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 several lines of evidence:

• Similarities in Cell Wall Composition: Both groups have cellulose-synthesizing complexes.

• Presence of Sporopollenin: A durable polymer that protects zygotes in charophytes and plant spores from desiccation.

• Genetic Evidence: DNA sequence data supports a close relationship.

Key Derived Characters of Plants (Embryophytes):

• Alternation of generations

• Multicellular, dependent embryos

• Walled spores produced in sporangia

• Multicellular gametangia

• Apical meristems

Example: Mosses exhibit alternation of generations, with distinct gametophyte and sporophyte stages.

Role of Fungi in Colonization of Land

Fungi played a crucial role in the colonization of land by plants through mutualistic relationships:

• Mycorrhizae: Symbiotic associations between fungal hyphae and plant roots, enhancing nutrient and water uptake.

• Decomposition: Fungi decompose organic matter, recycling nutrients in terrestrial ecosystems.

Structure and Nutrition of Fungi:

• Composed of hyphae forming a mycelium

• Absorptive heterotrophs: secrete enzymes to digest food externally

Example: Arbuscular mycorrhizal fungi penetrate plant root cells, increasing phosphorus uptake.

Major Groups of Plants and Their Characteristics

Nonvascular Plants (Bryophytes)

• Lack vascular tissue (xylem and phloem)

• Dominant gametophyte generation

• Examples: Mosses, liverworts, hornworts

Seedless Vascular Plants

• Have vascular tissue but do not produce seeds

• Dominant sporophyte generation

• Examples: Ferns, club mosses

Gymnosperms

• Vascular plants with "naked" seeds (not enclosed in ovaries)

• Examples: Conifers (pines, firs)

Angiosperms

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

• Most diverse group of plants

Key Adaptations for Life on Land

• Seeds: Protect and nourish the embryo

• Pollen grains: Allow fertilization without water

• Flowers: Specialized for sexual reproduction

• Fruits: Aid in seed dispersal

Plant Life Cycles and Reproduction

Alternation of Generations

Plants alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations.

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

• Gametophyte (n): Produces gametes by mitosis

• Fertilization produces a diploid zygote, which grows into a sporophyte

Identifying Structures and Ploidy

• Sporophyte: Diploid (2n)

• Gametophyte: Haploid (n)

• Spore: Haploid (n)

• Gamete: Haploid (n)

Angiosperm Reproduction

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

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

Sexual vs. Asexual Reproduction

Plant Structure and Organization

Plant Body Organization

• Root system: Anchors plant, absorbs water and minerals

• Shoot system: Stems, leaves, and flowers; supports and conducts photosynthesis

Major Plant Organs

• Roots: Absorption and anchorage

• Stems: Support and transport

• Leaves: Main site of photosynthesis

• Flowers: Reproduction (in angiosperms)

Plant Tissue Types

Plant Cell Types

• Parenchyma: Metabolism, storage, photosynthesis

• Collenchyma: Flexible support for young tissues

• Sclerenchyma: Rigid support, thick secondary walls with lignin

• Xylem: Water and mineral transport (tracheids, vessel elements)

• Phloem: Sugar and nutrient transport (sieve-tube elements)

Meristems and Plant Growth

• Apical meristems: Lengthen roots and shoots (primary growth)

• Lateral meristems: Increase thickness (secondary growth)

• Indeterminate growth: Plants can grow throughout their life

Resource Acquisition and Plant Nutrition

Essential Elements and Nutrients

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

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

Resource Acquisition Adaptations

• Root hairs increase surface area for absorption

• Mycorrhizae enhance nutrient uptake

• Leaf modifications (e.g., spines, tendrils) for specialized functions

Mineral Absorption: Cation Exchange

• Roots release H+ ions, displacing cations (e.g., K+, Ca2+) from soil particles

• Cations become available for uptake by root hairs

Equation for Cation Exchange:

Role of Bacteria and Fungi in Nutrition

• Rhizobium bacteria: Fix atmospheric nitrogen in root nodules of legumes

• Mycorrhizal fungi: Enhance phosphorus and water uptake

Angiosperm Reproduction and Adaptations

Flower Structure and Function

• Sepals: Protect flower bud

• Petals: Attract pollinators

• Stamens: Male organs (anther, filament)

• Carpels (Pistil): Female organs (stigma, style, ovary)

Pollination Mechanisms

• Abiotic: Wind, water

• Biotic: Insects, birds, mammals

Floral Adaptations: Color, scent, nectar, flower shape

Seed Adaptations

• Seed coat: Protects embryo

• Dormancy: Allows seeds to survive unfavorable conditions

Monocots vs. Eudicots

Key Terms and Definitions

• Alternation of generations: Life cycle with multicellular haploid and diploid stages

• Apical meristem: Growth region at tips of roots and shoots

• Cuticle: Waxy covering preventing water loss

• Mycorrhiza: Symbiotic association of plant roots and fungi

• Phloem: Transports sugars and nutrients

• Xylem: Transports water and minerals

• Stoma (stomata): Pores for gas exchange

• Seed: Embryo with food supply and protective coat

• Pollination: Transfer of pollen to ovule-containing part

• Double fertilization: Unique to angiosperms; forms zygote and endosperm

Additional info:

• For diagrams (e.g., Figure 28.3, 28.9, 29.2, 29.10), refer to your textbook for labeled structures and processes.

• Chapter 31.1–31.5 is listed but not detailed in the provided material; review textbook for plant responses to internal and external signals.