1262 Vaculars and Seedless Plants
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Transcript of 1262 Vaculars and Seedless Plants
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BIOL1262
Lecture 7
Vascular seedless plants II
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Evolution of today’s plants: note ferns
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Ancient (extinct) relative of vascular plants – a rhyniophyte
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Phylum Pteridophyta
Classification
Ferns & fern allies: most ferns, horsetails & whisk ferns
The major groups (orders) within this phylum are largelydetermined by:
1. whether homosporous (one type of spore) or
heterosporous (two types of spores) in life cycle.
most members are homosporous (few exceptions)
2. the structure and method of development of sporangia, i.e.whether they are
eusporangiate
leptosporangiate
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Seven Orders in Phylum Pteridophyta (ferns)
Eusporangiate and heterosporous Ophioglossales (e.g. Botrichum)
Marratiales
Leptosporangiate and homosporous
Filicales
Psilotales (whisk ferns, protostelic e.g. Psilotum)
Equisetales (horsetails, single genus Equisetum)
Leptosporangiate and heterosporous
Marsileales (water ferns)
Salvinales (water ferns)
Phylum Pteridophyta - Classification
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strobili
Equisetum - Horsetails
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Phylum Pteridophyta: ORDER Filicales
Contains the familiar ferns
more than 10,500 species 320 genera in 35 families
homosporous and leptosporangiate
Sporophyte has well-developed
body with:
true stems true leaves
true roots
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Development of Roots
probably from branches that penetratedunderground
developed further branching
exposed to different environment & selection
processes than stems
evolved over millennia to the forms present
today (different from shoots)
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Phylum Pteridophyta: Filicales
Roots are adventitious primary root develops from the
embryo but is short-lived
adventitious roots grow from the stem
Stems are generally siphonostelicrhizomes
Leaves/fronds are: megaphylls
most conspicuous structures of thesporophyte
commonly compound
with a rachis (extension of thepetiole)
pinnae (leaflets) [sing. pinna]
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Leaves/fronds (cont’d):
Show circinate vernation:-
type of leaf development
where the young leaves arecoiled;
the maturing leaf uncoils dueto more rapid growth on inner than outer surface.
Young leaves and rhizomesusually covered with hair or
waxy scales:- epidermal outgrowths; used in fern
classification
Phylum Pteridophyta: Filicales
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LIFE CYCLE OFHOMOSPOROUS FERN
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Phylum Pteridophyta: Order Filicales – Life Cycle
Sporophyte produces one type of spore (i.e. homosporous)
Spores are produced in sporangia on the lower surface of leaves OR specially modified leaves (sporophylls) ORseparate stalks (sporangiophores).
Sporangia occurs in clusters called sori.
may be yellow, orange-brown or blackish.
may appear as broad patches, lines or dots
Indusia cover the young sori (sing. indusium)
these are specialized epidermal outgrowths
they shrivel when sporangia are ready to shed spores
Stalked
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Types of sori
indusia covering
the sporangia
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The sporangium possesses:
tapetum food-rich layer of cells
nourishes the developing spores
thick wall annulus (differentially-thickened
cell walls stomium (thin-walled cells)
Dehiscence liberates (ejects)spores which are dispersed bywind
Phylum Pteridophyta: Order Filicales – Life Cycle
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Dehisced sporangia of homosporous fern
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Gametophyte:-
develops from the spores
found in moist places
flat, thin (one cell thick), leaf-like structure (prothallus)
free living
green and nutritionallyindependent
numerous rhizoids on lower surface.
Phylum Pteridophyta: Order Filicales – Life Cycle
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Gametophyte (cont’d)
Monoecious (both male and female gametangia on sameindividual)
Antheridia develop on ventral surface
occur among the rhizoids
consist of a sterile jacket containing sperm
Archegonia develop on ventral surface
occur near the apical notch. flask-shape
swollen lower portion (venter) sunken in gametophyteneck
Phylum Pteridophyta: Order Filicales – Life Cycle
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2. Transfer of gametes
& Fertilization
• When sperm are mature and there is
an adequate supply of water,
antheridia burst and sperm arereleased.
• The multi-flagellated sperm swim to
the archegonia
• Water is still essential for gametic
transfer
• A sperm enters through the neck
canal and fertilizes egg.
• The zygote begins to divide
immediately.
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3. Development of the young Sporophyte
• Through mitotic cell divisions embryo develops into ayoung sporophyte.
• At the early stage, it is dependent on gametophyte (receives
nutrition from the gametophyte through foot).
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Fern gametophyte with young developing sporophyte in notch
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Fern gametophyte with developing
sporophyte in notch
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4. Maturation of the
sporophyte
• The young sporophyte rapidly develops.
• The gametophyte disintegrates
once the sporophyte achieves alevel of photosynthesis that is
sufficient to maintain itself and it
has rooted in the soil.
• Sporophyte soon becomes mature.
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Spore production by sporophyte
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club mosses, spike mosses and
quill-worts
Phylum Lycophyta
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Phylum Lycophyta
Living members (10 genera) are herbaceous, generally less than 20 cm,
BUT large tree lycophytes were among dominant plants in coal-
forming forest of the Carboniferous period.
Possess only microphylls (simple leaves)
in contrast to the megaphylls of Pteridophyta and seed
plants often arranged spirally on stem.
Stems:
branched (dichotomous) or unbranched, erect, creeping or hanging.
Roots
dichotomously branched
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Lycopodiaceae: club mosses most familiar living members of
this phylum
mostly tropical species
sporangia aggregated in strobili
(sing. strobilus) or on
sporophylls
homosporous.
Isoetaceae: quill-worts
only one genus, Isoetes
heterosporous.
Lycopodium sp.
Phylum Lycophyta
Isoetessp.
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Selaginellaceae: spike mosses only one genus in this family
mostly tropical
grow mostly in moist places
(few grow in dry places)
heterosporous.
Creeping & erect stems
Roots arise where stem
branches
Dichotomously branched
Protosteles
Selaginella sp.
Phylum Lycophyta - Selaginella
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Stem bears four rows of leaves (microphylls)
2 smaller dorsally 2 larger ventrally
Sporophylls arranged instrobili at tips of branches.
each sporophyll bears a
single sporangium on itssurface.
¾ Sporophyll: modified leaf or leaf-like organ that bearssporangia.
Selaginella sp.
Phylum Lycophyta - Selaginella
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Megasporangia are borne
on megasporophyll
Microsporangia are borneon microsporophyll
Both kinds of sporangia
occur in the same strobilus
L.S. strobilus of
Selaginella sp.
megasporangium
megasporophyll
microsporophyll
microsporangium
Phylum Lycophyta - Selaginella
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microsporophyll
Microsporangiumcontainingnumerous
microspores
Megasporangiumcontaining four
megaspores
megasporophyll
Axis of strobilus
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Microsporangiaand
larger megasporangia
of
Selaginella sp.
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Life Cycle of
Selaginella
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Selaginella : Life cycle
Spore Production through meiosis of sporogenous tissue
- microspores produced in the microsporangium
- megaspores produced in the megasporangium
• Gametophytes develop (from mitotic divisions) within the
microspore/megaspore walls: much reduced
• Microgametophyte produces biflagellate sperm in the antheridia
• Megagametophyte produces eggs in the archegonia (nourished
by food previously stored in megaspore)
• Sperm must swim in water to reach egg for fertilization
• Zygote → sporophyte embryo (dependent on megagametophyte
initially) → independent mature sporophyte
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Important developments in Selaginella
Heterospory The megagametophyte stores food for the developing embryo
Improves survival in harsh terrestrial environment
Endospory The development of the gametophytes from spores takes place
on the parent sporophyte
Shift in the balance with gametophyte becoming dependenton the sporophyte as is seen in the seed plants
The new sporophyte generation begins development within themegagametophyte
Similar to the seed habit of seed plants
Range – greater; some species in arid habitats
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economic importance
Bryophyte mosses
Sphagnum peat – fuel,
(preservation of artefacts, bodies, seeds... )
Seedless vascular plants (Palaeozoic era)
coal