two types :   (a) Primary air pollutants : Air is polluted by poisonous gases and undesirable substances. They are released by burning fossil fuels. These substances are called primary air pollutants. The primary air pollutants are the following :

• Soot released from unburned fuel.
• Sulphur dioxide (SO2).
• Benzopyrene (hydrocarbon) released from cigarette smoke.
• Ammonia \[(N{{H}_{3}}).\]
• Oxides of nitrogen.
• Carbon monoxide (CO).
• Lead (Pb).

(b) Secondary air pollutants : Secondary air pollutants are poisonous substance formed from primary air pollutants. In bright sun light nitrogen, nitrogen oxides, hydrocarbons and \[{{O}_{2}}\] interact to produce more powerful photochemical oxidants like ozone \[({{O}_{3}}),\] peroxyacetyl nitrate (PAN), aldehydes, sulphuric acid, more...

The word ecosystem was coined by A.G. Tansley in 1935. According to Odum an ecosystem is the basic fundamental unit of ecology which includes both the organisms and the non-living environment each influencing the properties of the other and each is necessary for the maintenance of life. Structure : The structure of any ecosystem is formed of two components, namely :  (1) Abiotic factors : The abiotic factors of an ecosystem include the non–living substances of the environment. e.g., Water, soil, air, light, temperature, minerals, climate, pressure etc. The biotic factors of the ecosystem depend on the abiotic factors for their survival. (2) Biotic factors : The biotic factor include the living organisms of the environment. e.g., Plants, animals, bacteria, viruses etc. The biotic factors of an ecosystem are classified into three main groups, namely : (i) Producers : The organisms which carry out photosynthesis constitute the producers of an ecosystem. e.g., Plants algae and bacteria. (ii) Consumers : Consumers are organisms which eat or devour other organisms. The consumers are further divided into three or more types. They are primary consumers, secondary consumers and tertiary consumers. (a) Primary consumers : They eat the products like plants, algae and bacteria. The primary consumers are also called herbivores. (b) Secondary consumers : They kill and eat the herbivores. They are also called carnivores. As these carnivores directly depend on herbivores, they are specifically called primary carnivores. Fox, wolf, etc. are the secondary consumers in a terrestrial ecosystem. (c) Tertiary consumers : They kill and eat the secondary consumers. They are also called secondary carnivores. e.g., Lion, tiger, etc. (iii) Reducers or Decomposers : The decomposers are heterotrophs organisms that break up the dead bodies of plants and their waste products. They include fungi and certain bacteria. They secrete enzymes. The enzymes digest the dead organisms and the debris into smaller bits or molecules. These molecules are absorbed by the reducers. After taking energy, the reducers release molecules to the environment as chemical to be used again by the producers. (iv) Other heterotrophs (a) Scavengers or Detrivores : They feed on corpses, e.g., Vulture, Carrion Beetle. They help in quick disposal of dead bodies. In the process they also leave small fragments for decomposers. (b) Parasites : They obtain nourishment from a living host without capturing or killing the same. Parasites obtain food from all categories of organisms. Common parasites are bacteria, fungi some worm and some insects. Types of ecosystem The ecosystem may be large, as large as the world or small, as small as a cow dung ecosystem. The biosphere (The total life content of the world) is the major ecosystem. It comprises all other ecosystems. (1) Mega ecosystem : The biosphere is formed of four mega ecosystems. They are as follows : (i) Marine ecosystem : It is the largest ecosystem of earth. Fresh water ecosystem are two types : (a) Lotic : Running water ecosystem as river. (b) Lentic : Still water ecosystem such as pond or lake. more...

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cells into haploid gametophytic cells by meiosis and subsequent fusion of haploid gametes of opposite sex to form diploid zygote. The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants, all these steps of sexual reproduction occur within specialized reproductive organs, called the flowers. (1) Structure of the flower : Morphologically flower is a modified shoot meant for sexual reproduction of the plant. Typically, it is a condensed branch in which internodes have become condensed, bringing nodes very close to one another, and the leaves are modified to form floral whorl that directly or indirectly participate in the process of reproduction. The flower is commonly borne on short or long stalk called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus). (2) Parts of a flower : A typical angiospermic flower consists of four whorls of floral appendages attached on the receptacle : calyx, corolla, androecium and gynoecium. Of these, the two lower whorls (i.e., calyx and corolla) are sterile and considered as nonessential, accessory or helping whorls. The two upper whorls (i.e., androecium and gynoecium) are fertile and considered as essential or reproductive whorls. (3) Functions of a flower (i) Flowers are modifications of shoot to perform the function of sexual reproduction. The fertile leaves become microsporophylls (stamen) and megasporophylls (carpels) which bear anthers and ovules respectively. The anthers produce pollen grains and the ovules possess eggs. (ii) Flowers of most of the angiosperms are shaped variously to help diverse modes of pollination. (iii) Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and fertilization. (iv) The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after fertilization. (v) Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds. (4) Relative position of floral organs on thalamus : Depending upon the form of thalamus and the position of floral whorls with respect to the ovary, the flowers are of the following three types : (i) Hypogyny : In this case the thalamus is convex and ovary occupies the highest position on it. The outer three whorls, viz. sepals, petals and stamens inserted one above the other but below the ovary. Since the ovary lies above the other parts, it is described as superior and the rest of the floral whorls as inferior. A flower having hypogyny is called hypogynous. e.g., China rose, Brinjal, Mustard, etc. (ii) Perigyny : In some cases, the receptacle or the thalamus forms a swallow or deep cup-shaped structure around the ovary. The pistil is attached at the centre of the concave thalamus. The sepals, petals and stamens are attached at the margins of the thalamus, the flowers are said to be perigynous and ovary is half inferior or half superior. Different type of flowers show different degrees of perigyny. e.g., Rose, Pea, more...

The process of transfer of pollen grains, from an anther to the stigma of the same flower or of different flower. It is of two types : (1) Self pollination : This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types : (i) Autogamy : It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower. (ii) Geitonogamy : It is a kind of pollination in which the pollen from the anthers of one flower are transferred to the stigma of another flower borne on the same plant. It usually occurs in plants which show monoecious condition (unisexual, male and female flowers are borne on the same plant). Geitonogamy involves two flowers but these belong to the same parent plant. Merits

• Pollen grains are not wasted.
• The purity of the generation is maintained.

Demerits

• New and healthier varieties are not formed
• It results in weaker progeny, producing weaker seeds and plants.

Contrivances for self pollination : The major contrivances or adaptations which favours self pollination are :  (a) Bisexuality : Flowers should be bisexual or hermophrodite. (b) Homogamy : Anthers and stigma of the bisexual flowers of some plants mature at the same time. They are brought close to each other by growth, bending or folding to ensure self pollination. This condition is called homogamy. e.g., Mirabilis (Four O, clock), Catharanthus (= Vinca), Potato, Sunflower, Wheat, Rice, etc. (c) Cleistogamy : Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, e.g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicious, colourless and do not secrete nectar. (2) Cross pollination : Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy. Merits  

• Seeds are more and viable.
• Progenies are healthier.
• Adaptability is better.
• New varieties can be produced.

Demerits

• The process is not definite because plants depend on agencies.
• Large amount of pollen grains are wasted.

Contrivances for cross pollination : Nature favours cross  pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated. The main contrivances ensuring cross pollination are as follows : (i) Diclincy or Unisexuality : In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types :

• Monoecious plant : When male and female flowers are borne on the same plant. e.g., Maize, Cucurbits, Castor.
• Dioecious plant : When male and female flowers are borne on different plants. e.g., Carica papaya, Cannabis.

(ii) Dichogamy : In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. more...

The formation of fruits without fertilization is called parthenocarpy. Such fruits are either seedless or non-viable seeds. Parthenocarpy is of two types : (1) Natural parthenocarpy : When seedless fruits are produced without any special treatment from the ovaries in the absence of pollination and fertilization, the phenomenon is called natural parthenocarpy. e.g., Grapes, Banana, Pineapple and Noval oranges. (2) Induced parthenocarpy : When seedless fruits are produced by spraying the flowers with either water extract of pollen grains or growth promoting hormones such as Indole acetic acid (IAA), Naphthalene acetic acid (NAA), Gibberellic acid (GA), etc. the phenomenon is called induced parthenocarpy. e.g., Tomato, Black berry, Fig, Lemon, Apple, Orange, Pear. etc.

The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis. Microsporogenesis is well studied under following heads : (1) Structure of anther : The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores.     The pollen sacs are surrounded by following 4 layers : (i) Epidermis : This is the outermost single layered and protective. In Arceuthobium, cells of epidermis develops a fibrous thickening and the epidermis is designated as exothecium. (ii) Endothecium : Inner to epidermis, there is a single layer of radially elongated cells. Cells of endothecium develop fibrous thickening (made up of cellulose with a little pectin and lignin) which help in the dehiscence of anther. In between these cells, a few cells without thickening are also present. These thin walled cells collectively form the stomium. (iii) Middle layer : Three to four layers of thin walled cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells. (iv) Tapetum : This is the innermost layer of the wall. The cells are multinucleate(undergo endopolyploidy) and polyploid. Tapetal cells are nutritive. In these cells the Ubisch bodies are present which help in the ornamentation of microspore walls. A compound sporopollenin is secreted in the exine of microspore wall. According to Periasamy and Swamy (1966), developmentally the tapetum has dual nature. The tapetum is of two types (a) Amoeboid or Periplasmodial tapetum. (b) Secretory or Glandular tapetum. (2) Development of anther and formation of microspores (Pollen grains) : The young anther consists of homogenous mass of paranchymatous cells surrounded by epidermis. It soon becomes four lobed. In each of the four lobes, some of the hypodermal cells begin to act as archesporial initials. Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell. The primary parietal cell divides to form 3-5 wall layers, i.e., endothecium, middle layers and tapetum. The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes. Each microspore mother cell divides meiotically to form four haploid microspores or pollen grains and remains arranged in tetrads. The arrangement in the tetrads can be tetrahedral, isobilateral, linear, T-shaped and decussate. Now the microspores are separated from tetrad. In Drosera, Typha, Elodea, Hydrilla, etc. all the four pollen grains do not separate and thus form compound pollen grains. In the members of the family Cyperaceae (Cyprus), out of 4 pollen in a tetrad, 3 degenerate and one remains alive. So one meiosis produces one pollen. Sometimes more than four pollens are produced from one microspore mother more...

The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule. Megasporogenesis can be studied under following heads : (1) Structure of ovule (Megasporangium) : Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus.     The residual part of nucellus in the mature seed is called perisperm. In the centre of the nucellus is situated a female gametophyte known as embryo sac. Following are the conditions seen in ovule in relation to integuments : (i) Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons. (ii) Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons. (iii) Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc. (iv) Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of Euphorbiaceae. e.g., Castor (Ricinus). (v) Ategmic : In some parasites like Loranthus, Viscum, Santalum etc., there is no integument. Such an ovule is called ategmic. (2) Kinds of ovules : Depending upon the shape and orientation, the ovules of angiosperms are classified into following types :     (i) Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight line. This is most primitive type of ovules. e.g., Betel, Piper, Polygonum. (ii) Anatropous : The body of the ovule is completely inverted (turn at 180o angle ) so that micropyle and hilum come to lie very close to each other. e.g., 82% of angiosperm families. (iii) Hemianatropous : Ovule turns at 90o angle upon the funicle or body of ovule is at right angle to the funicle e.g., Ranunculus. (iv) Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down slightly. e.g., in members of Leguminosae and Cruciferae. (v) Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe e.g. Lemna, Poppy, Alisma. (vi) Circinotropous : The ovule is initially more...

The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884). (1) Germination of pollen grain on stigma and growth of pollen tube : Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity. G.B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two male gametes. Later, the vegetative cell degenerates. The pollen tube now reaches the ovule after passing through the style. (2) Entry of pollen tube into ovule : After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes : (i) Porogamy : When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily. (ii) Chalazogamy : The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. e.g., Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina. (iii) Mesogamy : The pollen tube enters the ovule through its middle part i.e., through integument (e.g., Cucurbita, Populus) or through funicle (e.g., Pistacia).     (3) Entry of pollen tube into embryo sac : The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus. The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion). The tip of pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of pollen tube enlarge and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid. (4) Double fertilization : The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger (1884). The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S.G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms more...

Endosperm is the nutritive tissue for the developing embryo and also the seedling. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion or fusion of a male gamete with secondary nucleus of the central cell. (1) Types of endosperm : On the basis of development, endosperm are of three types : (i) Nuclear endosperm : In the nuclear type of endosperm development, the primary endosperm nucleus divides by repeated mitotic free nuclear divisions without the formation of walls. It results in the formation of a large number of free nuclei in the central cell of the embryo sac. A big central vacuole develops in the embryo sac pushing all the nuclei to the peripheral cytoplasm. Finally cell wall formation takes place from the periphery of the embryo sac towards the centre leading to the formation of cellular endosperm tissue. In Coconut, the endosperm is multicellular in the outer part and free nuclear in the centre. Nuclear endosperm is the most common type of endosperm and mostly found in polypetalae. e.g., Cotton, Zeamays, Capsella etc. (ii) Cellular endosperm : In the cellular type of endosperm development, the first nuclear division of the primary endosperm nucleus is immediately followed by the wall formation. The first division results in the formation of two equal sized chambers : chalazal and micropylar chambers. The subsequent divisions are followed by regular cell wall formation. This type of endosperm formation is common in gamopetalae. e.g., Petunia, Datura. (iii) Helobial endosperm : In the helobial type of endosperm development, the endosperm is intermediate between cellular and nuclear types. The division of primary endosperm nucleus is followed by wall formation and as a result two chambers : micropylar and chalazal chambers, are formed. Generally the chalazal cell does not divide further and function as haustorium. Nucleus of the large micropylar cell divides by repeated free nuclear divisions and further development takes place in the same way as the nuclear endosperm. Helobial type of endosperm development is prevalent in monocotyledons. e.g., Erumurus. (2) Some terms related to endosperm (i) Ruminate endosperm : Mature endosperm with irregularity and unevenness in its surface is called ruminate endosperm. Rumination is caused by the activity of seed coat or by the endosperm itself. It is found in about 32 families of angiosperm. e.g., Annonaceae, Palmae, Myristicaceae, etc. (ii) Mosaic endosperm : In some cases, the tissue of endosperm is not homogeneous but there are patches of different colours. Such type of endosperm is called mosaic endosperm and was observed by Webber (1990) in Zea mays. In maize endosperm, red and white patches appear irregularly distributed. In Petunia and Tomato, endosperm shows two types of tissues – some consisting of diploid cells and some triploid cells. These two types of cells intermix to form mosiac. (iii) Xenia : The effect of pollen on endosperm is called xenia. This term was given by Focke (1881). e.g., Maize. (iv) Metaxenia : The effect of more...

(1) Development of embryo (Embryogeny) : The zygote after a period of rest develops into embryo. The process of development of mature embryo from diploid zygote is called embryogenesis. (i) In dicotyledons : The normal type of dicot embryo development has been studied in Shepherd's purse (Capsella bursapastoris) family Cruciferae. This is called as crucifer or onagrad type of embryo development. This development of embryo is endoscopic i.e., apex is downward or towards inside. The first division of zygote is transverse which produces a basal cell (cb) towards the micropyle and a terminal cell (ca) towards chalaza. The basal cell divides by transverse division and the terminal cell by a longitudinal division, so 4 celled T-shaped proembryo is produced. The two basal cells divide by transverse division and form 6-10 celled suspensor. The upper most cell of the suspensor is vasicular cell and lowest cell is called hypophysis which forms radicle and root cap. The two apical cells first divide by longitudinal division (at right angle to first one) and then by transverse and periclinal division. So sixteen celled globular embryo is produced. Due to differentiation of cotyledons globular embryo becomes heart shaped. Mature embryo in dicots consists of two lateral cotyledons, terminal plumule or stem tip and radicle or root tip.     (ii) In monocotyledons : The normal type of monocot embryo development has been studied in Sagittaria sagittaefolia. The early development of dicot and monocot embryos is similar upto globular stage. Later on differentiation starts. Suspensor is single celled and vascular. There is only one terminal cotyledon called scutellum (shield shaped). In grasses the second cotyledon is reduced called epiblast. The basal cell (cb) divides by a transverse wall into two cells ­– ci and m. The cell ci divides once again to form n and n’ cells. Of these n’ is the outermost which develops into suspensor. The cell n forms parts of root cap the cell m contributes to the remaining part of root cap and a part of the radicle. The terminal cell (ca) divides by two vertical walls, at right angles to one another. This results in the formation of a quadrant (q). Cells of the quadrant divide periclinally differentiating into the peripheral cells and the inner group of cells. The repeated divisions in both peripheral and central group of cells results in the formation of two regions –l and l’. Region l produces the lower part of cotyledon while upper part of cotyledon, hypocotyl and plumule are formed by l’ region.     (2) Polyembryony : Occurrence of more than two embryo in the seed is known as polyembryony. It was discovered by A.V. Leeuwenhock (1719) in Citrus. It may be : (i) Cleavage polyembryony : Due to cleavage of zygote or proembryo into two or more embryos and each split part develops into more...