Evolution and Diversity of Land Plants

Plant Evolution and Major Adaptations (7A)

Introduction to Land Plants

• Land plants (embryophytes) are plants that live on land. They evolved from green algae and had to solve new problems to survive outside water.

• Key adaptations:

• Water loss prevention: Plants developed a cuticle (a waxy layer on leaves and stems thân cây) to keep water inside, and stomata (tiny pores lỗ chân lông nhỏ) to control gas exchange.

• Support: Plants evolved three-dimensional tissues and lignified cells (cells with lignin, a strong substance) to stand upright and grow taller. Giúp tăng độ cứng và nâng đỡ cây → cây có thể đứng thẳng và cao hơn

• Transport tissue: Plants developed vascular tissues — xylem (moves water) and phloem (moves nutrients) — to transport materials throughout the plant.

• Alternation of generations Luân phiên các thế hệ: Plants have a life cycle with two main stages:

• Haploid stage (gametophyte): Makes eggs and sperm.

• Diploid stage (sporophyte): Makes spores. Tạo bào tử. (meiosis)

• Summary: Land plants evolved special features to survive on land: waxy cuticle, stomata, strong tissues, and transport systems. They have a unique life cycle with two stages (alternation of generations).

Core Definitions

• Plantae / Embryophytes: These are all land plants. They have embryos (baby plants) that grow inside the parent plant’s tissues.

• Bryophytes: These are simple land plants like mosses rêu, liverworts, and hornworts. They do not have special tubes (vascular tissue) to move water and nutrients, so they stay small and close to the ground.

• Tracheophytes Tế bào khí quản: These are more advanced plants. They do have tubes (vascular tissue: xylem and phloem) to move water and nutrients. This group includes ferns (seedless vascular plants), trees, and flowering plants.

• Sporophyte: This is the plant stage with two sets of chromosomes (diploid). It makes spores (tiny cells) by splitting its chromosomes in half (meiosis).

• Gametophyte: This is the plant stage with one set of chromosomes (haploid). It makes eggs and sperm (gametes) by copying its chromosomes (mitosis).

In short:

• Bryophytes: Simple, small, no tubes. (No Vascular)

• Tracheophytes: Advanced, tall, have tubes.

• Sporophyte: Makes spores. (Meiosis)

• Gametophyte: Makes eggs and sperm. (Mitosis)

Key Adaptations for Terrestrial Life

Structural and Reproductive Innovations Đổi mới cấu trúc và sinh sản

• Three-dimensional tissues: Plants grew more complex bodies, not just flat. This helps them have more surface area to absorb sunlight and nutrients.

• Apical meristems: These are special spots at the tips of roots and shoots where cells keep dividing, so plants can grow longer and taller.

• Cuticle: A waxy layer on the outside of leaves and stems. It keeps water from escaping, so the plant doesn’t dry out.

• Stomata: Tiny pores on leaves. They open and close to let air in and out, but also help control water loss.

• Specialized organs: Plants developed roots (for water and nutrients), stems (for support and transport), and leaves (for photosynthesis).

• Multicellular embryos: Baby plants (embryos) grow inside protective tissues, keeping them safe as they develop.

• Tough-walled spores: Spores are tiny cells that can survive dry conditions. Their tough walls help them spread and survive until they find a good place to grow.

• Lignin: A strong substance in cell walls, especially in vascular tissue. It helps plants stand tall and move water efficiently.

Summary:

• These features helped plants survive and grow on land, making them stronger, taller, and better at reproducing.

Plant Life Cycle: Alternation of Generations

Sporic Life Cycle

Plants alternate between two multicellular stages: the diploid sporophyte and the haploid gametophyte. This is known as alternation of generations or the sporic life cycle.

• Sporophyte (2n): Produces haploid spores by meiosis.

• Gametophyte (n): Produces gametes (sperm and eggs) by mitosis.

• Fertilization: Fusion of gametes forms a diploid zygote, which grows into a new sporophyte. Sự hợp nhất của giao tử tạo thành một hợp tử lưỡng bội, phát triển thành một bào tử mới.

Equation for meiosis in sporophyte:

$2n \xrightarrow{meiosis} n$

Equation for fertilization:

$n + n \rightarrow 2n$

Major Plant Groups

Overview of Plant Diversity

• Bryophytes: These are simple land plants like liverworts, mosses, and hornworts. They do not have tubes (vascular tissue) to move water and nutrients. Their life cycle is gametophyte dominant, meaning the main plant you see is in the haploid stage (makes eggs and sperm).

• Seedless vascular plants: These include lycophytes and pteridophytes (like ferns). They do have tubes (vascular tissue), so they can grow taller. Their life cycle is sporophyte dominant (the main plant is diploid and makes spores), but they do not have seeds.

• Seed plants: These are gymnosperms (like pine trees) and angiosperms (flowering plants). They have vascular tissue, are sporophyte dominant, and produce seeds. Angiosperms also have flowers and fruits.

• Evolutionary sequence: Plants evolved in this order:

  1. Nonvascular plants (bryophytes)

  2. Vascular plants (ferns and allies)

  3. Seed plants (gymnosperms and angiosperms)

  4. Flowering plants (angiosperms)

Summary:

• Bryophytes = simple, no tubes, gametophyte dominant.

• Seedless vascular plants = tubes, sporophyte dominant, no seeds.

• Seed plants = tubes, sporophyte dominant, seeds (flowers/fruits in angiosperms).

• Plants got more complex over time: first simple, then tubes, then seeds, then flowers.

Evolutionary sequence: Nonvascular plants → Vascular plants → Seed plants → Flowering plants.

Bryophytes

Characteristics and Examples

• Small, moisture-loving plants: Bryophytes (like mosses, liverworts, and hornworts) are tiny plants that need wet places to live. They don’t have tubes (vascular tissue) to move water, so they stay close to the ground and absorb water directly.

• Dominant gametophyte: The main plant you see is in the haploid stage (gametophyte), which means it has one set of chromosomes. The sporophyte (diploid stage) is small and stays attached to the gametophyte, depending on it for food and water. The sporophyte cannot live on its own . It gets all its water and nutrients from the gametophyte, because it doesn’t have roots or leaves to collect these things itself.

• Flagellated sperm (sperm have tails): Bryophytes need water for reproduction because their sperm have tails (flagella) and must swim to reach the egg.

Summary:

• Bryophytes are small, simple plants that live in moist places.

• The main plant is the gametophyte (haploid), and the sporophyte is tiny and attached.

• They need water for fertilization because their sperm swim.

Bryophyte Examples

• Liverworts: Flat thallus or leafy form; some reproduce asexually with gemmae.

• Mosses: Leafy gametophytes; sporophyte with stalk and capsule (sporangium).

• Hornworts: Flat thallus; long, horn-like sporophyte (sporangium).

Seedless Vascular Plants

Key Features

• Vascular tissue: Xylem and phloem for transport and support.

• No seeds: Reproduce via spores. Sinh sản thông qua bào tử.

• Sporophyte dominant: The familiar plant body is diploid.

Lycophytes

• Small modern plants with roots, stems, leaves, and sporangia on specialized leaves.

• Ancient lycophytes were much larger and formed extensive swamp forests.

• They produce spores in special structures called sporangia

Pteridophytes

• Includes ferns, horsetails, and whisk ferns.

• Ferns have large fronds; On the underside of fern leaves, you’ll find clusters of sporangia called sori (these look like little dots).

• Gametophytes are small and produce both antheridia (sperm) and archegonia (eggs).

Seed Plants

Innovations and Diversity

• Ovules, pollen, and seeds: Noãn, phấn hoa và hạt:

  • These features let plants reproduce without needing water for fertilization.

  • Pollen carries sperm and can travel through air, so plants don’t need water for sperm to swim.

  • Ovules are structures inside the plant where eggs are made and fertilized.

  • Seeds form after fertilization. They contain the embryo (baby plant), a food supply, and a protective coat.

• Seed:

  • Protects the embryo from drying out and damage.

  • Helps the plant spread to new places (dispersal). phân tán

  • Allows the embryo to wait (dormancy) (ngủ đông) until conditions are right for growth.

• Gymnosperms:

  • Seed plants without fruits (e.g., pine trees, conifers).

  • Seeds are "naked"—not enclosed in a fruit.

• Angiosperms:

  • Flowering plants.

  • Seeds are enclosed in fruits (like apples, tomatoes).

  • Flowers help attract pollinators.

  • Ex: Ong, bướm, chim, dơi, thậm chí cả gió

Summary:

• Seed plants evolved ways to reproduce without water, using pollen and seeds.

• Gymnosperms have seeds but no fruits; angiosperms have seeds inside fruits and flowers.

Figures and Diagrams Explained

Evolutionary Relationships

• Land plants are nested within the streptophyte algae lineage.

• "Critical innovations" (e.g., plasmodesmata, sporic life cycle, stomata, vascular tissue, seeds, flowers) mark evolutionary steps.

Alternation of Generations

• Algae: Haploid-dominant; diploid zygote undergoes immediate meiosis.

• Land plants: Diploid stage becomes multicellular sporophyte, producing many spores.

Gametophytes and Gametangia

• Antheridia: Structures producing sperm.

• Archegonia: Structures producing eggs.

• Protect gametes from drying and damage.

Bryophyte Structure

• Gametophyte is dominant and close to the ground.

• Sporophyte grows on and depends on the gametophyte.

• Common in moist habitats.

Liverworts and Hornworts

• Liverworts: Flat thallus or leafy body; gemma cups for asexual reproduction.

• Hornworts: Simple thallus; horn-like sporangium for spore dispersal.

Moss Sporangium

• Sporophyte with capsule (sporangium) produces and releases spores.

• Sporophyte is dependent on the gametophyte.

Seedless Vascular Plant Structure

• Ancient forms were large due to vascular tissue; modern forms are smaller but retain basic structure.

Fern Structure

• Sporophyte with fronds and sori (clusters of sporangia).

• Young leaves unroll (fiddleheads).

• Gametophytes are small and independent.

Summary Table: Major Plant Groups and Features

Group	Vascular Tissue	Dominant Generation	Seeds	Flowers/Fruits	Examples
Bryophytes	No	Gametophyte	No	No	Liverworts, Mosses, Hornworts
Seedless Vascular Plants	Yes	Sporophyte	No	No	Lycophytes, Ferns
Gymnosperms	Yes	Sporophyte	Yes	No	Conifers, Cycads
Angiosperms	Yes	Sporophyte	Yes	Yes	Flowering plants

Exam Tips and Study Strategies

• Remember the evolutionary sequence: land survival → transport → seeds → flowers.

• Be able to explain how each adaptation improved plant success on land.

• Key distinctions: bryophytes (no vascular tissue, gametophyte dominant), vascular plants (sporophyte dominant), seed plants (pollen, ovules, seeds), angiosperms (flowers and fruits).

Additional info: The alternation of generations is unique among major eukaryotic groups, and the evolution of seeds and flowers greatly increased plant diversity and ecological dominance.