Green Plant Phylogeny, Anatomy, and Transport Systems

Green Plant Phylogeny

Major Groups of Green Plants

Green plants share a common ancestor and are all photoautotrophs, meaning they use light energy to synthesize organic compounds. However, not all photoautotrophs are green plants. Green plants are divided into two major groups:

• Chlorophytes: Aquatic green algae that never colonized land.

• Charophytes: Gave rise to land plants; possess multicellular haploid and diploid stages and have diploid embryos.

Adaptations to Terrestrial Life

• Desiccation: Prevented by the development of a cuticle (waxy layer) that limits water loss but also restricts gas exchange.

• Stomata: Small openings on leaf surfaces that allow for regulated gas exchange.

• Transport Systems: Water and nutrients are transported from soil via specialized cells:

  • Tracheids: Water-conducting cells in tracheophytes.

  • Xylem: Transports water and minerals.

  • Phloem: Transports food (sugars and other organic compounds).

Haplodiplontic Life Cycle

Land plants exhibit a haplodiplontic life cycle, where both haploid and diploid stages are multicellular:

1. Haploid gametophyte produces gametes by mitosis.

2. Fusion of gametes (fertilization) forms a diploid zygote.

3. Zygote divides by mitosis to form multicellular sporophyte.

4. Sporophyte produces haploid spores by meiosis.

5. Haploid spores grow into gametophytes by mitosis.

• Gametophyte: Produces gametes.

• Sporophyte: Produces spores.

• Mosses: Most primitive land plants; gametophyte is dominant.

Major Plant Groups

Bryophytes

• Closest living descendants of the first land plants.

• Lack true vascular tissue; limited in size due to reduced transport ability.

• Examples: Mosses, liverworts, hornworts.

Tracheophytes

• Vascular plants with specialized tissues for transport.

• Include lycophytes, pterophytes, and seed plants.

Lycophytes (Club Mosses)

• Most ancient group of vascular plants.

• Have roots, stems, and vascular tissue but no seeds.

• Require water for fertilization; sporophyte is dominant.

Pterophytes (Ferns, Horsetails, Whisk Ferns)

• Have vascular tissue, roots, stems, and leaves.

• No seeds; require water for fertilization.

• Sporophyte is dominant.

Seed Plants

• Dominate terrestrial landscapes.

• Include gymnosperms (naked seeds) and angiosperms (seeds in fruits/flowers).

• Evolutionary advantages of seeds:

  • Protection of embryo

  • Food supply

  • Dispersal

  • Dormancy

• Pollen: Multicellular male gametophytes; pollen tube delivers sperm to egg, eliminating the need for water for fertilization.

Plant Anatomy

Monocots vs. Eudicots

Monocots and eudicots are two major groups of angiosperms, distinguished by several anatomical features:

Feature	Monocots	Eudicots
Flower Parts	Multiples of 3	Multiples of 4 or 5
Leaf Veins	Parallel	Net-like
Vascular Bundles	Scattered	Arranged in a ring
Root System	Fibrous	Taproot
Cotyledons	1	2

Meristems

• Regions of undifferentiated cells where growth occurs.

• Cells divide indefinitely by mitosis; one daughter cell differentiates, others remain meristematic.

• Types:

  • Apical meristems: At root and shoot tips; responsible for primary growth (lengthening).

  • Lateral meristems: Increase diameter (secondary growth); include vascular cambium and cork cambium.

  • Intercalary meristems: Lengthen internodes (spaces between leaves).

Plant Tissues

• Dermal Tissue: Protection; includes epidermis, cuticle, guard cells, trichomes, and root hairs.

• Ground Tissue: Storage, photosynthesis, support.

  • Parenchyma: Most common; thin walls, large vacuoles, alive at maturity.

  • Collenchyma: Irregularly thick walls, flexible support, alive at maturity.

  • Sclerenchyma: Thick, lignified walls, dead at maturity; includes fibers and sclereids.

• Vascular Tissue: Conducts water, minerals, and nutrients.

  • Xylem: Water and mineral transport; contains tracheids and vessels.

  • Phloem: Food transport; contains sieve cells/tube members (lack nuclei) and companion cells.

Organization of the Plant Body

• Organ systems: Root system and shoot system.

• Shoot system: Vegetative (internode, node, leaf, axillary bud) and reproductive (flower).

Leaves

• Main photosynthetic organ.

• Composed of three tissue types: dermal (epidermis, cuticle), ground (mesophyll), and vascular (veins with xylem and phloem).

• Most eudicot leaves have a flattened blade attached by a petiole.

Roots

• Functions: Anchorage and absorption.

• Regions:

  1. Root cap: Protects apical meristem; secretes lubricating substances.

  2. Zone of cell division: Apical meristem divides to produce new cells.

  3. Zone of elongation: Cells enlarge, vacuoles occupy most of cell volume.

  4. Zone of maturation: Cells differentiate into specialized types.

• Cross-section features:

  • Outermost: Epidermis (dermal tissue) with root hairs.

  • Inside: Cortex (ground tissue) for storage.

  • Innermost cortex: Endodermis with Casparian strip (suberin) to regulate water/solute entry.

  • Stele: All tissues inside endodermis; pericycle gives rise to branch roots.

  • Vascular tissue: Xylem and phloem arrangement differs in monocots and eudicots.

Stems

• Covered by epidermis with cuticle.

• Vascular tissue arrangement:

  • Eudicots: Vascular bundles in a ring; vascular cambium develops for secondary growth.

  • Monocots: Vascular bundles scattered; no vascular cambium or secondary growth.

• Secondary growth (mainly in woody eudicots/gymnosperms):

  • Vascular cambium produces secondary xylem (wood) inside and secondary phloem outside.

  • Annual growth rings reflect environmental conditions.

  • Cork cambium forms periderm (cork, cork cambium, phelloderm) replacing epidermis.

  • Bark includes all tissues outside vascular cambium.

  • Lenticels allow gas exchange in woody stems.

Plant Life Span

• Perennial: Live for multiple years.

• Annual: Complete life cycle in one year.

• Biennial: Complete life cycle in two years.

Transport in Plants

Water and Mineral Transport

• Water moves one way through xylem, from roots to leaves, and exits via stomata.

• Carbohydrates move through phloem, direction depends on plant needs.

Cellular Transport Mechanisms

• Water and solutes move via passive (diffusion, osmosis) and active transport, often using transport proteins.

• Plasmodesmata: Cytoplasmic connections between cells.

• Aquaporins: Water channel proteins facilitating water movement.

Water Potential

• Represents the free energy of water; drives osmosis.

• Measured in megapascals (MPa).

• Water moves from high to low potential.

Equation:

$\Psi = \Psi_P + \Psi_S$

• $\Psi$ = water potential

• $\Psi_P$ = pressure potential

• $\Psi_S$ = solute potential

Mineral and Water Absorption Pathways

• Root hairs: Increase surface area for absorption.

• Mycorrhizal fungi: Symbiotic relationship enhances absorption.

• Three pathways to xylem:

  1. Apoplastic: Through cell walls (non-living parts).

  2. Symplastic: Through cytoplasm via plasmodesmata.

  3. Transmembrane: Across cell membranes and vacuoles (most selective).

• Endodermis: Casparian strip (suberin) forces water/solutes through cell membranes for selective uptake.

• Guttation: Exudation of water from leaf tips due to root pressure at night.

Water Transport: Transpiration and Cohesion-Tension

• Transpiration: Evaporation of water from stomata creates tension, pulling water upward through xylem.

• Cohesion (water-water attraction) and adhesion (water-cell wall attraction) maintain water column.

• Water potential gradient: Highest in roots, lowest in leaves.

Cavitation

• Formation of air bubbles (cavitation) can break water column in xylem.

• Small diameter of tracheids/vessels increases tensile strength.

• Alternative pathways (pits) minimize damage.

• Tall trees are more susceptible to cavitation due to gravity.

Stomatal Regulation of Transpiration

• Stomata control water loss and gas exchange.

• Open during the day, closed at night.

• Guard cells regulate opening via turgor changes (ion uptake/loss).

Hormonal Control: Abscisic Acid (ABA)

• ABA promotes stomatal closing during water stress.

• ABA triggers ion efflux, water follows, guard cells lose turgor, stomata close.

• Stomata also close at high CO2, high temperature, or drought.

Phloem Transport: Translocation

• Translocation: Movement of carbohydrates (and other solutes) from sources (leaves) to sinks (growing tissues) via phloem.

• Phloem sap contains sugars, hormones, mRNA, amino acids, proteins, and ions.

Pressure Flow Hypothesis

• Describes movement of sugars in phloem:

  1. Sugars actively loaded into sieve tubes at source, lowering water potential.

  2. Water enters by osmosis, increasing turgor pressure.

  3. Pressure drives sap flow toward sink.

  4. At sink, sugars are actively removed, water follows by osmosis.

Equation (Pressure Flow):

$\text{Flow} = \frac{\Delta P}{R}$

• $\Delta P$ = pressure difference between source and sink

• $R$ = resistance to flow

Example:

During spring, sugars stored in roots (sink) are transported to developing leaves (source) to support new growth.

Additional info: The above notes expand on the original outline by providing definitions, context, and examples for key terms and processes, ensuring a comprehensive and self-contained study guide.