Wood Biology Notes

Overview of Wood

• Wood consists of secondary xylem derived from lateral meristems; found in dicots and conifers.

Key Concepts

• Different Properties: Dicot and conifer wood contain various cell types, leading to distinct mechanical and physical properties.
• Bark: A protective layer produced by cork cambial cells that surrounds the wood.

Evolution of Wood

• First Seed Plants: Initially, all were woody plants.
• Gymnosperms: Continued to evolve as woody structures.
• Angiosperms: Some evolved to be herbaceous (non-woody).
• Importance of wood's evolution is framed within the concept of having evolved multiple times in various lineages.

Types of Wood

• Three Categories:
  • Dicot Wood
  • Monocot Wood
  • Conifer Wood

Plant Cell Walls

• Primarily made of cellulose.
• May have a thickened secondary cell wall situated between the primary cell wall and plasma membrane.
• Plasmodesmata: Channels that connect adjacent cells, aiding in communication and transport of substances.
• Middle Lamella: Sticky layer linking adjacent cell walls together.

Organization of Wood Cells

• Wood cell walls consist of three layers in the secondary wall:
  • S1: Thin fibrils oriented along the grain.
  • S2: Thick fibrils that follow the grain direction.
  • S3: Thin fibrils perpendicular to the grain.
• Composition: Wood is comprised of 50% cellulose, 25% lignin, and 20% hemicellulose.

Plant Organs

• Definition: Organs are biological structures made of tissue groups performing similar functions.
• Types:
  • Vegetative organs: Stem, leaf, root.
  • Reproductive organs: Sepals, petals, stamens, carpels.

Stem Structure

• Types:
  • Non-woody: Found in monocots.
  • Woody: Present in dicots and conifers.
• Rough structure entails epidermis, cortex (dicots), vascular bundles, and ground tissue.

Growth Types

• All vascular plants exhibit primary growth, while woody dicots and gymnosperms also display secondary growth.

Anatomy of Wood

• Components include:
  • Pith: Center region, present in young stems.
  • Heartwood: Central, older wood, provides structural support.
  • Sapwood: Conducts water and nutrients.
  • Cambium Layer: Layer of actively dividing cells between xylem and phloem.
  • Bark: Protective layer, consisting of periderm, cork cambium, etc.

Growth Processes

• Growth Mechanism: Meristem cells divide and some differentiate into xylem and phloem cells. The cambium expands to allow for wood growth.

Cell Types in Wood

• Axial System:
  • Conifers: Composed of tracheids, fibers, and parenchyma.
  • Eudicots: Contains vessel elements, fibers, and parenchyma.
• Radial System: Includes ray parenchyma for radial transport and storage.

Xylem Tissue

• Composed of sclerenchyma cells, typically dead at maturity, pivotal for support.
• Types:
  • Tracheids: Long, narrow cells with tapered ends。
  • Vessel Elements: Shorter and wider, facilitating more efficient water flow.

Xylem Flow Dynamics

• Water Movement:
  • Through tracheids via pit pairs.
  • Through vessel elements via perforations and pit pairs, allowing water conduction upwards.

Conifer Wood Specifics

• Contains resin canals for defense against insects and pathogens, serving critical functions for the tree's health.

Variability in Wood Characteristics

• Different species show variability in vessel sizes and arrangements, impacting the wood's mechanical properties and uses.

Bark Composition

• Periderm: Includes cork cambium, cork, and phelloderm; protects the plant.
• Variability in bark features can include layers forming under stress, e.g., fire-resilient bark in certain species.

Miscellaneous

• Tutorials and Midterm Exam: Preparation material relates to concepts in vascular cambium, secondary phloem, and xylem from provided readings.

Key Figures

• Diagrams and figures throughout the transcript illustrate structures of wood and the conduction process of xylem facilitating further understanding of the materials discussed.