Evolutionary Origins of Land Plants

Distinguishing Land Plants from Algae

Land plants (embryophytes) evolved from green algal ancestors and developed unique adaptations for terrestrial life.

• Cuticle: A waxy layer that prevents water loss.

• Stomata: Pores that regulate gas exchange and water loss.

• Lignin: A complex polymer that strengthens cell walls, allowing upright growth.

• Multicellular reproductive organs: Protect gametes and embryos from desiccation.

Example: Mosses have a cuticle and stomata, while green algae do not.

Alternation of Generations

Life Cycle Dominance in Plant Groups

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

• Mosses (Bryophytes): Gametophyte dominant; sporophyte dependent on gametophyte.

• Ferns (Seedless Vascular Plants): Sporophyte dominant; independent gametophyte.

• Gymnosperms & Angiosperms (Seed Plants): Sporophyte dominant; gametophyte reduced and dependent.

Key Structures:

• Antheridia: Male gametangia producing sperm.

• Archegonia: Female gametangia producing eggs.

• Sporangia: Structures where spores are produced by meiosis.

Adaptations in Larger Land Plants: Pollen (male gametophyte) and seeds (embryo protection and dispersal) allow reproduction without water.

Spore Production: Mosses and ferns produce one type of spore (homospory); seed plants produce microspores (male) and megaspores (female) (heterospory).

Major Groups on the Plant Evolutionary Tree

Classification and Synapomorphies

• Green Algae (outgroup)

• Nonvascular Plants (e.g., mosses)

• Seedless Vascular Plants (e.g., ferns)

• Seed Plants (gymnosperms and angiosperms)

Synapomorphy: A shared, derived trait unique to a group (e.g., seeds in seed plants).

Evolutionary Adaptations: Cuticle, vascular tissue, seeds, flowers, and fruits improved survival, reproduction, and competition.

Flower Structure and Reproduction

Major Structures and Functions

• Sepals: Protect the flower bud.

• Petals: Attract pollinators.

• Stamens: Male organs (anther produces pollen).

• Carpels (Pistils): Female organs (ovary contains ovules).

Microspores: Develop into male gametophytes (pollen grains).

Megaspores: Develop into female gametophytes (embryo sacs).

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

Monoecious: Both male and female flowers on one plant (e.g., corn).

Dioecious: Male and female flowers on separate plants (e.g., holly).

Perfect Flowers: Contain both stamens and carpels.

Imperfect Flowers: Contain either stamens or carpels, not both.

Sexual vs. Asexual Reproduction: Sexual increases genetic diversity; asexual (e.g., stolons, rhizomes) is rapid and efficient in stable environments.

Plant Organs and Modifications

Roots, Stems, and Leaves

• Roots: Anchor plant, absorb water and minerals.

• Stems: Support, transport, and sometimes store nutrients.

• Leaves: Main site of photosynthesis.

Modified Structures: Tubers (storage stems), tendrils (climbing stems), bulbs (storage leaves), prop roots (support).

Photosynthesis and Cellular Respiration

Light-Dependent and Light-Independent Reactions

Photosynthesis converts light energy into chemical energy in two stages:

• Light-Dependent Reactions: Occur in thylakoid membranes; produce ATP and NADPH; split water to release O2.

• Light-Independent Reactions (Calvin Cycle): Occur in stroma; use ATP and NADPH to fix CO2 into sugars.

Photosystem I and II: Work together to drive electron transport and ATP/NADPH production.

Chlorophyll a and b: Main pigments absorbing light; accessory pigments expand the range of light absorption.

Relationship: Photosynthesis provides sugars for respiration; respiration releases energy for cellular work.

C3, C4, and CAM Pathways

• C3: Most plants; Calvin cycle directly fixes CO2.

• C4: Separates CO2 fixation and Calvin cycle spatially; adaptation to high light, temperature, and dryness.

• CAM: Separates CO2 fixation and Calvin cycle temporally (night/day); adaptation to arid environments.

Equation for Photosynthesis:

Water Movement in Plants

Water Potential and Osmosis

Water moves from areas of higher to lower water potential (Ψ), influenced by solute concentration and pressure.

Osmosis: Diffusion of water across a selectively permeable membrane.

Water Potential Equation:

where is solute potential and is pressure potential.

Mechanisms of Water Transport

• Cohesion-Tension Hypothesis: Water is pulled up xylem by evaporation (transpiration) and cohesion of water molecules.

• Capillary Action: Water rises in narrow tubes due to adhesion and cohesion.

• Root Pressure: Osmotic pressure pushes water upward at night or in small plants.

Plant Growth and Tissues

Primary vs. Secondary Growth

• Primary Growth: Increases length; occurs at apical meristems (tips of roots and shoots).

• Secondary Growth: Increases girth; occurs at lateral meristems (vascular cambium and cork cambium).

Vascular Cambium: Produces secondary xylem (wood) and phloem.

Cork Cambium: Produces protective outer bark.

Xylem: Transports water and minerals upward.

Phloem: Transports sugars and organic nutrients.

Root Structure and Growth Zones

• Zone of Cell Division: Contains apical meristem; cells actively divide.

• Zone of Elongation: Cells grow in length.

• Zone of Maturation: Cells differentiate into specialized types.

Monocot vs. Eudicot Roots and Stems

Key Terms and Definitions

• Gametophyte: Haploid, gamete-producing generation.

• Sporophyte: Diploid, spore-producing generation.

• Alternation of Generations: Life cycle alternating between gametophyte and sporophyte.

• Synapomorphy: Shared derived trait unique to a group.

• Cuticle: Waxy covering preventing water loss.

• Stomata: Pores for gas exchange.

• Lignin: Strengthening polymer in cell walls.

• Microspore/Megaspore: Precursors to male/female gametophytes.

• Pollen Grain: Male gametophyte in seed plants.

• Double Fertilization: Unique to angiosperms; forms zygote and endosperm.

• Endosperm: Nutritive tissue in seeds.

• Apomixis: Asexual seed formation.

• Monoecious/Dioecious: Both sexes on one plant / separate plants.

• Osmosis: Water movement across membranes.

• Water Potential: Drives water movement.

• Transpiration: Water loss via evaporation from leaves.

• Cohesion-Tension: Mechanism of water transport in xylem.

• Apical Meristem: Growth region at tips.

• Vascular Cambium: Lateral meristem for secondary growth.

• Cork Cambium: Lateral meristem for bark.

• Xylem/Phloem: Water/mineral and sugar transport tissues.

• Primary/Secondary Growth: Lengthening/thickening of plant organs.