Plant Phylogeny and Evolution

Phylogenetic Tree of Major Plant Groups

• Each node represents specific characteristics.

• Node 1: All vascular tissues (shared by all land plants).

• Node 2: Origin of vascular plants.

• Node 3: Seed plants (united by seeds).

Plant Evolution

• Characterized by adaptations facilitating life on land and success in terrestrial ecosystems.

Major Plant Groups

• Bryophytes: Seedless, non-vascular plants.

• Seedless Vascular Plants.

• Seed Plants:

  • Gymnosperms.

  • Angiosperms.

Transition to Terrestrial Environment

• Evolution associated with transition from aquatic to terrestrial environments.

• Early land plants had spores and sporangia.

• Derived structures (roots, vascular tissues, leaves) developed later.

Bryophytes

• Primitive, small, non-vascular plants.

• Require flagellated sperm to swim through moisture for fertilization.

• Spores for dispersal.

Ferns

• Early vascular plants.

• Vascular tissues indicated in red.

• Rudimentary leaves (leaflets or frondlets) for photosynthesis.

• Well-developed stems and roots for anchorage, water, and nutrient absorption.

Seed Plants

• Advanced roots, vascular tissues.

• Pollen and seeds for dispersal in non-aquatic environments.

• Drying at the end of the Carboniferous period led to the development of pollen to protect sperm from desiccation.

Bryophytes: Seedless Non-Vascular Plants

• Small, low due to lack of internal vascular system.

• Bryophytes include mosses, liverworts, and hornworts.

• Require moisture for fertilization (flagellated sperm needing to swim).

• Dominant gametophyte generation.

Liverworts

• Thallus: Gametophyte body.

• Sporophytes contained within gametophytes.

• Sporangia in sporophytes for spore formation.

• Local liverworts (e.g., Marchantia).

• Fleshy leaf-like gametophyte with sporophyte structures growing out.

Hornworts

• Sporophyte looks like long slender horns growing out of the mossy gametophyte.

• Spores released from the sporophytes.

Mosses

• Large green leaf-like structures (gametophyte).

• Sporophyte structure grows out of the gametophyte.

• Hair cap moss (hairy cap moss) example: brown stalks with enlarged capsule (sporophyte).

• Sporophytes produce spores to create new gametophyte.

Moss Life Cycle

• Gametophyte forms the main body of the moss.

• Gametophyte produces eggs and sperm.

• Zygote develops into mature sporophyte attached to gametophyte.

• Meiosis occurs in sporophyte to produce haploid spores, which are released to produce new gametophytes.

• Archegonia make eggs, antheridia make sperm (mitosis occurs in these haploid organisms).

• Sperm swim through a water film to reach the egg.

• Fertilization produces a zygote which develops into a sporophyte stalk structure.

• Fertilization: two haploid cells (sperm and egg) produce a diploid zygote.

• Mature sporophyte releases haploid spores (meiosis in sporophyte capsule).

Alternation of Generations in Bryophytes

• Mature female gametophyte produces haploid spores via sporophytes.

• Spores are released, producing new male and female gametophytes (haploid generation).

• Gametophytes contain antheridia (sperm) and archegonia (egg).

• Sperm propelled via raindrops or swimming through water film to reach the egg.

• Fertilization produces a zygote which grows into a new sporophyte.

• Sporophyte produces new spores via meiosis.

Mosses in Moist Environments

• Organic matter and debris build up as mosses live and die, leading to partial decay.

• Peat is produced when mosses partially decay in water.

• Peat: Partially decayed plant matter that can burn.

• Peat can be compressed to form coal (10:1 compression ratio).

• Peat was used as a fuel source in rural areas of Europe, England, Ireland, and Scotland.

• Buildup of peat during the Carboniferous period produced most of our existing coal deposits.

Vascular Plants

• Earliest vascular plant fossils: ~425 million years ago.

• Competition for sunlight hypothesized to drive the development of vascular tissues.

• Vascular tissues needed for taller plants to transport water and nutrients.

Types of Vascular Tissues

• Xylem: Transports water.

• Phloem: Transports sugars dissolved in water via passive movement of water.

• Evaporation pulls water up through the xylem.

• Vascular tissues also provide structural support.

Additional Features of Vascular Plants

• More well-developed roots for water uptake.

• More well-developed leaves for photosynthesis.

Early Vascular Plants

• Sporophyte not dependent on gametophyte for nutrition.

• Still have sporangia, sperm still need to swim to the egg.

• More well-developed roots and stems.

• Branching stems separate them from non-vascular plants (result of sunlight competition).

Fern Life Cycle

• Gametophytes are small and inconspicuous.

• Produce flagellated sperm that swim to fertilize eggs (require moist environments).

• Zygote develops within the female gametogenia and becomes an independent sporophyte.

• Sporophyte produces sporangia with cells undergoing meiosis to produce haploid spores.

Fern Life Cycle Details

• Mature sporophyte (fern fronds) has clusters of spores (sori) on the undersides of leaflets.

• Sporangia located within sori produce spores via meiosis.

• Spores disperse and mature into new gametophytes.

• Gametophytes typically have both male (antheridium) and female (archegonium) gametophyte cells.

• Sperm swim through a moist environment to reach the egg within the archegonium.

• Fertilization produces a zygote that develops into a new sporophyte.

Seedless Vascular Plants: Major Groups

Lycophytes

• Club mosses, spike mosses, quillworts.

• ~1,000 species.

• Simple rudimentary vascular system.

• Most ancient group of vascular plants.

• Widespread during the Carboniferous.

• Many primitive lycophytes went extinct at the end of the Carboniferous due to drying and warming.

• Often grow epiphytically on other plants.

• Small leaf-like structures, smaller ground-hugging plants with branching roots.

Examples of Lycophytes

• Spike moss (Selaginella).

• Quillwort.

• Club moss.

• Quillworts named for quill-like spikes.

• Club mosses have club-shaped cones called strobili.

Pterophytes

• Ferns and related structures: whisk ferns and horsetails.

• Require moist environments.

• Share various traits with other seed plants.

• Larger, overtopping growth; roots are more branching.

Whisk Ferns

• Relatively primitive.

• Lack true leaves or roots.

Horsetails

• Common in areas with standing water (wetlands, marshes, bogs).

Ferns

• Many varieties.

Whisk Fern Details

• Genus Psilotum.

• Branching stems; lack true leaves or roots.

  • Scale-like outgrowths may help with photosynthesis.

• Have sporangia producing spores.

Horsetail Details

• Adapted to living in areas with standing water.

• Hollow, air-filled stems.

• Separate vegetative versus reproductive stems.

Fern Details

• Stems project vertically, with secondary stems projecting horizontally, producing long leaflets (fronds).

• Fronds have spores in sporangia clustered on the undersides.

• Classification based on levels of branching.

Carboniferous Period Reproduction

• Artist's depiction: lots of standing water, very moist, lots of organic vegetation.

• Produced peat, which was then compressed into coal deposits.

• Carboniferous literally means coal-bearing or carbon-bearing.

• Removed large amounts of $CO_2$ from atmosphere and sequestered them in coal deposits.

• Burning fossil fuels (coal, peat, natural gas) puts carbon into the atmosphere at a faster rate than natural cycles.

• Global drying out at the end of the Carboniferous period brought an end to the dominance of moisture-requiring groups and provided selective pressures for seed plants.

Local Fern Species

• Sword fern: long individual fronds with leaflets.

• Lady fern.

• Bracken fern: tall, broad triangular frondlets.

• Deer fern.