Selaginella

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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
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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
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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
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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
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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
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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
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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