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 reproductive success and seed dispersal in angiosperms.

Major Plant Groups and Their Characteristics

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

• Seedless vascular plants: Ferns and relatives; have vascular tissue, dominant sporophyte, reproduce via spores.

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

• Angiosperms: Flowering plants with seeds enclosed in fruits; most diverse plant group.

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.

• Stems: Support leaves and reproductive structures; contain nodes and internodes.

• Leaves: Main site of photosynthesis; consist of blade and petiole.

• Apical bud: Growth at the tip of the stem.

• Axillary bud: Can form lateral branches.

Plant Tissue 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).

Plant Cell Types

• Parenchyma cells: Metabolism, storage, photosynthesis.

• Collenchyma cells: Flexible support for growing tissues.

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

• Tracheids and vessel elements: Water-conducting cells in xylem.

• Sieve-tube elements: Sugar-conducting cells in phloem.

Meristems and Plant Growth

• Apical meristems: Enable primary growth (lengthening of roots and shoots).

• Lateral meristems: Enable secondary growth (thickening of stems and roots).

• Indeterminate growth: Plants can grow throughout their life due to meristems.

Plant Nutrition and Resource Acquisition

Essential Elements and Nutrients

• Macronutrients: Required in large amounts (e.g., N, P, K, Ca, Mg, S).

• Micronutrients: Required in trace amounts (e.g., Fe, Mn, Zn, Cu, B, Mo, Cl, Ni).

Resource Acquisition Adaptations

• Root hairs: Increase surface area for absorption.

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

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

Role of Bacteria and Fungi in Plant Nutrition

• Nitrogen fixation: Conversion of atmospheric N2 to ammonia (NH3) by bacteria in root nodules (e.g., Rhizobium).

• Mycorrhizae: Fungi that increase water and mineral absorption for plants.

Plant Reproduction and Life Cycles

Angiosperm Reproduction

Angiosperms reproduce sexually through flowers, which contain both male and female reproductive organs. The process involves pollination, fertilization, seed development, and fruit formation.

• Five major steps: (1) Formation of male and female gametophytes, (2) Pollination, (3) Fertilization (double fertilization), (4) Seed development, (5) Fruit formation.

• Double fertilization: One sperm fertilizes the egg (zygote), another fuses with two nuclei to form endosperm (nutritive tissue).

Flower Structure

Flowers are specialized shoots with four types of organs: sepals, petals, stamens, and carpels. Each organ has a specific role in reproduction.

• Sepals: Protect the flower bud.

• Petals: Attract pollinators.

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

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

Pollination and Seed Adaptations

• Abiotic pollination: Wind or water disperses pollen.

• Biotic pollination: Animals (insects, birds, bats) transfer pollen.

• Seed coat: Protects the embryo.

• Dormancy: Allows seeds to survive unfavorable conditions.

Sexual vs. Asexual Reproduction

• Sexual reproduction: Increases genetic diversity but requires pollination and fertilization.

• Asexual reproduction: Offspring are genetically identical to the parent; rapid colonization but less genetic variation.

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, control apical dominance.

• Gibberellins: Stem elongation, seed germination, fruit development.

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

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

Plant Sensory Responses

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

• Photoperiodism: Response to day length (phytochromes).

• Circadian rhythms: Internal biological clocks (~24 hours).

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

Senescence and Abscission

• Senescence: Programmed cell or organ death (e.g., leaf drop, fruit ripening).

• Leaf abscission: Shedding of leaves, often regulated by ethylene.