Seedless - vascular - plants lecture

I. Review: non vascular plants

Earliest land plants - near water edges

  • limited by dehydration

  • no lignin

  • gametophyte dominant

  • sporophyte small, attached to gametophyte for nutrients

  • external fertilization

  • all homosporous = same size spores

II. NVP’s → Protracheophytes → SVP’s

  • protracheophytes are an intermediary group between the NVP’s and SVP’s

Evolutionary history

  1. NVP’s evolved from green algae in the charophytes

    • liverwort’s = most basal/closest to algae

    • hornworts = sister group to the vascular plants

      • evidence: molecular data

        • first persistent/photosynthetic sporophyte

  2. Protracheophytes: an extinct intermediary group between the NVP and SVP’s with dichotomous branching

    • have hydroid cells with some lignin

    • lack true tracheid’s

  • hydroids may have formed first tracheid’s with the addition of lignin and further specialization

  • the apical meristem switches to the top in embryology

  • SVP’s were the first to have true tracheids

III. SVP (Tracheophytes)

  • Explosion of plants on land - barren land became green

    • helped stabilize O2 in the atmosphere

    • basis for the web of terrestrial life

  • these changes led to movement farther away from water (not too far away)

A. Characteristics

Derived traits

  1. Tracheid’s (lignin)

    • first organ is the stem

    • ALL SVP’s have stem

  2. Sporophyte dominant

    • Tracheid’s/ lignin evolved in the sporophyte

    • sporophyte becomes big (increased reproduction due to increased amount of spores)

  3. Evolution of roots

    • allowed for moving away from water

  4. Gametophyte get’s smaller

    • important because its moving away from water

Ancestral Traits

  1. external fertilization

  2. spores (not seeds)

B. Extinct SVP phyla

  • 425 mya: tracheid’s in the fossil record (rhynie scotland)

    • rhyniophytes = the first SVP’s (all extinct)

      • ex: cooksonia

        • stem only

        • true tracheid’s (for support and conduction)

        • no roots or leaves

        • terminal sporangia (on the ends)

        • homosporous (all the spores look the same)

          • all NVP and SVP’s are homosporous (except for one)

        • endomycorrhizae in the stem

    • spore mortality was a problem

Rhyniophytes speciated into 2 major SVP clades

  1. Lycophytes (club moss)

    • early leaves that evolve = microphylls with 1 vascular strand

      • lignin grows into the leaf creating a microphyll

      • early leaves are microphylls that have 1 vascular strand created by lignin growing into it

    • Have roots

      • evolved from underground stems

      • rhizoids possibly became the root hairs

  1. Monilophytes (ferns)

    • defined by megaphylls = branching veins

    • leaves of today came from megaphylls (ferns of the past)

    • megaphylls arise from branches overlapping and fusing together

    • roots evolved from stems

      • evolved independently from roots in lycophytes

  • These two groups formed huge trees that dominated the landscape

    • they’re big because of the evolution of megaphylls

      • evolution of megaphylls was a huge selection pressure for plants (shading everything below them), plants got bigger and developed secondary growth to deal with it

SVP’s dominate in the carboniferous era

  • 425 mya = climate was tropical

    • rapid growth of SVP’s

  • 360 mya = carboniferous era

    • 100’ tall SVP trees

    • lepidonendron (makes secondary xylem only, unifacial vascular cambium)

    • giant horsetail trees

    • SVP remains became coal

    • increased oxygen lead to giant winged insects, large scorpions, 6 ft millipedes, giant amphibians

  • 300 mya = end of carboniferous era

    • climate cooled/arid

    • giant SVP’s died off (external fertilization needs water, when climate cooled the spores and gametophytes could not survive)

    • only small SVP’s left today

    • the climate cooled because…

      • CO2 decrease

      • giant trees were absorbing CO2 and storing it as wood

    • SVP’s croak! leading to gymnosperms rising

C. SVP’s alive today

  1. Lycophytes

    a. lycopodium

Derived traits in lycopodium over the rhyniophytes

  • roots

  • leaves (microphylls with one vascular strand)

  • strobilus (ancestral cone)

    • cone = sporangia + leaves clustered on branch tips

    • sporophyll = leaf with sporangium on them

Significance of the cone

  • huge increase in spore production

    • before cones, sporangia alone on the branch tips

    • reproductive rates of cones is much greater

  • for the first time in history we see…

    • roots, leaves, and strobilus

b. Selaginella

Derived traits in selaginella over lycopodium

  • heterospory = 2 different spore types

    • mega and microspores

Mega = female

Micro = male

Why is heterospory important

  1. big megaspore makes big gametophyte

    • increased food for the egg

small microspores make small gametophyte with billions of sperm

  • provides better dispersal

  1. first step to make seed and pollen in “higher” plants (higher = internal fert)

Why are pollen and seed important?

  • pollen protects the sperm (pollen = male structure

  • seed protects the egg (seed = female structure)

    • this led to internal fertilization

  • higher plants can live anywhere

SEEDLESS VASCULAR PLANTS DO NOT MAKE SEED AND POLLEN

  • they make megaspores and microspores which led to evolution of pollen and seed

Changes that come with heterospory

in homospory:

sporangium w/sporocytes (2N) → meiosis → spores (1N) → mitosis → gametophytes (1N)

in heterspory:

microsporangia w/microsporocytes (2N) → meiosis → microspores (1N) → mitosis → microgametophyte (1N)

megasporangia w/megasporocytes (2N) → meiosis → megaspores (1N) → mitosis → megagametophyte (1N)

  1. Monilophytes (ferns)

Derived traits in the monilophytes

a. megaphylls (vegetative)

  • large leaves with branching veins

  • arise coiled (fiddle heads)

    • protects the leaf tissue

      • arise from tangled rhizomes underground, coil protects them from damage

b. Sori (for reproduction)

  • clusters of sporangia on the underside of the leaf

  • sori (plural), sorus (singular)

sorus = cluster of sporangia

  • a greek helmet thing called annulus covers each sporangium

  • annulus folds back and then springs forward to throw spores (active spore dispersal)

  • true ferns have annulus

c. Indusium

  • cover protecting the sori (not always present)

  • its like a green thing in the middle

2 types of indusium

  1. outgrowth of leaf epidermis (growing right out of the leaf tissue)

  1. leaf margins curl over the sori

No indusium present? = naked sori

  • common in the tropics

Additional traits in the monilophytes

  • most ferns are homosporous

  • gametophyte is heart shaped, photosynthetic, and bisexual

Simple ferns

  • lack an annulus

  • DNA evidence puts psilotum and horsetails in the manilophytes

  1. psilotum (whisk fern)

    • tropical epiphyte (grows on other plants)

    • stem only

    • no roots or leaves

    • homosporous

  • sporangia on enations

  • enations = epidermal outgrowth

  • enations are not leaves because they dont have lignin

  • this is a reversal of evolution of megaphylls and roots because it became an epiphyte

  1. equisetum (horsetails)

    • jointed stem

    • silica walls (antipredation)

    • homosporous

    • medicinal plants (only in herb stores, not in wild bc they’re toxic)

    • used for sandpaper

  • vegetative sporophyte

    • whorled branches

    • microphylls reduced from megaphylls

      • rhizome that grows underground, grow out from under other plants and then pop out in the sun

  • reproductive sporophyte

    • strobilus (cone)

    • sporangiophore = circles on cone

Derived traits in equisetum

  1. sporangiophores prevent dehydration

  2. spores have elaters (arm like extensions for dispersal)