READING 8 - STRUCTURE OF WOODY PLANTS

Vascular Cambium and Woody Plants

1. Learning Objectives

• Explain advantages and disadvantages of secondary growth.

• Define vascular cambium, fusiform initials, and ray initials.

• Describe arrangement of cambial cells.

• Discuss the axial system of wood.

• Identify portions of growth rings.

• Define heartwood, sapwood, and reaction wood.

• Compare secondary phloem to secondary xylem.

• Contrast cork cambium and vascular cambium.

• Explain functions of lenticels.

• Discuss secondary root growth.

• Provide examples of anomalous secondary growth.

2. Overview of Woody Plants

• Woody plants are characterized by growth in height and width due to the formation of wood and bark.

• Trees, shrubs, and some herbs are examples of woody plants.

• Wood is produced in layers (growth rings) annually.

• The oldest trees can be extremely large, e.g., a cypress in Mexico with a diameter of 11.6 m.

3. Secondary Growth

3.1 Types of Secondary Tissues

• Secondary Xylem:

  • Composed of tracheids, vessels, fibers, parenchyma.

  • Growth rings divided into early wood (spring) and late wood (summer).

• Secondary Phloem:

  • Comprises conducting tissues (sieve tube members, companion cells).

  • Non-conducting cells for storage, similar in structure to secondary xylem rays.

3.2 Heartwood and Sapwood

• Heartwood:

  • Non-conductive, dark, aromatic due to accumulation of phenolic compounds.

  • Older xylem that no longer transports water.

• Sapwood:

  • Active in water conduction; contains living parenchyma cells.

4. Vascular Cambium

4.1 Structure and Function

• Composed of fusiform initials (elongated cells) and ray initials (cuboidal cells).

• Arrangements:

  • Axial system: derived from fusiform initials, responsible for lengthwise conduction.

  • Radial system: derived from ray initials, used for storage and lateral conduction.

4.2 Cambial Activity

• The cambium is a cylinder that grows by adding cells, producing more xylem than phloem.

• Secondary xylem accumulates inside the cambium, and fiber formation provides support.

5. Cork Cambium

5.1 Structure and Function

• Cork cambium arises from parenchyma in the outer cortex and produces cork cells (impermeable).

• Cores cells are water-resistant due to suberin.

• Formation of periderm: includes cork, phelloderm (if formed), and cork cambium.

6. Growth Rings

• Formed annually; consist of early wood (larger vessels in spring) and late wood (smaller vessels in summer).

• Indicates climate conditions based on thickness of growth rings.

7. Reaction Wood

• Tension Wood: (in angiosperms, produced on the upper side of branches)

  • Contains gelatinous fibers for support against gravity.

• Compression Wood: (in conifers, produced on the lower side)

  • Strengthened with lignin, wider growth rings below the limb.

8. Lenticels and Oxygen Diffusion

• Lenticels are raised pores in the bark that allow gas exchange (oxygen diffusion) into the inner tissues, crucial for respiration.

9. Anomalous Forms of Growth

• Some plants like sweet potatoes exhibit unusual secondary growth by forming multiple cambium layers leading to extensive parenchyma production.

10. Comparison of Secondary Growth in Different Plant Groups

• Gymnosperms: exhibit true secondary growth; all species have vascular cambium.

• Eudicots: Most have secondary growth with variable structures, some species lack secondary growth.

• Monocots: Typically do not undergo secondary growth like true woody plants, though adventitious roots can form to increase diameter without forming true wood.

11. Importance of Wood Structure

• Wood anatomy impacts functionality, adaptability, and uses of different trees.

• Variations allow adaptations to diverse ecological niches, providing strength and resilience against environmental stressors.

Vascular Cambium and Woody Plants

1. Learning Objectives

• Advantages and disadvantages of secondary growth.

• Definitions: vascular cambium, fusiform initials, ray initials.

• Describe cambial cell arrangement and axial system of wood.

• Identify growth ring portions: heartwood, sapwood, reaction wood.

• Compare secondary phloem and xylem; contrast cork and vascular cambium.

• Explain lenticel functions and secondary root growth; give examples of anomalous growth.

2. Overview

• Woody plants (trees, shrubs, herbs) grow in height and width through wood and bark formation, consisting of annual growth rings.

• Example: cypress in Mexico has a diameter of 11.6 m.

3. Secondary Growth

• Secondary Xylem: Contains tracheids, vessels, fibers; divided into early (spring) and late (summer) wood.

• Secondary Phloem: Conducting tissues (sieve tube members, companion cells) and storage cells.

Heartwood and Sapwood

• Heartwood: Non-conductive, dark, with phenolic compounds; older xylem.

• Sapwood: Actively conducts water; contains living parenchyma cells.

4. Vascular Cambium

• Structure: Includes fusiform initials (elongated cells) and ray initials (cuboidal cells).

• Function: Grows by adding cells, producing more xylem than phloem; supports growth via fiber formation.

5. Cork Cambium

• Arises from outer cortex parenchyma; produces impermeable cork cells (suberin).

• Forms periderm, including cork, phelloderm, and cork cambium.

6. Growth Rings

• Formed annually; indicate climate conditions via thickness comparison.

7. Reaction Wood

• Tension Wood (angiosperms): Supports branches against gravity with gelatinous fibers.

• Compression Wood (conifers): Strengthened with lignin; wider rings beneath limbs.

8. Lenticels and Oxygen Diffusion

• Raised pores in bark for gas exchange, vital for respiration.

9. Anomalous Growth

• Some plants, e.g., sweet potatoes, show unusual secondary growth via multiple cambium layers.

10. Comparison of Secondary Growth

• Gymnosperms: True secondary growth with vascular cambium.

• Eudicots: Variable structures; some lack secondary growth.

• Monocots: Usually do not undergo true secondary growth.

11. Importance of Wood Structure

• Anatomy of wood affects functionality and adaptability, providing strength against environmental stressors.

Vascular Cambium and Woody Plants

1. Learning Objectives

   • Explain the advantages (such as increased support, height, and competitive advantage) and disadvantages (including resource allocation and vulnerability to damages) of secondary growth.

   • Define vascular cambium (a layer of tissue that produces secondary xylem and phloem), fusiform initials (elongated cambial cells that contribute to vertical growth), and ray initials (cuboidal cambial cells focusing on lateral transport).

   • Describe the arrangement of cambial cells: fusiform initials primarily form the axial system while ray initials contribute to the radial system, allowing for efficient transport and storage.

   • Discuss the axial system of wood comprising vertically aligned cells for water transport and structural integrity.

   • Identify portions of growth rings: heartwood (older, non-conductive wood), sapwood (active xylem involved in water conduction), and reaction wood (formed in response to uneven growth).

   • Define heartwood, sapwood, and reaction wood, utilizing definitions that discuss their structural and functional differences.

   • Compare secondary phloem (responsible for food transport) to secondary xylem (responsible for water and mineral transport).

   • Contrast cork cambium (originates in the outer cortex, producing protective cork) with vascular cambium (responsible for growth in width).

   • Explain the functions of lenticels (allowing gas exchange for respiration) and their importance in maintaining internal metabolic processes.

   • Discuss secondary root growth (increased width of roots for stability and absorbance), highlighting the relevance of vascular cambium in root systems.

   • Provide examples of anomalous secondary growth (multiple cambium layers, atypical formation in some plants like sweet potatoes).

2. Overview of Woody Plants

   • Woody plants are characterized by their ability to grow in both height and width due to the formation of wood (secondary xylem) and bark (phloem and cork).

   • Common examples include various trees (like oaks and pines), shrubs (such as roses), and some herbs that develop woody characteristics over time.

   • Wood is produced in distinct layers known as growth rings, which can be counted to determine a tree's age and growth conditions.

   • The oldest trees can reach extraordinary sizes; for instance, a cypress in Mexico has a diameter of 11.6 m, showcasing impressive secondary growth.

3. Secondary Growth 3.1 Types of Secondary Tissues

   • Secondary Xylem: Composed of tracheids (for water transport), vessels (for efficient fluid movement), fibers (for structural support), and parenchyma (for storage of nutrients). Growth rings are categorized into early wood (develops in spring with larger vessels for rapid growth) and late wood (forms in summer with smaller, dense vessels for stability).

   • Secondary Phloem: Comprised of sieve tube members (for transporting sugars), companion cells (supporting metabolic functions), and various non-conducting storage cells which share structural similarities with secondary xylem rays.

   3.2 Heartwood and Sapwood

   • Heartwood: This central core of the tree is characterized by non-conductive, dark wood often rich in aromatic compounds due to the accumulation of phenolic compounds, which provide some resistance to decay. It is made up of older layers of xylem that no longer participate in water conduction.

   • Sapwood: The outer layer of secondary xylem, active in water conduction, contains living cells (parenchyma) that assist in the transport of water and nutrients.

4. Vascular Cambium4.1 Structure and Function

   • The vascular cambium consists of fusiform initials (elongated cells oriented vertically) and ray initials (cuboidal cells oriented laterally). This structure facilitates the organization of tissues and efficient functioning.

   • Arrangements: The axial system (formed from fusiform initials) is responsible for lengthwise conduction and generating the bulk of wood, while the radial system (formed from ray initials) focuses on storage and lateral conduction of nutrients.

   4.2 Cambial Activity

   • The cambium functions as a cylinder that continuously grows by adding new cells. It typically produces more xylem than phloem, allowing for the gradual increase in the diameter of the plant. The accumulation of secondary xylem occurs inside the cambium tissue, along with fiber formation that provides essential structural support.

5. Cork Cambium5.1 Structure and Function

   • The cork cambium is formed from parenchyma in the outer cortex and produces cork cells that are impermeable to liquids and gases, primarily due to the substance suberin.

   • The formation of the periderm includes cork (outer protective layer), phelloderm (inner layer formed in some plants), and the cork cambium itself, which all contribute to the protective functionalities of the bark.

6. Growth Rings

   • Growth rings are formed annually, with each ring consisting of early wood (larger vessels formed in spring for rapid growth) and late wood (smaller vessels formed in summer for strength).

   • The thickness of growth rings can indicate climate conditions, with wider rings often correlating with favorable growing conditions while narrower rings may indicate stressors such as drought or poor soil.

7. Reaction Wood

   • Tension Wood: Found primarily in angiosperms, it is produced on the upper side of branches to help support and maintain vertical growth, characterized by the presence of gelatinous fibers that provide additional strength against gravitational stress.

   • Compression Wood: Found in conifers, it is formed on the lower side of branches and is typically reinforced with lignin, resulting in wider growth rings beneath the limb to aid in supporting the overall structure of the tree.

8. Lenticels and Oxygen Diffusion

   • Lenticels are raised pores located in the bark that play a crucial role in facilitating gas exchange, allowing oxygen diffusion into the inner tissues to support cellular respiration and other metabolic processes vital for plant health.

9. Anomalous Forms of Growth

   • Certain plants, like sweet potatoes, exhibit unusual types of secondary growth by forming multiple layers of cambium, leading to extensive production of parenchyma and creating unique growth habits that diverge from typical patterns.

10. Comparison of Secondary Growth

• Gymnosperms: Possess true secondary growth, characterized by a complete presence of vascular cambium in all species.

• Eudicots: Generally have secondary growth, but structures can vary widely; some species do not exhibit secondary growth at all.

• Monocots: Typically do not undergo secondary growth like true woody plants do, although some may form adventitious roots that increase diameter without forming traditional woody structure.

11. Importance of Wood Structure

• The anatomy of wood significantly impacts the functionality, adaptability, and various uses of different tree species.

• The structural variations seen among different types of wood allow plants to adapt to diverse ecological niches and contribute to their strength and resilience against environmental challenges, highlighting the importance of secondary growth in plant evolution and ecology.