Plant Evolution and the Colonization of Land

Evidence for Plant Descent from Green Algae

Plants are believed to have evolved from green algae, specifically charophytes, based on similarities in cell wall composition, chloroplast structure, and the presence of the polymer sporopollenin that prevents desiccation of zygotes. Key derived traits of plants include multicellular, dependent embryos, alternation of generations, walled spores produced in sporangia, multicellular gametangia, and apical meristems.

• Embryophytes: All land plants share the trait of multicellular, dependent embryos.

• Cuticle: A waxy covering that prevents water loss.

• Alternation of Generations: Life cycle with multicellular haploid (gametophyte) and diploid (sporophyte) stages.

Alternation of Generations

Plants alternate between multicellular haploid and diploid generations. The gametophyte produces gametes by mitosis; the sporophyte produces spores by meiosis. Fertilization produces a diploid zygote, which grows into a sporophyte.

• Sporophyte (2n): Multicellular diploid form; produces haploid spores by meiosis.

• Gametophyte (n): Multicellular haploid form; produces gametes by mitosis.

• Spore: Haploid cell that can develop into a gametophyte.

Fungi and the Colonization of Land

Structure and Nutrition of Fungi

Fungi are heterotrophic organisms that absorb nutrients from their environment. Their bodies are composed of networks of hyphae, which collectively form a mycelium. Fungi play a crucial role in the colonization of land by forming mutualistic associations with plants (mycorrhizae), aiding in nutrient uptake.

• Hyphae: Filamentous structures that make up the body of a fungus.

• Mycelium: Densely branched network of hyphae.

• Mycorrhiza: Symbiotic association between fungi and plant roots.

Major Groups of Plants

Nonvascular Plants (Bryophytes)

Nonvascular plants, including mosses, liverworts, and hornworts, lack vascular tissue and are typically small and found in moist environments. They have dominant gametophyte generations and reproduce via spores.

• Dominant Generation: Gametophyte

• Reproduction: Spores; require water for fertilization

Seedless Vascular Plants

These plants, such as ferns, have vascular tissue (xylem and phloem) but do not produce seeds. The sporophyte is the dominant generation.

• Vascular Tissue: Xylem (water transport), Phloem (nutrient transport)

• Dominant Generation: Sporophyte

Seed Plants: Gymnosperms and Angiosperms

• Gymnosperms: Vascular plants with "naked" seeds not enclosed in an ovary (e.g., conifers).

• Angiosperms: Flowering plants with seeds enclosed in fruits (mature ovaries).

Plant Structure and Organization

Root and Shoot Systems

The plant body is organized into two main systems: the root system (anchors the plant and absorbs water/minerals) and the shoot system (stems, leaves, and flowers for photosynthesis and reproduction).

• Root System: Taproot, lateral roots, root hairs

• Shoot System: Stems (nodes, internodes), leaves (blade, petiole), buds (apical, axillary), flowers

Plant Tissues

• Dermal Tissue: Protective outer covering (epidermis, cuticle)

• Vascular Tissue: Xylem (water/mineral transport), Phloem (sugar/nutrient transport)

• Ground Tissue: Functions in storage, photosynthesis, and support (parenchyma, collenchyma, sclerenchyma cells)

Meristems and Plant Growth

Meristems are regions of undifferentiated cells that allow for indeterminate growth. Apical meristems contribute to primary growth (lengthening), while lateral meristems contribute to secondary growth (thickening).

• Apical Meristem: Growth in length

• Lateral Meristem: Growth in thickness (vascular cambium, cork cambium)

Plant Nutrition and Resource Acquisition

Essential Elements and Nutrient Uptake

Plants require essential elements (macronutrients and micronutrients) for survival. They absorb minerals from the soil through processes such as cation exchange, where hydrogen ions displace mineral cations from soil particles, making them available for uptake.

• Macronutrients: Needed in large amounts (e.g., N, P, K)

• Micronutrients: Needed in trace amounts (e.g., Fe, Zn)

• Cation Exchange: Process by which roots exchange H+ for mineral cations

Role of Bacteria and Fungi in Plant Nutrition

• Mycorrhizae: Fungi that enhance water and mineral absorption

• Nitrogen-Fixing Bacteria: Convert atmospheric N2 to ammonia (NH3), which plants can use

• Nodules: Structures on legume roots housing nitrogen-fixing bacteria

Plant Reproduction

Angiosperm Reproduction and Flower Structure

Angiosperms reproduce sexually through flowers, which contain both male (stamens) and female (carpels) reproductive organs. The life cycle includes double fertilization, where one sperm fertilizes the egg and another fuses with two nuclei to form endosperm.

• Stamen: Male organ (anther, filament)

• Carpel (Pistil): Female organ (stigma, style, ovary)

• Petals and Sepals: Attract pollinators and protect reproductive organs

• Ovule: Contains female gametophyte

• Fruit: Mature ovary that aids in seed dispersal

Pollination and Seed Adaptations

• Pollination: Transfer of pollen to the stigma; can be abiotic (wind, water) or biotic (animals)

• Seed Coat: Protects the embryo

• Dormancy: Allows seeds to survive unfavorable conditions

Sexual vs. Asexual Reproduction

• Sexual Reproduction: Increases genetic diversity; requires pollination and fertilization

• Asexual Reproduction: Offspring genetically identical to parent; rapid colonization but less diversity

Plant Responses to Internal and External Signals

Plant Hormones and Their Functions

• Auxin: Cell elongation, root formation, fruit growth

• Cytokinins: Stimulate cell division, delay aging

• Gibberellins: Stem elongation, seed germination

• Abscisic Acid (ABA): Promotes dormancy, closes stomata

• Ethylene: Fruit ripening, response to mechanical stress

Plant Sensory Responses

• Phototropism: Growth toward light (blue-light receptors)

• Photoperiodism: Response to day length (phytochromes)

• Circadian Rhythms: Internal biological clock (~24 hours)

• Triple Response: Slowing of stem elongation, thickening, and horizontal growth in response to mechanical stress

Senescence, Leaf Abscission, and Fruit Ripening

• Senescence: Programmed cell or organ death

• Leaf Abscission: Shedding of leaves, often regulated by ethylene

• Fruit Ripening: Coordinated by ethylene, making fruit palatable for dispersal

Comparison of Major Plant Groups