plant features: multicellular eukaryotes photosynthetic autotrophs (a few are parasitic...

Plant features:Multicellular eukaryotes

Photosynthetic autotrophs (a few are parasitic heterotrophs)

Mostly terrestrial (some aquatic) - requires new modes of nutrition, support, protection & reproduction

Multicellular gametangia, stomates

Characteristic cellular features:Chloroplasts with thylakoid membranes and chlorophyll (other eukaryotic organelles, including mitochondria, also present)

Starch as a storage compound (in plastids)

Cellulose walls (structural strength)

Cuticle (waxy coating) on the outside

Meet the Plants

Plants share several key features with their likely sister group, the charophyceans (green algae)

A common ancestor of modern charophyceans and plants moved from an aquatic habitat onto land

- this involves changes in size, gas exchange, protection for fragile embryos, protection from desication, ways for sperm to reach egg, mechanisms of dispersal (getting around)... etc

Bryophytes & Seedless Vascular Plants

Kingdom Plantae 1st appeared about 475 million years ago, along with terrestrial fungi

most plants retain a symbiotic association with fungi

Stages in Land Plant Evolution:

1) Early land plants (~400 MYA) bryophytes and seedless vascular plants

2) Diversification (~390 MYA) abundance of seedless vascular plants

1) Seeds (~ 300 MYA) Gymnosperms (naked-seeded plants)

2) Flowers and fruits (~ 130 MYA) Angiosperms (flowering plants)

3 contrasting opinions of what constitutes the “plant kingdom”

Standard definition: true plants have multicellular embryos

3 contrasting opinions of what constitutes the “plant kingdom”

should we include their closest algal relatives, the charophyceans, who share so many features with land plants?

should we include green algae (chlorophytes),who share the same photosynthetic machinery?

3 contrasting opinions of what constitutes the “plant kingdom”

Polypodium sp., a terrestrial plant with greater cell & tissue complexity

Unlike green algae, terrestrial plants have different cell and tissue types

Coleochaete orbicularis, a disc-shaped charophycean showing simple structure

Features shared by plants and charophyceans:Primary endosymbiosis (double membrane around chloroplast)Photosynthetic pigments (chlorophyll a, b and carotenoids)Starch storage in plastidsProduction of cell walls from cellulose Cell plate formation during cell divisionSporic life cycle

New features in terrestrial plants:Specialized tissues + organs (roots, stems, leaves) derived from the apical meristem, the growing tip of a shootAlternation of multicellular generationsMulticellular gametangia Walled spores produced in sporangiaMulticellular, dependent embryos

Evidence for common ancestor

of green algae & plants

Plants organs derived from apical meristem

Above ground Below ground

Alternation of Generations

All plants have multicellular haploid stages, and multicellular diploid stages, which produce each other

gametophyte (N)

sporophyte (2N)

Walled spores

Sporophyte (2N) stage has multicellular structures called sporangia that produce the haploid spores through meiosis

spores = haploid cells that cangrow into the gametophyte bymitosis

protective outer layerallows spores to resist dry conditions

Multicellular Gametangia

Gametophyte (N) stage has multicellular structures called gametangia that produce the haploid gametes

Archegonium: vase-shaped female organ holds single egg cell

Antheridium: male organ releases sperm

Multicellular, Dependent Embryos

Multicellular embryos develop from zygote inside tissue of female parent, which provide embryo with nutrients during development

Placental transfer cells, analogous to mammalian placenta

Non-vascular plants (7%)mosses, liverworts, hornworts - no roots or true leaves

Vascular plants (93%) have a system of tubes that transport water + nutrients throughout the plant

Seedless vascular plants ferns + horsetails

Seed plantsgymnosperms + angiosperms

Trends in Plant Evolution:

Increasing adaptation to a terrestrial environment

Progressive reduction ofgametophyte, increasingdominance of sporophyte

Increased protection(esp. of reproductive parts)

Increased height, structuralstrength, and more complex transport tissues

Campbell & Reece 2002

Key innovations are special features that allow one group to speciate

6500 12,000

250,000

1000100

12,0

00 550

# of living species in different plant groups

100 1

1) Early land plants (~438-408 MYA, Paleozoic Era)

Bryophytes and 1st vascular plants appeared at same time

Bryophytes features (e.g. moss)

Short

Lack elaborate vascular tissue

No true leaves, roots, or stems.

Dominant gametophyte

Flagellated (=swimming) sperm

Bryophytes

(plants lacking vascular tissues)

They are all short -- why?

Typically associated with moist habitats

Most parts only 1 cell thick

Bryophyte life cycle

gametophytes are anchored into soil by threadlike protonema and single-celled rhizoids

sporophytes = brown stalks growing out of the female gametophyte

Bryophyte life cycle

1) spores can disperse by wind – why is dispersal important?

- allows colonization of new (maybe better) habitats

- allows “escape” for offspring if local conditions turn bad

- prevents inbreeding

2) sperm have to swim to reach egg – this requires environmental water

- limits where bryophytes can grow

- think about how this limited the ability of bryophytes to take over the world of dry land, compared to plant groups that evolved

later

- on the plus side: no need for pollinators, just rain or splashes

3) what features of the bryophyte body also limit:

- the size of these plants?

- the environment in which they can live?

Sphagnum sp.

Forms extensive peat bogs in northern latitudes (arctic regions) around world

Seedless vascular plants appeared ~ 420 MYA

Seedless vascular plant features (e.g., ferns) - Dominant sporophyte - Most lack true roots, stems, and leaves - Primitive vascular tissues

Diversification culminated in Carboniferous Period (~300-350 MYA), when our modern-day coal deposits were first formed

damp conditions favored the growth of huge forests of seedless vascular plants, limiting opportunities for seed plants until a global drying out

Fossil Cooksonia, a seedless plant with water-conducting tissue in stems, but no true roots or leaves

Primitive vascular plants had branching sporophytes that were independent of the gametophyte for growth

Enable multiple sporangia per individual

Lycopodium sp.

Psilotum sp.

Equisetum sp.(horsetail)

Polypodium sp. (fern)

Seedless Vascular Plants – have primitive vascular tissues

Phylum Pterophyta

Phylum Lycophyta

Fern Life cycle

Complex tissues and organs 1st appeared in seedless vascular plants

xylem - tubes of dead cells w/ lignin-reinforced cell walls, used for transporting water + dissolved minerals

phloem - living cells that transport sugars + other organic nutrients

roots - organ for uptake of nutrients from soil...-

leaves - increase surface area for photosynthesis

Microphylls, single vein Megaphylls, w/ complex branching - found in Lycophytes, the vascular system club mosses

Major evolutionary development: modified leaves called sporophylls on which sporangia grow

Fern sporophylls bear many sori, small clusters of sporangia on the underside of the sporophyll (look for these in lab)

In groups we will see later, sporophylls diversified in shape and structure; stopped looking like leaves

in gymnosperms

in angiosperms