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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– 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 protective covering of the plantp g p

• 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• 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 h id fi t t f t• Tracheids were first type of water-conducting cell– More primitive (less specialized) than vessel

elements (flowering plants)

Only Conifers

Angiosperms have both Tracheids and Vessels

Vascular tissues• Primary xylem, phloem, & pith

– Central cylinder or stele– Several types of steles recognized

• ProtosteleMost ancient– 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’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 systemsLikely 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), and some 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– 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.• HomosporousHomosporous

– 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 axilsborne in axils of fertile microphylls

Lycopodium lagopusbranches terminated by sporophyllsgrouped into strobili

Selaginellaceae: Selaginella (only genus)

• Similar to some Lycopodiaceae– Microphylls & sporophylls arranged in strobili

• Have small, scale-like outgrowth – Ligule near base of upper surface of microphyll & g 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-like microphylls on upper surfaceQuill 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. I C t Ri l 1 000 i• In Costa Rica alone > 1,000 species

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

• FilicalesL t i t f– Leptosporangiate ferns

• Marsileales & Salviniales– Water ferns

• Psilotales– Whisk ferns

Ophioglossales & Marattiales• Ophioglossales

– Botrychium (the grape ferns)– Ophioglossum (adder’s tongue)

• Both genera produce a single leaf/year from g g ythe 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 fungi which resembles y g

gametophytes of Psilotales• Ophioglossum reticulatum has highest

chromosome number known in any living organism– Diploid complement of 1260 chromosomes

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OphioglossalesBotrychium parallelum: lower vegetative portion is divided.

Ophioglossum: lower leaf undivided

Filicales – Leptosporangiate fern

• Nearly all familiar ferns – Homosporous & leptosporangiate

• Most have siphonostelic rhizomesP d t f l h– Produce new sets of leaves each 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

DicksoniaTransverse 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• Extension of the leaf stalk or 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 sporocarpsstructures called 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 threein groups of three.

– Sperm are multiflagellated and require water to swim to egg.

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Gametophyte of Psilotum nudumBisexual: bears both antheridia & archegonia

• Tmesipteris grows as an epiphyte on tree ferns and other plants.– Leaves are supplied with a single unbranched

vascular bundlevascular 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• 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 i f h hi i• Alternation of heteromorphic generations• Living SVP’s classified into two phyla

– Lycopodiophyta– Pteridophyta

• Lycophytes have microphylls– Others phyla members have megaphylls