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Premium member Presentation Transcript Life –Cycle patterns in Plant Kingdom : Life –Cycle patterns in Plant Kingdom AMIT SARAF Dept of Botany, Elphinstone College, Mumbai, India 400032 firstname.lastname@example.org AMIT SARAF,Elphinstone College Life – Cycle (Alternation of generation) : Life – Cycle (Alternation of generation) Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing eggs). AMIT SARAF,Elphinstone College Sporophytic Phase vs Gametophytic Phase : Sporophytic Phase vs Gametophytic Phase Within the plant kingdom the dominance of phases varies. Nonvascular plants, algae, mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns , gymnosperms to angiosperms. AMIT SARAF,Elphinstone College Homospory and Heterospory : Homospory and Heterospory Homosporous plants produce bisexual gametophytes which develops from the spores of same size Ferns are a classic example of a homosporous plant. Heterosporous plants produce separate male and female gametophytes and their gametophytes germinate from spores of different sizes. The male gametophyte produces sperm, and is associated with smaller or microspores. The female gametophyte is associated with the larger or megaspores AMIT SARAF,Elphinstone College Plant Adaptations to Life on Land : Plants faced many challenges on terrestrial environment. Drying out. living in very moist environments (bryophytes) production of a waterproof surface (the cuticle in plants, cork layers and bark in woody trees). Gas exchange. The plant solution to gas exchange is a new structure, Stomata. The guard cells allow gas exchange by diffusion through the stomata. Support. Adaptations to this include specialized plant cells and mechanical tissues that support the plant. Conduction. Adaptations to this include the specialized conducting tissues xylem and phloem in plants. Some multicellular algae and bryophytes also have specialized conducting cells. Reproduction. developing specialized delivery systems like pollen tubes to get the sperm cells to the egg. Plant Adaptations to Life on Land AMIT SARAF,Elphinstone College Life cycle patterns in Algae : Life cycle patterns in Algae HAPLONTIC Somatic phase (plant) is haploid (gametophytic) while the diploid phase (sporophytic) is represented by zygote. Gametophyte (n) alternates with one-celled zygote or sporophyte (2n) e.g. Spirogyra, Oedogonium Chara. DIPLONTIC Somatic phase(plant) is diploid (sporophytic) while the haploid(gametophytic) phase is represented by gametes. Zygote develops into diploid sporophytic plant by mitosis e.g. Fucus, Saragassum AMIT SARAF,Elphinstone College Life cycle patterns in Algae : Life cycle patterns in Algae ISOMORPHIC Morphologically identical somatic phases showing alternation of generation Here gametophytic and sporophytic phases are exactly similar e.g. Ulva, Ectocarpus,Dictyota HETEROMORPHIC Alternating haploid (n) and Diploid (2n) somatic phases are morphologically different. e.g. Laminaria, Urospora. AMIT SARAF,Elphinstone College Life cycle patterns in Algae : Life cycle patterns in Algae HAPLOBIONTIC 3 phases in life-cycle, 2 haploid & 1diploid e.g. Batrachospermum, Coleochaete In Batrachospermum, two morphologically different haploid phases ( gametophyte & carposporophyte) alternates with Zygote (2n) DIPLOBIONTIC 3 phases in life-cycle, 2 diploid & 1 haploid e.g. all Rhodophyceae except Nemalionales. Polysiphonia, shows Triphasic life cycle involving alternation of two diploid phases (carposporophyte & tetrasporophyte) with one haploid phase AMIT SARAF,Elphinstone College Bryophytes : Bryophytes Bryophytes are small, nonvascular plants that first evolved approximately 500 million years ago. The earliest land plants were most likely bryophytes. Bryophytes lack vascular tissue and have life cycles dominated by the gametophyte phase The lack of conducting cells limits the size of the plants, generally keeping them under 5 inches high. Roots are absent in bryophytes, instead there are root-like structures known as rhizoids. Bryophytes include the hornworts, liverworts, and mosses. AMIT SARAF,Elphinstone College Slide 10: Bryophytes The moss life cycle The haploid gametophyte phase is free-living and photosynthetic. The diploid sporophyte grows from and is nourished by the gametophyte. AMIT SARAF,Elphinstone College Slide 11: Vascular plants first developed during the Silurian Period, about 400 million years ago. The earliest vascular plants had no roots, leaves, fruits, or flowers, and reproduced by producing spores. Cooksonia is a typical early vascular plant. It was less than 15 cm tall, with stems that dichotomously branched. Dichotomous branching appears a primitive or ancestral trait in vascular plants. Some branches terminated in sporangia that produced a single size of spore. Development of vascular plant AMIT SARAF,Elphinstone College Slide 12: Rhynia gwynne-vaughanii (L) stem cross section from the Rhynie Chert in Scotland Reconstruction of Aglaophyton major Rhynia major, has since been reclassified as Aglaophyton major. Primitive vascular plants AMIT SARAF,Elphinstone College Slide 13: Primitive vascular plants Psilotum is a "primitive" vascular plants similar to the fossil rhyniophytes and psilophytes. They lack true leaves and roots (like the Bryophytes), and thus represent the simplest of vascular plants AMIT SARAF,Elphinstone College Development of strobilus : A strobilus is a series of sporangia and modified leaves closely grouped on a stem tip. The leaves in strobili are soft and fleshy as opposed to the hard, modified leaves in cones. Strobili are characterized by a central axis (anatomically a stem) surrounded by spirally arranged or decussate structures that may be modified leaves or modified stems. Leaves that bear sporangia are called sporophylls, while sporangia bearing stems are called sporangiophores. Recent phylogenetic reconstructions suggest that axially condensed flower-like structures evolved in seed plants from either simple or compound strobili. The simple-strobilus model of flower evolution is widely applied to the angiosperm flower, interprets the inflorescence as a compound strobilus. Development of strobilus AMIT SARAF,Elphinstone College THE STROBILUS MODEL of flower evolution : THE STROBILUS MODEL of flower evolution AMIT SARAF,Elphinstone College Presumed evolution of sporophyll : Leaves that contained vascular tissue- sporophylls - shown considerable advancement. The leaves in lycophytes (Pteridophytes) are known as microphylls. The term microphylls does not imply any size constraint, but rather refers to the absence of a leaf gap in the vascular supply of the stem at the point where the leaf vascular trace departs. Ferns and other plants have megaphylls, leaves that produce this leaf gap. Presumed evolution of sporophyll AMIT SARAF,Elphinstone College Microphyll & Megaphyll leaf : Microphyll & Megaphyll leaf Proposed steps in the evolution of the microphyll & megaphyll leaf. AMIT SARAF,Elphinstone College Pteridophytes (homosporous) : Pteridophytes (homosporous) Most Pteridophytes produces isospores that germinate in the soil and produce a bisexual gametophyte, Prothallus These spores are all approximately of the same size AMIT SARAF,Elphinstone College Slide 19: Pteridophytes (heterosporous) Selaginella and Isoetes are heterosporous, small spores (microspores) that germinate to produce the male gametophyte; and larger spores (megaspores) that germinate to produce the female gametophyte. The production of separate unisexual gametophytes, is thought an important step toward the seed development. AMIT SARAF,Elphinstone College Gymnosperms : Gymnosperms Evolution of Seed Plants Remarkable milestone achieved during the evolution of gymnosperm was restriction of gametophyte development on the sporophyte. Thus gametophyte was dependent on sporophyte Female gametophyte(ovule) grew on the sporophyte and has given rise to seed. Thus seed plants came into existence . AMIT SARAF,Elphinstone College Gymnosperms : Gymnosperms AMIT SARAF,Elphinstone College Angiosperms : Flowering plants, the angiosperms, were the last of the seed plant groups to evolve. Made appearance before 100 million years ago during the middle of the Age of Dinosaurs (late Jurassic) All flowering plants produce flowers and if they are sexually reproductive They produce a diploid zygote and triploid endosperm. The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers Angiosperms AMIT SARAF,Elphinstone College Typical Angiospermic life-cycle : Typical Angiospermic life-cycle AMIT SARAF,Elphinstone College Flowers : Flowers Flower is a modified shoot. It is a collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium). AMIT SARAF,Elphinstone College Androecium : Androecium Evolutionary pathway for the development of the anther The individual units of the androecium are the stamens, which consist of a filament which supports the anther. Stamens are thought to represent modified sporophylls (leaves with sporangia on their upper surface). An evolutionary series from primitive angiosperms (like Austrobaileya) which have leafish stamens to others with "normal" stamens (Lilium) is given below. AMIT SARAF,Elphinstone College Anther : Anther The anther contains microsporangia within which microspores (pollen) are produced by meiosis. T.S . of anther reveals 4 layers protecting sporangial tissue Epidermis single layered, protective in function Endothecium Characteristic fibrous band, dev. during pollen dehiscence Middle layer ephemeral structure, crushed during sporangial growth Tapetum Nursing dev sporangial tissue, Amoebic & secretive contribute in exine wall thickening AMIT SARAF,Elphinstone College Pollen : Pollen Pollen grains contain the male gametophyte (microgametophyte) phase of the plant. Pollen grains are produced by meiosis of microspore mother cells that are located along the inner edge of the anther sacs (microsporangia) The outer part of the pollen is the exine, which is composed of a complex polysaccharide, sporopollenin. Inside the pollen are two (or, at most, three) cells that comprise the male gametophyte. The tube cell (or tube nucleus) develops into the pollen tube. The germ cell divides by mitosis to produce two sperm cells(male gamete). Division of the germ cell can occur before or after pollination. AMIT SARAF,Elphinstone College Gynoecium : Gynoecium The gynoecium consists of the stigma, style, and ovary containing one or more ovules. These three structures are often termed a pistil or carpel. In many plants, the pistils will fuse for all or part of their length Hypothesized steps in the evolution of the carpel. AMIT SARAF,Elphinstone College Ovary : Ovary The ovary contains one or more ovules, which in turn contain one female gametophyte, also referred to in angiosperms as the embryo sac. Ovule (megasporangium) is protected by one or two integuments. Inside integument, nucellus develops a megaspore mother cell which gives rise to 4 megaspore by meiosis. Functional megaspore gives rise to embryo sac (female gametophyte) AMIT SARAF,Elphinstone College Embryo Sac : Embryo Sac Female gametophytes of flowering plants develop within the ovule There are usually eight (haploid) celled seven nucleated structure a) Egg Apparatus containing one egg, two synergids flanking the egg (at micropyle end ) b) two polar nuclei in the center of the embryo sac (central cell) c) three antipodal cells (at the chalazal end of the embryo sac from the egg). AMIT SARAF,Elphinstone College Double Fertilization : Double Fertilization After pollination, the pollen tube grows through the stigma and style toward the ovules in the ovary. The germ cell in the pollen grain divides and releases two sperm cells which move down the pollen tube. Once the tip of the tube reaches the micropyle end of the embryo sac, the tube grows through into the embryo sac and releases two sperm cells into one of the synergids One sperm cell fuses with the egg, producing the zygote which will later develope into the next-generation sporophyte. The second sperm fuses with the two polar bodies located in the center of the sac, producing the nutritive triploid endosperm tissue. AMIT SARAF,Elphinstone College Embryo Development : Embryo Development Stages of growth and development of the embryo AMIT SARAF,Elphinstone College Reference books : Reference books College Botany Vol I & II Das ,Dutta, Ganguly & Kar. Embryology of Angiosperms Bhojwani & Bhatnagar The Science of Biology, 4th Edition Sinauer Associates and WH Freeman Apart from these standard textbooks, internet itself is the ultimate source of information. 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