UT Arlington BIOL BIOL 3427 - ch20 (9 pages)

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Pages:
9
School:
University of Texas at Arlington
Course:
Biol Biol 3427 - Plant Science
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BRYOPHYTES Chapter 20 ADAPTATIONS ENABLING THE MOVE TO LAND 1 A layer of sporopollenin protects charophytes from desiccation sporopollenin is found in the spore wall of land plants 2 Danger of desiccation required new adaptations transport tissue cuticle large compact multicellular body etc 3 Simultaneous evolution of cuticle and stomata with guard cells 4 Support against gravity 5 Reproduction became adapted to periods of moisture so sperms could swim 6 Gametes and spore mother cells became protected from dryness by layers of cells 7 Evolution of xylem and phloem to transport materials to and from different parts of the plant the atmosphere and the soil 8 Alternation of generations a characteristic life cycle Alternation of generation does not occur in the charophyceans This suggests that alternation of generation arose independently in land plants A life cycle characterized by a multicellular haploid gametophyte stage followed by a multicellular diploid sporophyte stage Multicellular dependent embryos The zygote is retained surrounded by tissues of the gametophyte The parental tissue provides the embryo with nutrients Placental transfer cells present in the embryo and sometimes in the gametophyte as well enhance the transfer of nutrients 9 Spores produced in sporangia haploid reproductive cells that become a multicellular haploid gametophyte by mitosis The multicellular sporangium contains sporocytes the cells that undergo meiosis to form spores 10 Plants are eukaryotic multicellular mostly autotrophic organisms with haploid diploid life cycles which retain embryo within female sex organ on parent plant the cell wall contains cellulose CHARACTERS OF NONVASCULAR PLANTS About 17 000 species worldwide divided into three Divisions Bryophyta the mosses Hepatophyta the liverworts and Anthocerophyta the hornworts Their life cycle is similar but the three groups may not be closely related The bryophytes may form a polyphyletic group Bryophyta refers to the phylum of mosses only bryophytes refer to the three phyla mentioned above In many respects bryophytes are transitional between charophycean green algae and the vascular plants discussed later in the course Characteristics 1 Small embryophytes found in moist environments lack woody tissue and xylem and phloem they usually form mats spread over the ground 2 Gametophyte generation is dominant sporophyte is parasitic on the gametophyte 3 Bryophytes have cuticle stomata and multicellular gametangia and sporangia that allow them to survive on land 4 Their bodies are composed of true parenchyma derived from three dimensional growth usually from an apical meristem 5 Bryophytes need water to reproduce and most species lack vascular tissue xylem and phloem 6 Water transport is mostly through capillary action diffusion and cytoplasmic streaming They lack true roots stems and leaves The gametophyte of mosses is a one cell thick filament known as the protonema that eventually produces buds having meristematic tissue These meristems produce an upright structure called the gametophore These gametophytes are one to a few cells thick and obtain nutrients and water by direct absorption from the environment Most mosses do not have conducting tissue Some species have specialized cells that conduct water and nutrients but lack lignin in their cell walls The gametophores are anchored by fragile rhizoids Rhizoids are either single elongated cells as those found in liverworts and hornworts or filaments of cells as those of mosses Rhizoids are not made of tissues and do not absorb any significant amount of water In that way they differ from roots Bryophytes have smallest and simplest sporophyte of any group The sporophyte remains attached to the gametophyte throughout its lifetime dependent of the gametophyte for food water and minerals The mature sporophyte of mosses consists of a foot embedded in the archegonium a seta or stalk is present in the phylum Bryophyta and a capsule or sporangium The cap or calyptra closes the peristome or opening or the capsule Division Bryophyta Mosses are a large phylum containing about 12 000 species Many new species are discovered every year in the tropics The phylum is divided into three classes Sphagnosida peat mosses Andreaeopsida granite mosses and Bryopsida true mosses The class Bryidae contains about 9 500 species CHARACTERISTICS Protonema gives rise to gametophores leafy shoots that produce gametangia Protonema chlorophyllous with multicellular rhizoids Gametophyte may or may not branch Gametophores derive from an apical meristem that has a large apical cell Derivative cells subdivide and produce the tissues of stems and blades in rather precise arrangement Gametophore leafy with more than three ranks of blades Axis with thick walled outer cortical cells and a central strand of smaller cells Lamina sessile entire seldom lobed one cell thick Midrib composed of several layers Laminar cells elongated hyaline cells dead large barrel shaped retain large quantities of water In the family Polytrichaceae the lamina bears long sheets of cells lamellae on the upper surface that greatly increases the volume of photosynthetic tissue Cuticle exists only on the upper surface of the blades the underside lacks cuticle and is capable of absorbing water directly from the environment Stomata are absent on blades but present on the sporangial wall Paraphyllia present in some species small branched uniseriate structures Water transport 1 In some mosses primarily in the family Polytrichaceae the innermost cortex is composed of cells called hydroids that conduct water and dissolved minerals Elongated cells that loose their cytoplasm at maturity End walls are partially digested but are not removed completely Each hydroids are aligned with those above and below it 2 Species that have hydroids typically have leptoids Leptoids resemble sieve cells Elongated cells Lack nuclei at maturity but retain some cytoplasm Have prominent interconnections with adjacent cells 3 Adjacent parenchyma cells are unusually cytoplasmic and rich in enzymes just as are companion cells The majority of mosses lack hydroids and leptoids Water is conducted long the exterior of their stems by capillary action In species that lack leptoids sugar is transported by slow transport between the cells At the base of the axis rhizoids anchor the plant but do not absorb water or nutrients Multicellular filaments They lack chloroplasts Development Spores germinate and send out a


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