Chapter 26: The Colonization of Land

The Greening of the Earth

For much of Earth's early history, the land was devoid of life. The colonization of land by plants, fungi, and animals transformed terrestrial environments and enabled the development of complex ecosystems.

• Cyanobacteria and protists likely existed on land about 1.2 billion years ago.

• Plants, fungi, and animals moved onto land within the last 500 million years.

• The first forests formed approximately 385 million years ago.

Fungi & Plants: Partners in Colonization

Fungi and plants played crucial roles in the colonization of land, often forming symbiotic relationships that enhanced survival and nutrient acquisition.

• Fungi are more closely related to animals than to plants, but colonized land as partners with plants.

• Plants supply oxygen and are the primary source of food for land animals.

• Fungi decompose organic material, making nutrients available to plants.

• Animals colonized land after plants and fungi.

Algal Ancestry of Land Plants

Land plants evolved from green algae, specifically charophytes, which share several key traits with modern plants.

• Plants are multicellular, eukaryotic, photosynthetic autotrophs with cell walls made of cellulose and chloroplasts containing chlorophyll a and b.

• Charophytes in the clade Zygnematophyceae are the closest living relatives of land plants.

• Shared traits include rings of cellulose-synthesizing protein complexes and structurally similar flagellated sperm.

• DNA analysis (nuclear, chloroplast, mitochondrial) supports the relationship between plants and charophytes.

Adaptations for Life on Land

The terrestrial environment offered new resources but also posed challenges, such as water scarcity and lack of structural support. Plants evolved several adaptations to thrive on land.

• Advantages: unfiltered sunlight, abundant CO2, nutrient-rich soil.

• Challenges: scarcity of water, need for structural support.

• Sporopollenin: a protective polymer surrounding zygotes and spores, preventing desiccation.

Defining the Plant Kingdom

Systematists debate the boundaries of the plant kingdom. For practical purposes, plants are defined as embryophytes—organisms with multicellular, dependent embryos.

Derived Traits of Land Plants

Land plants possess several key traits absent in their algal ancestors.

• Alternation of generations

• Multicellular, dependent embryos

• Walled spores produced in sporangia

• Apical meristems

Alternation of Generations

Plant life cycles alternate between two distinct multicellular generations: gametophyte and sporophyte.

• The gametophyte is haploid and produces gametes by mitosis.

• Fusion of gametes forms a diploid sporophyte, which produces haploid spores by meiosis.

Multicellular, Dependent Embryos

Embryos are retained within the female gametophyte, receiving nutrients via placental transfer cells. This dependency is a defining feature of embryophytes.

Walled Spores Produced in Sporangia

Sporophytes produce spores in multicellular organs called sporangia. Spore walls contain sporopollenin, making them resistant to harsh environments.

Apical Meristems

Apical meristems are regions of cell division at the tips of roots and shoots, enabling growth throughout a plant's life.

Other Common Plant Traits

• Cuticle: waxy coating preventing water loss and microbial attack.

• Stomata: pores for gas exchange (CO2 and O2).

Early Land Plants

Fossil evidence indicates plants colonized land at least 470 million years ago. Early plants had specialized tissues for water transport, cuticles, stomata, and branched sporophytes, but lacked true roots and leaves.

Plant-Fungal Symbioses

Fungi are heterotrophs that absorb nutrients, often forming symbiotic relationships with plants.

• Fungi consist of networks of branched hyphae adapted for absorption.

• Mycorrhizae: symbiotic association where fungal hyphae transfer nutrients to plants, aiding colonization of land.

Origins and Adaptations of Fungi

Fungi are more closely related to animals than to plants, with multicellularity evolving independently in their ancestors.

• Oldest undisputed fungal fossils date to 460 million years ago.

• Fungi are heterotrophs that feed by absorption, secreting hydrolytic enzymes to break down complex molecules.

• Cell walls composed of chitin prevent cells from bursting during osmosis.

• Fungi form interwoven masses of hyphae called mycelium, maximizing surface area for absorption.

Maximizing Surface Area

Organisms maximize surface area through branching, flattening, folding, and projections, increasing efficiency in nutrient and gas uptake.

Fungal Diversity

Fungi are diverse, with about 100,000 known species and estimates up to 1.5 million.

Fungal Life Cycles

Fungi reproduce both sexually and asexually, with highly variable life cycles.

• Plasmogamy: union of cytoplasm from two haploid parent mycelia.

• Karyogamy: fusion of haploid nuclei, producing diploid cells.

• The diploid phase is brief, followed by meiosis to produce haploid spores.

• Sexual and asexual reproduction generate genetic variation.

• Molds produce haploid spores by mitosis; yeasts divide asexually.

Highlights of Plant Evolution

Plant evolution is marked by the origin of key adaptations and diversification into major groups.

• Origin of plants: 470 million years ago.

• Origin of vascular plants: 425 million years ago.

• Vascular tissue: specialized for water and nutrient transport.

• Most modern plants are vascular; non-vascular plants are called bryophytes.

• Origin of seed plants: 360 million years ago.

Plant Diversity: Bryophytes and Vascular Plants

Bryophytes are nonvascular plants, represented by three clades: liverworts, mosses, and hornworts. Vascular plants evolved later, enabling greater size and complexity.

• Bryophyte gametophytes are larger and longer-living than sporophytes.

• Vascular plant sporophytes are larger and more complex.

• Vascular tissue includes xylem (water/mineral transport, strengthened by lignin) and phloem (organic product transport).

• Roots anchor plants and absorb water/nutrients; leaves are primary photosynthetic organs.

• Leaves: Microphylls (single vein), Megaphylls (branched vascular system).

Seed Plants: Gymnosperms & Angiosperms

Seed plants evolved adaptations for reproduction and dispersal, enabling colonization of diverse habitats.

• Seed: embryo + food supply + protective coat.

• Seeds can remain dormant and are dispersed by wind, water, or animals.

• Two clades: Gymnosperms (naked seeds) and Angiosperms (seeds in ovaries).

• Common features: ovules, pollen, reduced gametophytes.

Gametophyte-Sporophyte Relationships in Seed Plants

• Gametophytes are microscopic and develop within parent sporophyte tissues.

• Parent sporophyte protects and nourishes gametophyte.

Fertilization in Gymnosperms

Gymnosperm reproduction involves ovules, pollen, and seeds.

• Ovule: egg-producing female gametophyte surrounded by integument.

• Pollen grain: male gametophyte from microspore, surrounded by protective wall.

• Pollination: transfer of pollen to ovule; does not require water.

• Seed: sporophyte embryo + food supply + protective coat.

• Seeds offer advantages over spores: multicellularity, protection, dormancy, stored food.

Origins and Examples of Gymnosperms

• Seed plant features evolved before seeds (380 million years ago).

• Oldest seed plant fossils: genus Elkinsia, 360 million years ago.

• Gymnosperms replaced lycophytes and ferns during the Permian period.

• Adaptations: thick cuticles, needle-like leaves, reduced water loss.

• Conifers dominate northern forests.

Angiosperms: Flowering Plants

Angiosperms are the most diverse and widespread plants, characterized by flowers and fruits.

• Flowers: specialized shoots with four types of modified leaves (floral organs).

• Floral organs: sepals (enclose flower), petals (attract pollinators), stamens (produce pollen), carpels (produce ovules).

• Stamen: filament + anther (pollen production).

• Carpel: ovary (contains ovules) + style + stigma (pollen reception).

• Pistil: single carpel or fused carpels.

• Seeds develop from ovules; ovary matures into fruit.

• Fruits protect seeds and aid dispersal (wind, water, animals).

Origin and Diversity of Angiosperms

Angiosperms originated at least 140 million years ago and rapidly diversified.

• Earliest angiosperm fossils: 130 million-year-old pollen grains.

• Early angiosperms may have been herbaceous and aquatic (Archaefructus), or woody.

• Phylogeny: ancestors diverged from gymnosperms about 305 million years ago.

• Living relatives: Amborella trichopoda, water lilies, star anise.

• Major groups: Monocots (one cotyledon, 70,000 species), Eudicots (two cotyledons, 170,000 species).

Plants & Fungi in Ecosystems

The colonization of land by plants and fungi altered the physical environment and enabled new biotic interactions.

• Lichens: symbiotic association between a photosynthetic microorganism and a fungus; important pioneers in soil formation.

• Plants stabilize soil, add nutrients, and release oxygen via photosynthesis.

• Plants and fungi affect chemical cycling: plants absorb nutrients, fungi decompose dead organisms.

Biotic Interactions: Mutualism and Parasitism

Biotic interactions between plants and fungi can be mutualistic or parasitic.

• Mutualism: both species benefit (e.g., mycorrhizae, endophytes).

• Endophytes: fungi living inside plant tissues; may produce toxins deterring herbivores and defending against pathogens.

• Parasitism: fungi absorb nutrients from host cells without providing benefits; ~30% of fungi are parasites or pathogens.

• Example: Cryphonectria parasitica causes chestnut blight.

Summary Table: Major Plant Groups and Features

Key Equations and Concepts

• Alternation of Generations:

• Surface Area Maximization:

Example: Mycorrhizal fungi increase phosphate uptake in plants, while plants provide carbohydrates to fungi.

Example: Conifers (gymnosperms) dominate boreal forests, while angiosperms include most crop plants.

Additional info: The notes provide a comprehensive overview of the evolutionary history, adaptations, and diversity of plants and fungi, emphasizing their ecological roles and biotic interactions.