Biology Chapter 32: Plant Evolution

Chapter 32: The Evolution of Plants

• Plants are complex multicellular autotrophs, primarily found on land. Nearly 300,000 species are currently known.

Adapting to Terrestrial Living

• Early plant ancestors, green algae, were aquatic and not well adapted to land.
• Moving to land required adaptations to overcome environmental challenges like obtaining minerals, conserving water, and reproducing without water.

Adapting to Terrestrial Living (continued)

• A waxy cuticle prevents water loss to the air, allowing plants to thrive on land.
• Water uptake occurs primarily through roots while water loss to air through this cuticle is minimized.
• Stomata (pores) allow gas exchange (taking in CO2, releasing O2) despite the cuticle.

Adapting to Terrestrial Living (continued - Reproduction)

• Sexual reproduction on land faced unique challenges.
• Early plants needed a water film for sperm to fertilize an egg.
• Pollen evolved later, enabling the transfer of gametes without water.

Adapting to Terrestrial Living (Alternation of Generations)

• Plants alternate between diploid (sporophyte) and haploid (gametophyte) generations.
• The diploid sporophyte produces haploid spores.
• Haploid spores develop into the gametophyte, which produces gametes.

Plant Evolution

• Key evolutionary advances in land plants include:
  • Alternation of generations: The sporophyte became the dominant generation.
  • Vascular tissue: Transports water and nutrients throughout the plant, providing support.
  • Seeds: Provide nutrients and protection for the embryo until conditions are favorable for growth.
  • Flowers and fruits: Improved mating opportunities and seed dispersal in stationary organisms.

Nonvascular Plants

• The first successful land plants lacked a vascular system.
• Only three phyla (liverworts, hornworts, mosses) lack a vascular system, limiting their size.

Moss Life Cycle

• Mosses are a type of non-vascular plant.
• Their life cycle involves a complex transition between a diploid and haploid phase.

Evolution of Vascular Tissue

• Seven plant phyla have vascular systems.
• Vascular tissue consists of specialized cells forming a network throughout the plant.
• Xylem transports water and minerals.
• Phloem transports carbohydrates.

Evolution of Vascular Tissue (continued)

• Early vascular plants grew taller due to primary growth at the tips of stems and roots.
• Later vascular plants developed secondary growth, allowing for increased diameter (wood).

Seedless Vascular Plants

• Two major phyla: ferns (fronds) and club mosses.
• In ferns, the sporophyte generation is larger and more advanced than the gametophyte.

Evolution of Seed Plants

• Seeds are a crucial adaptation for life on land, protecting the embryo during its vulnerable stage.
• Seed plants have male and female gametophytes, which develop entirely within the sporophyte.
• Male gametophytes (pollen grains) arise from microspores.
• Female gametophytes(containing the egg) are within ovules, developed from megaspores

Evolution of Seed Plants (continued)

• Water is no longer required for fertilization.
• Pollination (by insects, wind, etc.) transfers pollen to an ovule.
• The pollen tube carries sperm to the egg.

Evolution of Seed Plants (Gymnosperms)

• Gymnosperms include conifers, cycads, gnetophytes, and ginkgo.
• Conifers produce their seeds in cones; most conifers have needle-like leaves for enhanced water conservation.
• Cycads have short stems and palm-like leaves.
• Gnetophytes are unusual plants.
• Ginkgoes are mostly characterized by their fan-shaped leaves.

Evolution of Seed Plants (Gymnosperms) - Life Cycle

• Conifers are notable for a multi-year cycle that involves two key types of cones: pollen cones and seed cones.
• Pollen is dispersed, often by the wind, to seed cones, where fertilization takes place.
• The resulting seeds are typically also wind-dispersed.

Rise of the Angiosperms

• Angiosperms (flowering plants) are the most diverse group on earth (90% of plant species).
• They use flowers to attract pollinators (insects and animals).

Rise of the Angiosperms (Flower Structure)

• Flowers consist of four whorls attached to a receptacle: sepals, petals, stamens, and carpels.

Rise of the Angiosperms (Double Fertilization)

• Angiosperms reproduce through a process called double fertilization.
• The first sperm fertilizes the egg, forming a zygote.
• The second sperm fuses with polar nuclei, forming a triploid endosperm. -This endosperm is nutritive tissue within the seed.

Rise of the Angiosperms (Fruits)

• Fruits are mature ovaries that surround the ovules/seeds.
• They aid in seed dispersal, often by attracting animals.

Double Fertilization (continued)

• Endosperm tissue is often the nutritive material for the seed's early development.
• In some angiosperms, endosperm is also stored later as cotyledon in the seed.

Double Fertilization (Dicots vs. Monocots)

• Dicots typically have two cotyledons, while monocots have one.

Fruits (dispersal)

• Fruits aid in seed dispersal, sometimes attracting animals or facilitating wind dispersal.