This essay sample essay on Ailing Planet Earth offers an extensive list of facts and arguments related to it. The essay’s introduction, body paragraphs and the conclusion are provided below.
Plant Kingdom (Biology)| Close| | Study Material| | Division Algae Classification within Angiosperms * Artificial system of classification * Given by Linnaeus * Based on vegetative characters and androecium structures * Gave equal importance to vegetative and sexual characteristics * Natural system of classification * Based on morphology, anatomy, embryology, and phytochemistry * Given by George Bentham and Joseph Dalton Hooker * Phylogenetic system of classification – based on evolutionary relationship * Numerical Taxonomy * Based on all observable characteristics * Numbers and codes assigned to all characters Easily carried out using computers * Cytotaxonomy ? Based on cytological information such as chromosome number, structure, behaviour * Chemotaxonomy ? Based on chemical constituents of plant to resolve doubts and confusions Division Algae * Includes chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (freshwater and marine) organisms *
Some occur in association with fungi (lichens) and animals (on sloth bear). * Size ranges from microscopic unicellular forms such as Chlamydomonas to colonial forms such as Volvoxand to filamentous forms such as Ulothrix and Spirogyra. Massive plant-like bodies are seen in some marine forms (such as kelps). Economic Importance * Carbon dioxide fixation on earth is majorly carried out by algae. * Important as primary producers of energy-rich compounds Example ? Sargassum, Laminaria, and Porphyra used as food * Some brown and red algae species produce water-holding hydrocolloids. Example ? Algin (brown algae) and carrageen (red algae) * Agar produced by Gelidium and Gracilaria is used to grow microbes and in preparation of ice creams and jellies. * Chlorella and Spirulina are protein-rich unicellular algae, used as food supplements.
Why Earth Is Called Ailing Planet
They are also known as space food. * Major classes of algae: Chlorophyceae * Commonly called green algae * May be unicellular, colonial, or filamentous * Grass green in colour due to abundance of chlorophyll a and b * Chloroplast of most of the Chlorophyceae contains pyrenoids. * Pyrenoids ? Storage bodies containing proteins in addition to starch * Food storage occurs in the form of oil droplets in some algae. * Cells have rigid cell wall: inner layer made of cellulose, outer layer made of pectose Phaeophyceae (Brown algae) * Primarily marine forms * Show great variation in size and form Range from simple-branched, filamentous forms (Ectocarpus) to profusely branched forms such as kelps (may reach a height of 100 m) * Possess chlorophyll a, c, carotenoids, and xanthophylls * Vary in colour from olive green to various shades of brown (depending on amount of xanthophyll and fucoxanthin) *
Food stored as complex carbohydrates such as laminarin or mannitol * Vegetative cells have cellulosic wall covered on the outside by gelatinous coating of algin. * Cell contains a centrally located vacuole and nucleus in addition to plastids. Union of gametes takes place in water or within oogonium (oogamous species). * Gametes are pyriform (pear-shaped). Example ? Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus Rhodophyceae (Red algae) * Commonly called red algae due to the presence of red pigment, r-phycoerythrin * Mainly marine forms with bulk mass inhabiting warmer areas * Occur in well-lighted regions i. e. , close to the surface of water and also in deeper areas * Red thalli of most of these species are multicellular. Some have complex body organization. * Food is stored as Floridian starch similar to amylopectin and glycogen in structure.
Example ? Polysiphonia, Gelidium, Gracilaria, Porphyra Division Bryophyta * Known as amphibians of plant kingdom since they live on land, but depend on water for sexual reproduction * Usually occur in cool, damp, and shady areas * Play an important role in plant succession on bare rocks/soils * Plant body more differentiated than algae * Thallus-like plant body is attached to substratum by unicellular or multicellular rhizoids. * Lack true roots, stem and leaves; may possesses root-like, stem-like, and leaf-like structures * Sporophyte is dependent on gametophyte for food.
Hence, it remains attached to the gametophyte. * Few cells of sporophyte undergo meiosis to produce spores (haploid). * Spores germinate to form gametophyte. Economic Importance * Provide food for herbaceous mammals, birds, and insects * Peat provided by Sphagnum is used as fuel. * Sphagnum is also used as packing material in trans-shipment of living material because of their water-holding capacity. * They form dense mats on the soil and hence prevent soil erosion. * Mosses along with lichens form the pioneer community in land and desert succession. Classes of bryophytes Liverworts Grow in moist, shady habitats * Plant body is thalloid. * Thallus is dorsiventrally appressed to the substrate. * Leafy members have tiny leaf-like appendages on stem-like structures Mosses Gametophyte *
Predominant stage Sporophyte * More elaborate than liverworts * Consists of foot, seta, and capsule * Capsule contains spores. * Spores formed by meiosis * Elaborate mechanism of spore dispersal Example ? Funaria, Polytrichum, and Sphagnum Division Pteridophyta General Characteristics * The dominant plant body is sporophyte. * First terrestrial plants to possess xylem and phloem Found in cool, damp, shady places * Have well-differentiated true stem, leaves, and roots * Leaves may be microphylls as in Selaginella or macrophylls as in ferns. * Sporophytes bear sporangia, which develop in association with leaves called sporophylls. * In some pteridophytes, sporophylls form distinct, compact structures called strobili or cones (Selaginella, Equisetum). * Sporangia produce spores by meiosis in spore mother cells. * Spores germinate to form small, multicellular, free-living photosynthetic thalloid gametophyte called prothallus. *
Major classes: Gametophyte Require cool, damp, shady places to grow * Also require water for sexual reproduction * Zygote produces well-differentiated, multicellular sporophyte. Sporophyte * Example of heterospory ? Selaginella and Salvinia Division Gymnospermae * Word gymnosperms, gymnos ? naked, sperma ? seeds * Ovules not enclosed by any ovary wall * Seeds formed after fertilization are not covered (i. e. , naked). * Include medium-sized trees, shrubs, and tall trees * Contains the world’s largest plant Sequoia – the giant redwood * Plants have tap roots. Roots in some genera show symbiotic associations. Mycorrhiza shows association of fungi with Pinus roots. * Coralloid roots of Cycas show association withN2-fixing Cyanobacteria. * Leaves are well-adapted to withstand extreme conditions. In conifers, needle-like leaves with thick cuticle and sunken stomata reduce surface area and water loss. * Spores produced within sporangia, borne on sporophylls, which form strobili or cones * Male and female strobili may be borne on same tree (Pinus) or on different trees (Cycas). * Megaspore mother cell divides mieotically to form four megaspores. * Megaspore mother cell is a differentiated cell of nucellus.
Nucellus protected by envelopes is known as an ovule. * Male and female gametophytes do not have independent existence, hence remain within sporangia. * Steps in fertilization: Pollen grain released from microsporangium v Carried by air currents v Come in contact with ovules v Discharge of pollen content on mouth of archegonia v Fertilization v Formation of zygote v Development of naked seed * For Example ? Pinus and Cycas Division Angiosperms * Large group of plants inhabiting a wide range of habitats * The pollen grains and ovules are developed in structures called flowers. * Seeds enclosed by fruits Range from tiny, almost microscopic, specimens (Wolfia) to tall trees like Eucalyptus * Two main groups are: *
Monocotyledons ? having one cotyledon in their seeds * Dicotyledons ? having two cotyledons in their seeds * The male sex organ in a flower is a stamen. * Each stamen consists of: * a slender filament * an anther at the tip * The anther produces pollen grains by meiosis. * The female sex organ is a pistil or carpel * Each pistil consists of: * an ovary * a style * a stigma * The ovary encloses one or more ovules. * Within the ovule (the highly reduced female gametophyte) embryo sacs are present. Embryo sac is a seven-celled, eight-nucleated structure. Embryo sac contains * One egg cell * Two synergids * Three antipodal cells * One central cell * The polar nuclei fuse to form a secondary nucleus (diploid). * Pollen grains, after dispersal from anthers are carried by the wind or other agents to the stigma of the pistil; termed as pollination *
Fertilisation in angiosperms is termed as double fertilisation. * The synergids and antipodals degenerate after fertilisation. * The ovules develop to form seeds, and the ovaries develop into fruits. LIFE CYCLE OF AN ANGIOSPERM Plant Life Cycles There is alternation of generations between haploid gametophyte and diploid sporophyte in the life cycle of a plant. * In plants, both haploid and diploid cells can divide by mitosis. * Hence, there are two different plant bodies ? haploid and diploid. * The haploid plant body produces gametes by mitosis and represents a gametophyte. * Mitotic division is encountered in diploid cells when zygote divides by mitosis to produce sporophytic plant body after fertilization. * This sporophyte produces haploid spores by meiosis. * Spores in turn undergo mitosis to form haploid plant body.
Types of Life Cycles in Plants Haplontic Life cycle * In this, sporophyte is represented by one-celled zygote. * There is no free living sporophyte. * Zygote undergoes meiotic division to produce spores, which divide mitotically and form gametophyte. * Gametophyte is the dominant phase in this life cycle as it is dominant, free living, and photosynthetic. * Algae such as Spirogyra and some species of Chlamydomonas have this type of life cycle. Diplontic Life Cycle * In this case, diploid sporophyte is the dominant phase as it is free living and photosynthetic. * Gametophyte is single to few-celled. * Example ?
All seed-bearing plants, gymnosperms, and angiosperms, some algae-like Fucus Haplodiplontic Life Cycle * Intermediate condition * Both gametophyte and sporophyte are free-living and multicellular, but have different dominant phases. * In Bryophytes, haploid gametophyte is dominant, independent, and photosynthetic. It alternates with short-lived multicellular sporophyte totally or partially and is dependent on gametophyte for nutrition and anchorage. * In Pteridophytes, diploid sporophyte is dominant, independent, and photosynthetic. It alternates with short-lived haploid gametophyte, which is independent of sporophyte.