Plant Evolution and Adaptations

• Land plants are multicellular autotrophs adapted for terrestrial life.
• Bryophytes feature a dominant gametophyte generation.
• Seedless vascular plants possess evolved roots, stems, and leaves.
• Lycophytes are characterized by a dominant sporophyte generation.
• Pterophytes include ferns and their relatives.
• Seed plants marked a significant advancement in plant evolution.
• Gymnosperms are plants bearing naked seeds.
• Angiosperms are flowering plants.
• Plants evolved from freshwater algae.

Major Transformations in Plant Evolution

• Evolution of alternation of generations (gametophyte and sporophyte phases)
• Development of vascular tissue
• Evolution of pollen and seeds
• Development of flowers
• Evolution of stomata

Transition from Water to Land

• Adaptations were necessary for terrestrial survival.
• Ways to limit water loss and control gas exchange.
• Mechanisms to protect against UV damage to DNA.
• Ways to transport water within the plant body.
• Tissues to resist wind and gravity.
• Mechanisms to protect and disperse reproductive structures.
• Life cycles that promote genetic diversity.

Advantages of Aquatic Life

• Water is abundant and easily accessible for absorption.
• Water filters UV light.
• Water provides structural support.
• Reproductive cells (gametes, spores, pollen, seeds) are easily distributed.

Challenges of Terrestrial Life

• Water is limited and evaporates quickly.
• Plants developed a cuticle to prevent drying out.
• The cuticle limits gas exchange; plants evolved stomata that open and close.
• Plants developed pigments to absorb UV wavelengths from sunlight.

General Plant Life Cycle

• Plants exhibit alternation of generations.
• One generation is haploid (n), the other is diploid (2n).
• The dominant generation varies among plant groups.
• Haploid and diploid individuals can be multicellular.
• Spores are produced through meiosis.
• The sporophyte (2n) produces 2n spores.
• Gametes are produced through mitosis.
• The gametophyte produces 1n gametes.

Plant Sporophyte vs Gametophyte

• Sporophyte produces diploid (2n) spores.
• Gametophyte produces 1n gametes.

Plant Diversity Through Time

• Plants evolved three primary methods for pollen/spore transfer: water, wind, and animal/insect.
• Animal/insect pollination became effective about 100 million years ago.

Bryophytes (liverworts, mosses, hornworts)

• Bryophytes evolved in parallel with vascular plants via convergent evolution.
• Possess specialized cells for water and carbohydrate transport.
• They are the closest living descendants of the first land plants.
• Adaptations include being short (usually under 7 cm).
• Limited height due to lack of vascular tissue and true roots.
• Have primitive water-conducting cells without lignin.
• Possess rhizoids as a basic anchoring system for water absorption, but less efficient than roots.
• Dominant gametophyte generation is present.
• Sporophyte is attached to and dependent on the gametophyte.
• Require water for sexual reproduction.

Liverworts

• Thalli of liverworts have separate female and male gametophytes as the dominant generation.
• Produced when haploid spores germinate.
• Thallus is typically flattened and liver-shaped.
• Some species have aerial, umbrella-like gametophytes.

Mosses

• Gametophytes are small, leaflike structures.
• Stomata are only present on the capsule.
• Anchored by rhizoids.
• They are among the most abundant plants.
• Highly sensitive to air pollution.
• Moss has separate female and male gametophytes.
• Female gametophytes produce gametes in archegonia.
• Male gametophytes produce gametes in antheridia.
• Fertilization requires water.
• A zygote forms from the sperm fertilizing the egg.
• The zygote develops into a sporophyte that remains attached to the gametophyte.
• The sporophyte develops a sporangium where spores are produced.
• Mature spores disperse and germinate into male or female gametophytes.
• The gametophyte is haploid (n), with archegonia (female) and antheridia (male).
• The zygote is diploid (2n).
• The sporophyte is diploid (2n) within the sporangium.
• Spores are haploid (n).

Hornworts

• Sporophyte is attached to the gametophyte.
• Gametophyte is typically less than 2 cm.
• Sporophyte is photosynthetic and bears stomata.
• Spores are found in the hollow column of the sporophyte.

Key Traits of Vascular Plants

• Vascular tissue includes xylem and phloem.
• Xylem transports water and minerals from the roots (one-way movement).
• Phloem transports hormones and sugars throughout the plant (two-way flow).
• Stems allow plants to grow taller.
• Leaves provide more surface area for gas exchange and photosynthesis.
• Roots enable plants to gather water and nutrients from the soil.
• The cuticle helps prevent drying out.
• Stomata control gas exchange.
• Seedless vascular plants disperse by spores and rely on swimming sperm, requiring water.
• Gymnosperms and angiosperms disperse by seeds and pollen.
• The earliest vascular plants appear in the fossil record 420 million years ago.
• Xylem and phloem provided a significant advantage.
• Enabling the plants to grow larger and stay hydrated.
• Early plants lacked leaves, had small branching structures for photosynthesis, and had roots unlike modern roots.
• Leaves and roots evolved independently in lycophytes, ferns, horsetails, and seed plants.
• Lycophytes have leaves with only one vein.
• They can be terrestrial or epiphytic (living on other plants).
• Some lycophytes are desiccation tolerant, surviving in ponds that dry up.
• Resurrection plants can lose 99% of their water, then rehydrate and live.
• Living lycophytes possess vascular cambium, indicating ancestral relatives were trees.
• Ferns and horsetails form a monophyletic group and are the sister group to seed plants.
• Ferns have distinctive leaves that uncoil from tightly wound fiddleheads.
• This group includes horsetails, whisk ferns, and adder's tongue ferns.
• Whisk ferns have photosynthetic stems without leaves or roots.
• Both the gametophyte and sporophyte are photosynthetic.
• Eggs are produced in archegonium on the gametophyte.
• Sperm is produced in antheridium on the gametophyte.
• Spores are produced in sporangium on the sporophyte.
• Sorus is a cluster of sporangia.
• Bryophytes (mosses) disperse spores via wind/air.
• In both, the zygote is diploid.
• Ferns have rhizoids at both stages; mosses only on gametophyte.
• Ferns are sporophyte dominant, whereas mosses are gametophyte dominant.
• Ferns have gametangia on one gametophyte, while mosses have gametangia on different gametophytes.
• Bryophytes possess one sporophyte per gamete, and sperm is dispersed via water.
• Some ferns produce inclusia that partially encloses spores for protection, an evolutionary precursor to seeds.
• Whisk ferns live on other plants, have forked green stems, and have lost leaves and roots.
• Gymnosperms and angiosperms are the two groups of seed plants.
• Seed plants no longer require water for fertilization.
• Seeds consist of a 2n seed coat, remnants of the female gametophyte (n), and a 2n embryo.
• Male gametophytes never exposed to the environment.
• Relationship between the sporophyte and gametophyte is reversed from that of bryophytes.
• reduced to a few cells dependent on the sporophyte (gametophyte is small, sporophytes are massive).
• Seeds produced can disperse away from the parent plant.

Gymnosperms

• Approximately 1,000 species of gymnosperms exist.
• Gymnosperms produce pollen and seeds but not flowers.
• They produce cones instead.
• Cycads are gymnosperms that produce cones.
• They live in nutrient-poor soils because of a symbiotic relationship with nitrogen-fixing bacteria.
• There are about 300 species found in tropical or subtropical areas.
• Cycads are pollinated by insects and their seeds are dispersed by birds and mammals.

Gymnosperms - Ginkgos

• Ginkgos are represented by only one species dating back 270 mya.
• Display distinctive fan-shaped leaves.
• They have fleshy seeds and are great for urban environments because they can tolerate air pollution.
• Ginkgos exist as only male trees.
• Seed comes only on female trees.
• Fleshy seed cones start fermenting when they fall.

Gymnosperms - Gnetophytes

• Gnetophytes have only three genera: Welwitschia, Gnetum, and Ephedra.
• Welwitschia is found only in SW African deserts.
• Gnetum is found only in tropical rainforests.
• Ephedra is found in arid regions.

Gymnosperms - Conifers

• Conifers are the tallest and oldest trees on the planet.
• The group includes pines, junipers, and redwoods.
• Their xylem consists almost entirely of tracheids.
• Many have resin canals to deter insects and fungi.
• Conifers use wind for pollen dispersal, produce cones.
• Many are fire resistant and require fire to open cones.

Gymnosperm Life Cycle

• In the gymnosperm life cycle, the sporophyte stage is dominant.
• Both male and female cones are found on the same sporophyte (pine).
• Microsporangium contain microsporocytes (2n).
• Microsporocytes produce microspores (n) via meiosis.
• Microspores produce a two-cell pollen grain (n) via mitosis.
• Megasporangium contain megasporocyte (2n).
• Megasporocyte produces four megaspores (n) via meiosis.
• One megaspore is retained and develops into a female gametophyte (n).
• The gametophyte produces 2-6 archegonium, each containing an egg.
• Pollen grains (n) travel via wind to female cones.
• Pollen grain develops a pollen tube that enters the female gametophyte.
• Fertilization takes up to 15 months to occur.
• Sperm and egg (haploid) form a zygote, which is diploid (2n).
• The embryo develops inside the seed.

Angiosperms

• Angiosperms comprise an extremely diverse group overall.
• Early branching groups were less diverse.

Characteristics Promoting Diversification of Angiosperms

• More efficient at growth and reproduction.
• Interaction of flowers with pollinators.
• Vessel elements for support and for transporting water and nutrients.
• Xylem transports water and nutrients up from the root.
• Phloem transports sugars and hormones up and down.

Angiosperms - Monocots and Eudicots

• Monocots comprise approximately 25% of all angiosperms.
• Huge variety of forms and habitats.
• Contain one embryonic seed leaf.
• Do not develop vascular cambium.
• Leaves have parallel veins and surround the stem.
• Continuously start new roots from their stems.
• Flowers typically have parts in multiples of three.
• Examples are rice, corn, and wheat.

Angiosperms - Eudicots

• Eudicots comprise ~160,00 species, accounting for approximately 75% of angiosperms.
• Well represented in the fossil record due to pollen.
• Pollen grains feature three openings.
• Contains two embryonic seed leaves.
• Examples include coffee and milkweeds.
• Monocots have only one embryonic seed leaf, whereas eudicots have two.
• Monocots have parallel leaf veins; eudicots have branched or webbed veins.
• Economically important monocots include rice, corn, and wheat.
• Economically important eudicots include tomato, blueberries, and coffee.
• Some angiosperm flowers are "perfect," having both female and male parts on the same flower.
• Some flowers are female or male but found on the same individual, termed monoecious.
• Some flowers are female or male but found on different individuals, termed dioecious.