Plant Anatomy Lecture Notes

Plant Anatomy

Plant Tissue Systems and Cell Types

  • Dermal Tissue

    • Consists mainly of epidermis covered by a waxy cuticle.

    • Functions to protect the plant and minimize water loss.

    • Epidermis: A single layer of cells.

    • Cuticle: A waxy layer that covers the epidermis.

    • Guard Cells: Cells that regulate the opening and closing of stomatal pores, allowing gas exchange.

    • Stomatal Pore: An opening in the epidermis for gas exchange.

  • Ground Tissue

    • Parenchyma Tissue: Involved in storage, photosynthesis, and tissue repair.

    • Collenchyma Tissue: Provides support, particularly in young stems and leaves.

    • Sclerenchyma Tissue: Composed of thick walls; provides strength and support. Includes fiber cells and sclereids.

  • Vascular Tissue

    • Xylem: Transports water and dissolved minerals.

    • Contains Tracheids and Vessel Elements:

      • Tracheids: Long, narrow cells; have pits for fluid movement.

      • Vessel Elements: Shorter, wider cells; allow more efficient water transport.

    • Phloem: Transports carbohydrates and other organic compounds.

    • Contains Sieve-tube Elements (main transporting cells) and Companion Cells (support sieve-tube elements).

    • Sieve Plate: A porous structure that allows the flow of sap between the sieve-tube elements.

Plant Morphology

  • Nodes and Internodes

    • Nodes: Areas of the stem where leaves, buds, and branches arise; contain meristematic tissue.

    • Internodes: Segments of the stem between nodes.

  • Terminal Buds: Located at the tips of stems and branches; where primary tissue is produced.

  • Axillary Buds: Located in the leaf axil and can develop into lateral branches.

Plant Primary Growth and Development

  • Primary Growth

    • Results from primary meristems such as apical meristems.

    • Increases the length of the plant body by producing primary tissues.

  • Leaf Primordia: Young, developing leaves.

  • Axillary Bud Primordia: Will develop into axillary buds which give rise to branches.

  • Apical Meristem: Composed of dividing cells; one cell adds to the plant body, while another remains in the meristem.

Anatomy of Stems

  • Epidermis: Outermost layer of the stem.

  • Cortex: Ground tissue between epidermis and vascular bundles.

  • Vascular Bundles: Structures containing xylem and phloem; with specific arrangements in eudicot and monocot stems.

    • Eudicot Stem: Vascular bundles form a ring with ground tissue partitioned into pith (inner) and cortex (outer).

    • Monocot Stem: Vascular bundles are scattered throughout the ground tissue; no distinct pith and cortex.

  • Key Components:

    • Pith: Ground tissue located towards the center of the stem in eudicots.

    • Cortex: Ground tissue located between the epidermis and vascular bundles.

    • Sclerenchyma: Fibrous cells providing structural support.

Modified Stems

  1. Rhizomes: Horizontal stems growing just below or along the surface; e.g., ginger.

  2. Tubers: Enlarged rhizomes specialized for storing food; e.g., potatoes.

    • “Eyes” on potatoes are clusters of axillary buds marking the nodes.

  3. Bulbs: Vertical, underground shoots with enlarged leaves that store food; e.g., onions.

  4. Stolons: Horizontal stems; “runners” that enable asexual reproduction, e.g., strawberry plants.

Primary Plant Root Structures

  • Growth in root length concentrated near the root tip in three overlapping zones:

    • Zone of Cell Division: Active cell division occurs here.

    • Zone of Elongation: Cells lengthen, pushing the root deeper into the soil.

    • Zone of Maturation: Cells differentiate into various types.

  • Epidermis: Outermost layer of root.

  • Cortex: Composed of parenchyma cells; involved in storage.

  • Vascular Cylinder contains xylem and phloem inside the root:

    • Endodermis: Inner boundary of the cortex; regulates water and nutrients entering the vascular cylinder.

    • Pericycle: Layer of cells outside the vascular cylinder that can give rise to lateral roots.

Leaf Anatomy

  • Structure and Function:

    • Large surface area for light absorption.

    • Stomata: Allow CO2 entry and minimize water loss through the cuticle.

    • Veins: Vascular bundles connecting xylem and phloem; help maintain leaf shape.

  • Mesophyll: Contains layers of parenchyma cells for photosynthesis:

    • Upper Epidermis: Covered by cuticle; protects the leaf.

    • Lower Epidermis: Contains stomata and guard cells.

    • Trichomes: Small hair-like structures that reduce water loss.

Modified Leaves

  1. Tendrils: Modified leaves for clinging; e.g., pea plant.

  2. Spines: Modified leaves for protection; e.g., cactus spines.

  3. Storage Leaves: Modified for water storage in succulents.

  4. Bracts: Modified leaves that attract pollinators; e.g., red parts of the poinsettia.

  5. Reproductive Leaves: Produce plantlets which fall and root in the soil; e.g., Kalanchoe daigremontiana.

Secondary Growth

  • Meristems Involved:

    • Vascular Cambium: Produces new xylem and phloem cells, increasing stem diameter.

    • Cork Cambium: Produces cork to the outside and other cells to the inside, forming the periderm. Periderm replaces the epidermis and cortex in woody stems and roots.

  • Cork Cells: Dead at maturity; contribute to bark.

  • Secondary Xylem: Forms wood, while secondary phloem contributes to bark.

Tree Trunk Anatomy

  • Parts include:

    • Phloem Ray: Transport nutrients.

    • Cork: Protective layer.

    • Cork Cambium: Layer producing cork.

    • Lateral Ray: Radial transport of nutrients between xylem and phloem.

  • Annual Rings: Indicate the age of the tree based on yearly growth patterns.