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: Plants with multicellular, dependent embryos.

• Cuticle: A waxy covering that prevents water loss.

• Stomata: Pores for gas exchange.

Alternation of Generations

Plants exhibit a life cycle known as alternation of generations, involving both multicellular haploid (gametophyte) and diploid (sporophyte) stages. This adaptation allows for increased genetic diversity and survival in terrestrial environments.

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

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

• Fertilization: Fusion of gametes forms a diploid zygote, which develops into a sporophyte.

Key Adaptations for Life on Land

• Seeds: Protect and nourish the embryo, allowing for dormancy and dispersal.

• Pollen grains: Enable fertilization without water by transporting sperm via air or animals.

• Flowers and Fruits: Enhance pollination and seed dispersal, especially in angiosperms.

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. Their dominant life stage is the gametophyte.

• Key features: No true roots, stems, or leaves; rely on diffusion for water and nutrient transport.

Seedless Vascular Plants

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

• Lignin: Provides structural support for upright growth.

Gymnosperms

Gymnosperms are seed plants that produce "naked" seeds not enclosed in an ovary. Most are cone-bearing trees (conifers).

• Examples: Pines, firs, spruces.

Angiosperms

Angiosperms, or flowering plants, produce seeds enclosed within a fruit (mature ovary). They are the most diverse group of plants.

• Flowers: Specialized for sexual reproduction.

• Fruits: Aid in seed dispersal.

Plant Structure and Organization

Overview of the Plant Body

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).

• Roots: Anchor the plant, absorb water and minerals, store food.

• Shoots: Support leaves and reproductive structures; include stems, leaves, and flowers.

• Leaves: Main site of photosynthesis.

Plant Tissues and Cell Types

• Dermal tissue: Protective outer covering (epidermis, cuticle).

• Vascular tissue: Xylem (water/mineral transport), phloem (sugar/nutrient transport).

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

Meristems and Plant Growth

Meristems are regions of undifferentiated cells that enable indeterminate growth. Apical meristems lengthen roots and shoots (primary growth), while lateral meristems thicken stems and roots (secondary growth).

Plant Nutrition and Resource Acquisition

Essential Elements and Mineral Nutrition

• Macronutrients: Required in large amounts (e.g., nitrogen, phosphorus, potassium).

• Micronutrients: Needed in trace amounts (e.g., iron, zinc).

Resource Acquisition Adaptations

• Root hairs: Increase surface area for absorption.

• Mycorrhizae: Mutualistic associations with fungi that enhance nutrient uptake.

• Cation exchange: Process by which roots exchange H+ ions for mineral cations in soil.

Role of Bacteria and Fungi

• Nitrogen fixation: Conversion of atmospheric N2 to ammonia by bacteria in root nodules (e.g., Rhizobium).

• Mycorrhizae: Fungi that increase water and mineral absorption.

Plant Reproduction

Angiosperm Reproductive Structures

Flowers are the reproductive organs of angiosperms, typically consisting of four types of organs: sepals, petals, stamens, and carpels.

• Stamen: Male organ (anther + filament); produces pollen.

• Carpel (pistil): Female organ (stigma, style, ovary); produces ovules.

• Sepals: Protect flower bud.

• Petals: Attract pollinators.

Angiosperm Life Cycle

• Double fertilization: Two sperm cells fertilize the egg and central cell, forming a zygote and endosperm.

• Seed formation: Embryo, endosperm, and seed coat develop within the ovule.

• Fruit formation: Ovary matures into fruit, aiding seed dispersal.

Pollination and Seed Dispersal

• Abiotic pollination: Wind or water transfers pollen.

• Biotic pollination: Animals (insects, birds) transfer pollen; flowers often have adaptations to attract pollinators.

• Seed adaptations: Seed coat for protection, dormancy for survival in adverse conditions.

Plant Hormones and Responses

Major Plant Hormones and Their Functions

• Auxin: Cell elongation, root formation, phototropism.

• Cytokinins: Stimulate cell division, delay aging.

• Gibberellins: Promote stem elongation, seed germination.

• Abscisic acid (ABA): Induces dormancy, closes stomata during water stress.

• Ethylene: Fruit ripening, leaf abscission, response to mechanical stress.

Plant Responses to Stimuli

• Phototropism: Growth toward light (mediated by auxin).

• Gravitropism: Growth in response to gravity.

• Triple response: Slowing of stem elongation, thickening, and horizontal growth in response to mechanical stress (ethylene-mediated).

• Circadian rhythms: Internal biological clocks with ~24-hour cycles.

Light Receptors

• Blue-light photoreceptors: Mediate phototropism and other responses.

• Phytochromes: Regulate seed germination, shade avoidance, and flowering based on red/far-red light.

Comparing Monocots and Eudicots

• Monocots: One cotyledon, parallel leaf veins, scattered vascular bundles, fibrous roots, floral parts in multiples of three.

• Eudicots: Two cotyledons, net-like leaf veins, vascular bundles in a ring, taproot system, floral parts in multiples of four or five.

Summary Table: Major Plant Groups and Features