Transcript Selaginella
The Early Tracheophytes Chapter 23 Tracheophytes • First tracheophytes were Rhyniophytes – Found in fossil beds of Rhynie, Scotland – Characteristics • Small • Lacked leaves and roots • Dichotomously branching rhizomes with rhizoids attached to them • Vertical aerial stems with sporangia at tips (sporophyte phase of life cycle) • Only a few gametophyte fossils have been found • Simple stem anatomy – Evidence of endosymbiotic fungi in stems Tracheophytes – Rhyniophyte group gave rise to all other land plants • not monophyletic Tracheophytes • Tracheophyte innovations – Important in colonization of land • Dichotomously branching sporophyte with multiple terminal sporangia • Free-living, nutritionally independent sporophyte that is prominent in the life cycle • Reduced gametophyte • Lignified vascular tissue (xylem) in sporophyte Relationships Among Early Tracheophytes • Divided into two major clades – Lycophytes – All other tracheophytes • Two major lineages – Seed plants – Monilophytes » Ferns » Horsetails » Whisk ferns Lycophytes • Line originated in Devonian or Silurian period • Earliest known members called Zosterophyllophyta – Now extinct – Lacked leaves and roots – Unique distinguishing morphological feature • Sporangia attached to stems in lateral rather than terminal position Lycophytes • Line reached peak of diversity and ecological importance in Coal Age • Produce leaf called a microphyll – Defined by presence of single vascular bundle • Group today consists of three lineages – Lycopodium (and related genera) – Selaginella – Isoetes Lycophytes • Lycopodium – Familiar as evergreen trailing plants used in making wreaths – Abundant spores • Highly flammable – Once used by magicians and photographers • Used to coat latex items – Gloves and condoms – Spores irritating to skin, so no longer used Lycophytes • Lycopodium – Lycopodium clavatum • Experimentally shown to have hypoglycemic effects – Stem anatomy • Interconnected strands of xylem with phloem between them • Xylem has tracheids • Phloem contains sieve cells and parenchyma cells • No true endodermis Lycophytes • Lycopodium – Roots arise at apical shoot meristem and emerge on underside of horizontal stem – Homosporous life cycle • Only one type of spore is made • Gametophytes are bisexual • Sporangia produced on top surface of sporophylls (leaves bearing sporangia) • Sporophylls may be aggregated into strobili (singular, strobilus) which are conelike structures Lycophytes • Lycopodium – Homosporous life cycle • Haploid spores produced by meiosis inside sporangia • Spores are shed, germinate on ground, develop into gametophytes – Typically long lived, subterranean, require endosymbiotic fungi to survive • Antheridia and archegonia form on surface of gametophyte Lycophytes • Lycopodium – Homosporous life cycle • Biflagellate sperm liberated from antheridia swim through water to fertilize eggs in archegonia • Resulting zygote develops into embryo • Embryo has – – – – Short primary root Leaf primordia Shoot apex Well-developed foot • Sporophyte – Initially dependent upon gametophyte, becomes self-sustaining Lycophytes • Selaginella – Single living genus, Selaginella (spike moss) – Mainly tropical – Several commercially grown as ornamental plants • Selaginella lepidophylla (resurrection plant, rose of Jericho) • Selaginella willdenovii (peacock fern) • Selaginella braunii (treelet spike moss) Lycophytes • Selaginella – Microphylls often arranged in four rows or ranks • One row of large leaves on either side of stem, two rows of smaller leaves on top side of stem – Stem and leaves resemble miniature cypress branches – All leaves possess ligule on top side • Ligule secretes protective fluids during leaf development Lycophytes • Selaginella – Rhizophore • • • • Organ produced at meristems at branch points Unique to Selaginella Has characteristics of both stem and root Grows downward to soil and gives rise to true roots • Can give rise to stem under certain conditions Lycophytes • Selaginella – Heterosporous life cycle • Sporophytes produce two types of spores – Megaspores produced by megasporangia – Microspores produced by microsporangia • Sporangia located in axil of sporophylls – Always aggregate into strobili Lycophytes • Selaginella – Heterosporous life cycle • Megasporangia – Filled with diploid megasporocytes – One divides by meiosis to produce four large megaspores – Megaspores divide mitotically to form megagametophyte » When mature, spore wall cracks open » Archegonia develop in cushion of gametophyte tissue Lycophytes • Selaginella – Heterosporous life cycle • Microsporangia – – – – Filled with up to several hundred diploid microsporocytes Sporocytes divide by meiosis Produce microspores Microspores divide mitotically to form microgametophyte » Layer of cells inside spore wall forming an antheridium and mass of sperm cells in center Lycophytes • Selaginella – Heterosporous life cycle – – – – – Sperm liberated when microspore wall becomes wet Sperm swim toward mature archegonia Union of egg and sperm produces diploid zygote cell Diploid zygote cell divides and differentiates into embryo Embryo does not become dormant, continues to grow into fully mature sporophyte Lycophytes • Heterospory – Probably evolved in Selaginella – Megagametophyte provides nutrition and protection for zygote, embryo, and young sporophyte – Represents necessary step toward seeds Lycophytes • Isoetes – Commonly called quillwort or Merlin’s grass – Typically grow submerged in water for part or all of life cycle – Plant body • Lobed cormlike structure that undergoes secondary growth and produces roots • Tuft of microphylls that resemble grass leaves – Microphylls filled with large air chambers, have prominent ligules Lycophytes • Isoetes – Heterosporous • Sperm are multiflagellate (most other living lycophytes have biflagellate sperm) – Fossil record suggests Isoetes is living member of ancient lepidodendroid group Monilophytes • Includes all other seedless tracheophytes except lycophytes • Consists predominantly of plants commonly called ferns • Typically herbaceous today – Previously were tree size – Were important members of Coal Age swamp forests Monilophytes • Secondary growth occurs in different way than in lycophytes • Produce leaf called a megaphyll – More than one vascular strand – Extensive branching in leaf – Vascular strands cause leaf gap (interruption) in xylem of stem where they branch off to enter leaf – Thought to have resulted from modification of branch system Monilophytes • Whisk ferns – Psilophytes – No known fossil record – Two living genera • Psilotum • Tmesipteris – Restricted to South Pacific and Australia – Grow in tropical or subtropical regions, often as epiphytes Monilophytes • Whisk ferns – Psilotum • Lacks roots • Has dichotomously branched rhizome system covered with rhizoids – Cortex cells of rhizome infected with mycorrhizal fungi • Aerial stems that bare enations – – – – Have pith with fibers, surrounded by cylinder of xylem Endodermis with Casparian strip Cortex Epidermis with thick cuticle and many stomata Monilophytes • Whisk ferns – Psilotum • Homosporous • Gametophytes lack chlorophyll and associate with endomycorrhizal fungi – Tmesipteris • Epiphyte with dangling branches • Lacks roots but has leaves Monilophytes • Ophioglossalean ferns – Closest relative of psilophytes – Group of about 75 species • Genera – Botrychium (grape fern) – Ophioglossum (adder’s tongue fern) » Has the greatest number of chromosomes of any plant » 2n being as high as 1,260 in some species Monilophytes • Ophioglossalean ferns – Unusual leaves divided into two segments • Spikelike fertile segment with sporangium embedded in it • Sterile segment expanded for photosynthesis – Leaves not coiled when young – Stems upright rather than horizontal Monilophytes • Ophioglossalean ferns – Roots run horizontally through soil and produce shoot buds at intervals • Strong mycorrhizal relationships • Lack root hairs Monilophytes • Horsetails – Sphenophytes – Only one living genus, Equisetum – Worldwide distribution except for Australia and New Zealand – Contains silica in stem epidermis • In pioneer days, stem was used to scrub pots and pans • Commonly called scouring rush Monilophytes • Horsetails – Some may be toxic to humans and livestock • Contain enzymes that break down thiamine – Medicinal uses • Treat urinary and kidney problems • Reduce bleeding – Originated in Devonian period • Were important members of Coal Age swamp forests Monilophytes • Horsetails – Sporophytes easily recognized by jointed and ribbed stems, whorled appendages – Stem anatomy • Large central cavity surrounded by ring of vascular bundles and smaller cavities called vallecular canals • Smaller canals called carinal canals in center of each vascular bundle • Stems are hollow except at nodes Monilophytes • Horsetails – Sporangia are produced in strobili on structures called sporangiophores – Homosporous (produces one kind of spore) • Spores are green, thin-walled, with long, ribbonlike elaters attached to spore wall • Elaters coil and uncoil in response to humidity – Help disperse spores when sporangium splits open at maturity Monilophytes • Marattialean ferns – Similar in appearance to true ferns • Compound leaves (fronds) that are coiled when young – Have upright stems and distinctive sporangium – Largely tropical – Extensive fossil record • Important element in Coal Age swamp forest flora Monilophytes • True ferns – Make up majority of living monilophytes – At least 12,000 species known – Unique feature of true ferns leptosporangium • Originate from single cell in leaf • Strip of thick-walled cells called annulus flicks spores out of sporangium • Grouped in clusters called sori (may be protected by structure called an indusium or by edge of leaf curling over them) Monilophytes • True ferns – Pteridium aquilinum, most widespread plant on Earth – Fossil record extending back to Devonian period – Important members of coal swamp flora during Carboniferous period Monilophytes • True ferns – Sporophyte • Typically grow from underground perennial rhizome • Roots and leaves arise from nodes – Young leaves form coiled fiddleheads • Leaf structure – Well-developed epidermis with stomata – Mesophyll may be differentiated into palisade and spongy layers – Secondary and tertiary leaflets (pinnae and pinnules) develop on petiole extension called a rachis Monilophytes • True ferns – Sexual reproduction • Sporophyte matures in 1 to 10 years • Sporangia develop • Temperate zones – Spores released in fall • Tropics – Released any month of the year • Spores of many species require light for germination Monilophytes • True ferns – Sexual reproduction • Germinating spore produces (usually) heartshaped thallus • Rhizoids on lower surface anchor thallus • Archegonia and antheridia develop • Archegonia produces attractant that guides sperm toward them • When egg is fertilized, plasma membrane of egg changes so no other sperm can penetrate Monilophytes • True ferns – Sexual reproduction • Diploid zygote cell develops into embryo • Embryo has foot, shoot, and root regions • Usually only one or two zygotes will mature into embryos on one gametophyte • Embryo develops into sporophyte and becomes nutritionally independent of gametophyte Monilophytes • True ferns – Alternative means of reproduction • Miniature plantlets can form on mature leaves, break off, grow into new plants • Walking ferns form new plants when tip of frond touches soil • Gametophytes can also reproduce vegetatively • Apospory reproduction without spores – Produce diploid gametophytes directly out of sporophyte tissue (usually leaf tissue) • Apogamy reproduction without gametes – Gametophytes produce sporophytes without any fusion of gametes Monilophytes • Ferns – Ecological and economic importance of ferns • Provide bulk of biomass in some tropical forests • Dominate understories of some temperate conifer forests • Some are weeds smother other vegetation, clog waterways, poison livestock – Lygodium (climbing fern) – Pteridium aquilinum (bracken) Monilophytes • Ferns – Ecological and economic importance of ferns • Generally avoided by animals because of poisons or unpalatable chemicals present • Humans – – – – – Some consume fiddlehead Leaves used in basket-making Fronds mixed in flower arrangements Popular indoor houseplant and outdoor landscaping plant Gametophytes are excellent subject for research on physiology and plant development