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Seedless Vascular Plants
Ch 17
Evolution of Vascular Plants
• Efficient fluid-conducting systems
– Xylem & Phloem allowed for food & water
transport.
• Ability to synthesize lignin (cell walls)
– Adds rigidity
– Sporophyte reaches great heights
• Profuse branching via apical meristems
• Sporophytes produce multiple sporangia.
• Gametophytes are free-living and require
water for motile sperm to swim to the egg.
Early Devonian Landscape
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Organization of Vascular Plant Body
• Cell differentiation gave rise to:
– Root system (anchor & mineral uptake)
– Shoot system (photosynthesis)
• Dermal tissue system
– Outer p
protective covering
g of the p
plant
• Vascular tissue system
– Conductive tissues (xylem & phloem)
– Embedded in ground tissue system
• Differences in roots, shoots, & leaves
– Distribution of vascular & ground tissues
– Will discuss in CH’s 24, 25
1° vs. 2° growth
• Primary growth
– Growth occurs near root & shoot tips.
– Tissues arising from this “primary tissues”
– Part of plant where occurs is “primary plant body”
• Secondary growth
– Growth that thickens the stem & roots
– Activity of lateral meristems
– Vascular cambium
• Secondary vascular tissues
– Second lateral meristem
• Cork cambium
• Tissues arising from this “secondary tissues”
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Conducting cells of xylem
• Tracheary elements
– Lignified wall thickenings
– Well-preserved in fossils
• T
Tracheids
h id were first
fi t type
t
off watert
conducting cell
– More primitive (less specialized) than vessel
elements (flowering plants)
Angiosperms have both
Tracheids and Vessels
Only Conifers
Vascular tissues
• Primary xylem, phloem, & pith
– Central cylinder or stele
– Several types of steles recognized
• Protostele
– Most ancient
– Found in extinct groups
– Typical stele found in most roots
• Siphonostele
– Found in stems
– Marked by strands leading to leaves (leaf traces)
and gaps (leaf gaps) in siphonostele.
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Root & Leaf Evolution
• Roots have retained many of the ancient
structural characteristics no longer present in
stems.
• Leaves arise as protuberances (leaf
primordia) from the apical meristem of the
shoot.
• Evolutionarily speaking
– Two distinct types of leaves
• Microphylls
• Megaphylls
Microphylls (“small leaf”)
• Associated with protosteles
• Evolved as superficial lateral outgrowths of
the stem.
– Began as small scalelike or spinelike
outgrowths called ‘enations’
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Megaphylls (“larger than micro”)
• Associated with stems that have
siphonosteles or eusteles.
– Therefore associated with leaf gaps and leaf
trace gaps.
– Likely evolved from entire branch systems
Reproductive systems
• Homosporous
– Only one kind of spore produced after meiosis
– Homospory found in pteridophytes
(horsetails) and some lycophytes.
(horsetails),
lycophytes
• Fun fact:
– Sporophytes of ferns are heterozygous
• NOT the result of self-fertilization
• Heterosporous
– Production of two types of spores in two
different kinds of sporangia.
• Microspores
p
– Microsporangia
– Give rise to male gametophytes
• Megaspores
– Megasporangia
– Give rise to female gametophytes
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Phyla of SVP’s
• Extinct Phyla at end of Devonian
– Rhyniophyta
– Zosterophyllophyta
– Trimerophytophyta
• Living representatives
– Lycopodiophyta
– Pteridophyta
Rhyniophyte
Zosterophyllophyte
Trimerophyte
Make sure to read these sections in book
Phylum Lycopodiophyta
• 10 – 15 living genera ~ 1,200 species
• Lycophyte clade
• Euphyllophyte clade
– Ferns + allies & Seed Plants
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Lycopodiaceae: Club Mosses
• All but two living genera of living
lycophytes belong to this family.
• Both stem & root are protostelic.
• Homosporous
– Sporangia occur singly on the upper surface
of fertile microphylls called sporophylls.
• Spores of Lycopodiaceae give rise to
bisexual gametophytes
– Depending on genus
• Green irregularly lobed masses
• Subterranean, nonphotosynthetic, mycorrhizal
structures
• Maturation of archegonia & antheridia in
gametophyte
– 6 – 15 years
– Self-fertilization rates very low
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Sporophylls and strobili
Huperzia
Lack strobili
Sporangia are
borne in axils
of fertile
microphylls
Lycopodium lagopus
branches terminated
by sporophylls
grouped into strobili
Selaginellaceae: Selaginella (only genus)
• Similar to some Lycopodiaceae
– Microphylls & sporophylls arranged in strobili
• Have small, scale-like outgrowth
– Ligule
g
near base of upper surface of microphyll
y &
sporophyll
•
•
•
•
Stem & root are protostelic
Heterosporous with unisexual gametophytes
Megasporangia borne by megasporophylls
Microsporangia borne by microsporophylls
Selaginella lepidophylla (resurrection plant)
Completely dries out & when it rains “revives”
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Selaginella rupestris (rock spikemoss) with strobili
Selaginella kraussiana, a prostrate creeping plant
Selaginella willdenovii. Shade loving, climbs to 7 meters
Note the rhizomes!!!!
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Isoetaceae: only genus is Isoetes
• Quillworts
• Sporophyte
– Short, fleshy underground stem
– Quill
Quill-like
like microphylls on upper surface
– Roots on lower surface
• Heterosporous
– Megasporangia borne at base of
megasporophylls
– Microsporophylls located nearer the center
Isoetes storkii
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Pteridophyta
• Includes the “true ferns”, whisk ferns, and
horsetails.
• Only 380 species occur in U.S. & N.A.
• In
I Costa
C t Rica
Ri alone
l
>1
1,000
000 species
i
Two kinds of sporangia
• Eusporangiate or Leptosporangiate
• Eusporangium
– Parent cells located on surface of tissue from
which sporangium is produced.
– Forms inner and outer wall
• Outer layer builds up several-layer wall of sporangium.
• Inner layer gives rise to mass of irregularly oriented
cells from which the spore mother cells arise.
• Take home message:
– Eusporangia have a multicellular origin!!!!!!
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• Leptosporangia arise from a single superficial
cell
– Divides transversely or obliquely
• Outer cell ultimately gives rise to an
elaborate, stalked sporangium
• Nutritive structure two cell layers thick called
tapetum
• Inner mass differentiates into spore mother
cells, then undergo meiosis; producing four
spores each.
• The sporangia are stalked
– Each contains a special layer of thick-walled
cells called an annulus.
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4 very different kinds of ferns
• Ophioglossales & Maratiales
– Eusporangiate
• Filicales
– Leptosporangiate
L t
i t ferns
f
• Marsileales & Salviniales
– Water ferns
• Psilotales
– Whisk ferns
Ophioglossales & Marattiales
• Ophioglossales
– Botrychium (the grape ferns)
– Ophioglossum (adder’s tongue)
• Both g
genera produce a single
g leaf/year
y
from
the stem; consisting of two parts:
– A vegetative portion or blade
– A fertile segment
• Botrychium (fertile segment is dissected same way as
vegetative) bears two rows of eusporangia.
• Ophioglossum (fertile segment undivided) bears two
rows of sunken eusporangia.
• Gametophytes of each are subterranean,
tuberous, elongate structures with
numerous rhizoids
– Also, endophytic
y fungi
g which resembles
gametophytes of Psilotales
• Ophioglossum reticulatum has highest
chromosome number known in any living
organism
– Diploid complement of 1260 chromosomes
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Ophioglossales
Ophioglossum: lower leaf
undivided
Botrychium parallelum: lower
vegetative portion is divided.
Filicales – Leptosporangiate fern
• Nearly all familiar ferns
– Homosporous & leptosporangiate
• Most have siphonostelic rhizomes
– Produce
P d
new sets
t off leaves
l
each
h year
– Produces a true root which soon withers
– Leaves (fronds) are megaphylls which
represent the most conspicuous part of the
sporophyte.
Maidenhair fern:
Transverse section of rhizome
Dicksonia
Transverse section of rhizome
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• Fronds are compound
– Lamina divided into leaflets or pinnae
– Attached to the rachis
• Extension of the leaf stalk or petiole
petiole.
• Young leaves are coiled (circinate)
– ‘fiddleheads’
– This type of development ‘circinate vernation’
Sori
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• Sori are covered by outgrowths
– Indusia; which may shrivel when sporangia
are ripe.
• Gametophyte develops into heart shaped,
membranous structure called prothallus
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Water ferns
• Orders Marsileales & Salviniales
• Marsilea leaves resemble 4-leaf clover
– Drought-resistant bean shaped reproductive
structures called sporocarps.
sporocarps
• Salviniales (Azolla & Salvinia)
– Leaves are borne in 3’s
– 1 of 3 hangs down in water
• ‘roots’ bear sporangia
Marsilea polycarpa
Salvinia
Psilotales – rootless & eusporangiate
• Psilotum (whisk fern) and Tmesipteris
• Psilotum is homosporous
– Spores produced in sporangia and aggregate
in groups of three
three.
– Sperm are multiflagellated and require water
to swim to egg.
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Gametophyte of Psilotum nudum
Bisexual: bears both antheridia & archegonia
• Tmesipteris grows as an epiphyte on tree
ferns and other plants.
– Leaves are supplied with a single unbranched
vascular bundle
– Otherwise is essentially similar to Psilotum
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Equisetales – 2nd major line of
Pteridophytes
• Species Equisetum (horsetails)
• Roots originate at the nodes of rhizomes
– Important in vegetative propagation
• Homosporous
– Sporangia borne in groups of 5 to 10 along
umbrellalike structures
• Sporangiophores which are clustered into strobili
• When spores mature; sporangia contract
• Elaters (thickened bands) coil when moist; uncoil
when dry – aiding in spore dispersal
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Summary
• Primary vascular tissues arranged in steles
– Three basic types
•
•
•
•
Roots & Leaves evolved in different ways
Either homosporous or heterosporous
Al
Alternation
i off h
heteromorphic
hi generations
i
Living SVP’s classified into two phyla
– Lycopodiophyta
– Pteridophyta
• Lycophytes have microphylls
– Others phyla members have megaphylls
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