The Early Tracheophytes

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1 The Early Tracheophytes
Chapter 23

2 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

3 Tracheophytes Rhyniophyte group gave rise to all other land plants
not monophyletic

4 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

5 Relationships Among Early Tracheophytes
Divided into two major clades Lycophytes All other tracheophytes Two major lineages Seed plants Monilophytes Ferns Horsetails Whisk ferns

6 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

7 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

8 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

9 Lycophytes Lycopodium Lycopodium clavatum Stem anatomy
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

10 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

11 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

12 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

13 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)

14 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

15 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

16 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

17 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

18 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

19 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

20 Lycophytes Heterospory Probably evolved in Selaginella
Megagametophyte provides nutrition and protection for zygote, embryo, and young sporophyte Represents necessary step toward seeds

21 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

22 Lycophytes Isoetes Heterosporous
Sperm are multiflagellate (most other living lycophytes have biflagellate sperm) Fossil record suggests Isoetes is living member of ancient lepidodendroid group

23 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

24 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

25 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

26 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

27 Monilophytes Whisk ferns Psilotum Tmesipteris Homosporous
Gametophytes lack chlorophyll and associate with endomycorrhizal fungi Tmesipteris Epiphyte with dangling branches Lacks roots but has leaves

28 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

29 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

30 Monilophytes Ophioglossalean ferns
Roots run horizontally through soil and produce shoot buds at intervals Strong mycorrhizal relationships Lack root hairs

31 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

32 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

33 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

34 Monilophytes Horsetails
Sporangia are produced in strobili on structures called sporangiophores Homosporous (produces one kind of spore) Spores are green, thin-walled, with long, ribbon-like elaters attached to spore wall Elaters coil and uncoil in response to humidity Help disperse spores when sporangium splits open at maturity

35 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

36 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)

37 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

38 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

39 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

40 Monilophytes True ferns Sexual reproduction
Germinating spore produces (usually) heart-shaped 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

41 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

42 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

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44 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)

45 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