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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 ppgprotective covering of the 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 • ThidTracheids were first t ype of wat er- 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.

• 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 – 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 near base of upper surface of microphyll & 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 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 C os ta Ri ca a lone > 1, 000 spec ies

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 – LtLeptosporang itiate ferns • Marsileales & Salviniales – Water ferns • Psilotales – Whisk ferns

Ophioglossales & Marattiales

• Ophioglossales – Botrychium (the grape ferns) – Ophioglossum (adder’s tongue) • Both gggyenera produce a single leaf/year 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 fungi 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 Botrychium parallelum: lower Ophioglossum: lower leaf vegetative portion is divided. undivided

Filicales – Leptosporangiate fern

• Nearly all familiar ferns – Homosporous & leptosporangiate • Most have siphonostelic rhizomes – PdProduce new se ts of fl 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: Dicksonia Transverse section of rhizome 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. • 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. • 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. – 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 • Alternat ion o f heteromorp hic generat ions • Living SVP’s classified into two phyla – Lycopodiophyta – Pteridophyta • Lycophytes have microphylls – Others phyla members have megaphylls

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