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.

• Stomata: Pores that allow gas exchange.

Alternation of Generations

Plants exhibit a life cycle known as alternation of generations, involving both multicellular haploid (gametophyte) and diploid (sporophyte) stages.

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

• Gametophyte (n): Produces haploid gametes by mitosis.

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

Major Groups of Plants

• Nonvascular plants (Bryophytes): Mosses, liverworts, hornworts; lack vascular tissue.

• Seedless vascular plants: Ferns and relatives; have vascular tissue but no seeds.

• Gymnosperms: Seed plants with "naked" seeds (not enclosed in ovaries), e.g., conifers.

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

Structure and Organization of the Plant Body

Root and Shoot Systems

The plant body is organized into two main systems: the root system and the shoot system. The root system anchors the plant and absorbs water and minerals, while the shoot system supports leaves, flowers, and fruits for photosynthesis and reproduction.

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

• Shoots: Consist of stems, leaves, and flowers; responsible for photosynthesis and reproduction.

• Leaves: Main photosynthetic organs.

• Nodes and Internodes: Nodes are points where leaves attach; internodes are stem segments between nodes.

• Apical and Axillary Buds: Apical buds are at the tip of stems; axillary buds can form branches.

Plant Tissues and Cell Types

• Dermal Tissue: Outer protective covering (epidermis, cuticle).

• Vascular Tissue: Conducts water, minerals, and nutrients (xylem and phloem).

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

• Xylem: Transports water and minerals from roots upward.

• Phloem: Transports sugars and organic nutrients throughout the plant.

• Parenchyma Cells: Metabolism, storage, and regeneration.

• Collenchyma Cells: Flexible support for growing tissues.

• Sclerenchyma Cells: Rigid support, thick secondary walls with lignin.

Meristems and Plant Growth

Meristems are regions of undifferentiated cells that allow plants to grow throughout their lives (indeterminate growth). Apical meristems enable primary growth (lengthening), while lateral meristems enable secondary growth (thickening).

Plant Nutrition and Resource Acquisition

Essential Elements and Mineral Nutrition

Plants require essential elements for survival, growth, and reproduction. These include macronutrients (needed in large amounts, e.g., nitrogen, phosphorus, potassium) and micronutrients (needed in trace amounts, e.g., iron, zinc).

• Cation Exchange: Roots release H+ ions to displace mineral cations from soil particles, making them available for absorption.

• Mycorrhizae: Mutualistic associations between plant roots and fungi that enhance nutrient uptake.

• Nitrogen Fixation: Conversion of atmospheric nitrogen to ammonia by bacteria, often in root nodules of legumes.

Plant Reproduction and Flower Structure

Angiosperm Reproduction and Life Cycle

Angiosperms (flowering plants) reproduce sexually through a process involving flowers, pollination, fertilization, seed formation, and fruit development. The life cycle includes double fertilization, where one sperm fertilizes the egg and another fuses with two nuclei to form endosperm.

• Pollination: Transfer of pollen from anther to stigma (can be abiotic or biotic).

• Seed Adaptations: Seed coat protects the embryo; dormancy allows survival in unfavorable conditions.

• Sexual vs. Asexual Reproduction: Sexual reproduction increases genetic diversity; asexual reproduction produces genetically identical offspring.

Structure of a Flower

A typical flower consists of four main organs: sepals, petals, stamens, and carpels. Each organ has a specific function in reproduction.

• Sepals: Protect the flower bud before it opens.

• Petals: Attract pollinators with color and scent.

• Stamens: Male reproductive organs (anther and filament) that produce pollen.

• Carpels (Pistil): Female reproductive organs (stigma, style, ovary) that produce ovules.

Plant Responses to Internal and External Signals

Plant Hormones and Responses

Plants sense and respond to environmental and internal cues using hormones. Major plant hormones include:

• 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, response to mechanical stress, leaf abscission.

Plant Movements and Environmental Responses

• Phototropism: Growth toward light (mediated by blue-light receptors).

• Gravitropism: Growth in response to gravity (roots grow down, shoots grow up).

• Thigmomorphogenesis: Growth response to mechanical stimulation (e.g., wind).

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

• Photoperiodism: Response to day length, controls flowering in some plants.

Tables

Comparison of Major Plant Groups

Summary of Plant Hormones

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard introductory biology textbooks.