Vascular Cambium – Structure and Function, Seasonal activity

What is the Vascular Cambium?

• The vascular cambium is a vital tissue in plants, particularly in dicotyledons and gymnosperms, where it plays a crucial role in secondary growth. This cylindrical layer of meristematic cells lies between the xylem and phloem and is responsible for producing secondary vascular tissues, which contribute to the plant’s increased girth over time.
• The formation of the vascular cambium begins with the fascicular cambium, which originates within the bundles of primary vascular tissue. In most dicotyledons and gymnosperms, some cells of the procambium remain meristematic after primary growth is complete, eventually developing into this fascicular cambium. As the plant continues to grow, additional bands of meristem, known as the interfascicular cambium, form between these bundles from the interfascicular parenchyma. Together, the fascicular and interfascicular cambium create a continuous ring of cambium, often resembling a hollow cylinder that extends through the nodes and internodes of the plant stem.
• This continuous cambial ring is significant because it marks the beginning of the secondary growth phase. Within this ring, the cambium segregates into two distinct types of cells: fusiform initials and ray initials. The fusiform initials are elongated cells that produce secondary xylem (wood) on the inner side and secondary phloem on the outer side. The ray initials, on the other hand, are smaller and give rise to radial files of cells known as rays, which are essential for the lateral transport of nutrients and water across the plant’s stem.
• The vascular cambium functions by dividing in a tangential plane, meaning it produces new cells that contribute to the thickening of the plant’s axis. The secondary xylem, formed on the inner side of the cambium, accumulates over time and becomes the wood that provides structural support. Conversely, the secondary phloem, formed on the outer side, remains functional in the transportation of nutrients.
• The cambial activity is continuous throughout the life of the plant, with the cambium maintaining its position between the xylem and phloem. This consistent positioning allows for the ongoing production of secondary tissues, which is essential for the plant’s growth and adaptation to environmental changes.

Location of Vascular Cambium in Plants

The vascular cambium is a key component of secondary growth in plants, responsible for producing secondary vascular tissues. Its location varies among different plant types, as detailed below:

• In Herbaceous Dicots
  • Vascular Bundles: In herbaceous dicots, which do not undergo significant secondary growth, the vascular cambium is found within the vascular bundles. These bundles are typically arranged in a scattered pattern throughout the stem.
    • Arrangement: The cambium appears as a narrow, continuous ring within each vascular bundle, often likened to beads on a necklace.
    • Function: In these plants, the cambium’s primary function is to produce secondary xylem and phloem, albeit to a limited extent compared to woody plants.
• In Woody Plants
  • Cylindrical Configuration: In woody dicots and gymnosperms, the vascular cambium forms a continuous, cylindrical layer around the stem or root.
    • Continuous Ring: This cylindrical cambium is positioned between the primary xylem and primary phloem, encircling the stem and extending into branches and roots.
    • Development: As secondary growth progresses, the cambium continuously adds new layers of secondary xylem inward and secondary phloem outward, contributing to the increase in stem diameter.
• In Seed-Producing Gymnosperms
  • Similar Arrangement: Gymnosperms, such as conifers, exhibit a similar vascular cambium arrangement to that of woody dicots.
    • Cylindrical Layer: Here, the cambium forms a continuous ring, contributing to the formation of wood and secondary phloem.
    • Needles: In coniferous plants, vascular cambium can also be found in the needles, although its role is more limited compared to that in stems and roots.
• In Monocots
  • Absence of Vascular Cambium: Monocots typically lack a vascular cambium because they do not undergo significant secondary growth.
    • Primary Growth Only: Instead, these plants primarily increase in size through primary growth, with vascular bundles scattered throughout the stem but without the formation of a cambial ring.

Structure of Vascular Cambium

The vascular cambium, a crucial meristematic tissue in plants, is composed of two primary types of cells: fusiform initials and ray initials. These cells are specialized to support the production of secondary xylem and phloem, which are essential for the plant’s growth and structural integrity. The structure and function of these cells are intricate and serve distinct roles in the cambium’s overall activity.

1. Fusiform Initials

• Shape and Size: Fusiform initials are elongated cells with tapered ends, resembling flat shoelaces. Their length varies depending on the species and the age of the plant. As the plant matures, these cells tend to increase in length.
• Cell Faces: These cells typically have around 18 contact faces with neighboring cells, though the number can range from 8 to 32. This structure allows for a complex network of connections within the cambium.
• Division and Differentiation: Fusiform initials undergo periclinal division, where the new cells form in radial files. The inner cells differentiate into secondary xylem, while the outer cells form secondary phloem. Some fusiform initials remain undifferentiated, maintaining their meristematic properties and contributing to the cambial zone.
• Function: These cells are essential for producing the structural tissues of the plant. The xylem, formed towards the center, provides support and water transport, while the phloem, formed towards the periphery, is involved in nutrient distribution.

2. Ray Initials

• Shape and Size: Ray initials are smaller than fusiform initials, typically isodiametric, meaning they have equal dimensions in all directions. They are usually about two or three times as tall as they are wide, making them appear more compact.
• Formation: Ray initials can arise in several ways, including lateral division of fusiform initials, conversion of the end of fusiform initials into ray initials, or reduction of fusiform initials to single ray initials. This flexibility in formation ensures the proper development of rays within the cambium.
• Conversion: In some plants, particularly gymnosperms and dicots, fusiform initials can convert into ray initials and vice versa. This ability helps maintain the balance between rays and fusiform cells, preventing large parenchyma areas that could weaken the wood structure.
• Function: Ray initials are crucial for the horizontal transport of nutrients and water across the stem. They form rays, which are groups of cells that act as conduits between the inner and outer regions of the plant.

3. Dormancy and Reactivation

• Dormancy: Under stressful conditions, the vascular cambium can enter a dormant state, halting cell division. During this period, some xylem and phloem mother cells become quiescent and partially differentiated.
• Reactivation: When favorable conditions return, such as in spring, the cambial cells resume division, spurred by the basipetal movement of the hormone auxin. This reactivation allows the plant to quickly produce new tissues and continue its growth cycle.

The vascular cambium’s structure is highly specialized, with fusiform and ray initials playing distinct yet complementary roles. This organization allows the cambium to efficiently produce the secondary tissues that are vital for the plant’s growth and survival.

Types of Cambium

Cambium is a meristematic tissue responsible for the secondary growth of plants, leading to the formation of new vascular tissues. The classification of cambium types is based on the arrangement of fusiform cells in tangential sections. There are two primary types: storied (or stratified) cambium and non-storied (or non-stratified) cambium.

1. Storied or Stratified Cambium

• Arrangement of Cells: In storied cambium, fusiform cells are arranged in distinct tiers or stories. This means that the cells align horizontally at the same levels, creating a layered appearance when viewed tangentially.
  • Structural Features: The ends of fusiform cells in adjacent stories generally overlap only slightly, forming a zigzag pattern. This alignment creates a structured appearance in the cambial tissue.
  • Development: Ray initials can contribute to this type by either losing fusiform cells located between them or by transverse divisions that convert fusiform cells into rows of ray initials. This results in structural elements extending radially.
• Characteristics:
  • Tissue Organization: The organization into stories allows for a more defined layering within the cambium.
  • Examples: This type is often observed in woody dicotyledons, where the organized structure facilitates efficient secondary growth.

2. Non-storied or Nonstratified Cambium

• Arrangement of Cells: In non-storied cambium, fusiform cells overlap extensively without a clear lateral alignment. This results in a seemingly random distribution of cell ends.
  • Structural Features: The overlapping of cell ends is more extensive compared to storied cambium, with no distinct horizontal alignment.
  • Characteristics: Fusiform initials in non-storied cambium can reach lengths up to 6200 µm in vessel-less dicotyledons, making them generally longer than those in storied cambium.
• Prevalence and Adaptation:
  • Occurrence: Non-storied cambium is more common and can be found in a wider range of plant species, including those with vessel-less structures.
  • Functional Adaptation: The less organized arrangement allows for greater flexibility in cell formation and differentiation, accommodating varied growth conditions.

Seasonal Activity of Vascular Cambium

The vascular cambium, a key meristematic tissue responsible for secondary growth in plants, exhibits seasonal variations in its activity. These variations are influenced by environmental factors such as temperature, light, and water availability, which affect the growth patterns of the cambium and its derivatives.

1. Variation in Fusiform Initials
   • Cell Size and Activity: At the end of the growing season, fusiform initials, which are elongated cambial cells, tend to be shorter compared to those formed at the beginning of the season.
     • Growth Rate: This reduction in cell length is attributed to the decreased rate of cambial activity as the season progresses.
     • Seasonal Impact: Environmental factors that limit growth, such as reduced light and lower temperatures, influence the size and function of these cells.
2. Spiral Grain Formation in Conifers
   • Orientation Changes: In coniferous trees, the vascular cambium’s activity can lead to the formation of a spiral grain pattern in the wood. This pattern results from periodic changes in the orientation of cell walls during anticlinal divisions.
     • Panel Formation: These changes cause the formation of spiral panels or domains of cells within the xylem.
     • Environmental Influence: Variations in cambial activity and orientation can be influenced by seasonal climatic changes, contributing to the spiral grain effect.
3. Cytokinesis and Cell Division
   • Longitudinal Division: Fusiform initials undergo longitudinal division during the formation of new vascular tissues. This process involves cytokinesis, where the cell plate and phragmoplast play crucial roles.
     • Cytokinesis Mechanism: The phragmoplast, a structure that guides the formation of the cell plate, is essential for dividing the fusiform initials into new cells.
     • Division Patterns: Periclinal divisions, where cells divide parallel to the cambial surface, are observed in the cambium of various species such as Cryptocarya, leading to the formation of secondary xylem and phloem.

Functions of Vascular Cambium

The vascular cambium plays a crucial role in the growth and development of plants, particularly in the production of vascular tissues and the initiation of secondary growth. Below are the key functions of the vascular cambium:

1. Production of Vascular Tissues

• Xylem Formation: The vascular cambium generates secondary xylem towards the interior of the plant. This xylem is responsible for conducting water and minerals from the roots to the rest of the plant. As secondary xylem accumulates, it forms the bulk of the wood in woody plants, providing both structural support and water conduction.
• Phloem Formation: On the exterior side, the vascular cambium produces secondary phloem. This tissue is essential for the transport of nutrients, particularly the products of photosynthesis, from the leaves to other parts of the plant. Over time, older phloem layers become stretched and non-functional, but the newer layers remain active in nutrient transport.

2. Initiation of Secondary Growth

• Lateral Growth: The vascular cambium is a lateral meristem, meaning it contributes to the plant’s lateral or radial growth. Through its activity, the cambium increases the thickness or girth of stems and roots, a process known as secondary growth. This growth is essential for the plant’s ability to support increased height and biomass.
• Wood Formation: In woody plants, the vascular cambium’s continuous production of secondary xylem forms wood. The older layers of secondary xylem become part of the plant’s structural core, while only the most recent layers are involved in active water and mineral conduction.

3. Formation of a Cylindrical Structure

• Development of a Cambial Cylinder: Initially, the arrangement of primary phloem and xylem in roots prevents the vascular cambium from forming a complete circle. However, over time, varying rates of cell division result in the formation of a cylindrical structure, which contributes to the plant’s radial growth and stability.
• Expansion of Conductive Capacity: As the cambial cylinder forms and expands, it increases the plant’s capacity to transport water, minerals, and nutrients. The secondary xylem’s continuous addition supports greater vertical growth by enhancing the plant’s ability to conduct essential resources from the roots to the upper parts.

4. Contribution to Structural Support

• Strengthening the Plant: The secondary xylem, formed by the vascular cambium, not only conducts water and minerals but also provides significant structural support. As the plant increases in diameter, the additional xylem strengthens the stem or root, allowing the plant to withstand various environmental stresses.
• Replacement of Non-functional Cells: Over time, older xylem and phloem cells lose their functionality. The vascular cambium compensates for this by continuously producing new cells. For instance, old xylem cells may no longer conduct water due to broken water columns or air-filled tracheids, and old phloem cells may break due to stretching. The cambium’s activity ensures that these cells are replaced with new, functional ones.

5. Role in Diameter Increase

• Coordination with Cork Cambium: The vascular cambium works in conjunction with the cork cambium to increase the diameter of the plant. While the vascular cambium contributes to the inner growth by adding xylem and phloem, the cork cambium adds layers to the outer protective tissue, enabling the plant to grow wider while maintaining protection against external elements.
• Pushing Outward Growth: As the vascular cambium produces new cells, the older primary xylem and phloem are pushed outward. This displacement is necessary to accommodate the expanding secondary tissues and maintain the plant’s overall structural integrity.

What is Secondary growth in Plant?

• In plants, growth can be classified into two main types: primary growth and secondary growth. Primary growth is responsible for the increase in length of the plant’s axis, such as stems and roots, and is associated with the formation of primary tissues. This type of growth originates from the apical meristem, which is located at the tips of roots and shoots.
• On the other hand, secondary growth refers to the increase in the girth or thickness of the plant’s axis. Unlike primary growth, which contributes to the plant’s height, secondary growth is responsible for widening the plant. The tissues formed during this process are known as secondary tissues. Secondary growth is typically absent in monocotyledons and pteridophytes, as these plants do not exhibit significant cambial activity.
• Secondary growth is most commonly observed in dicotyledonous plants, where it initiates in both the intra- and extrastelar regions of the stem. The process involves several key steps, beginning with the formation of the cambial ring. This ring is a layer of meristematic cells that encircles the stem and is crucial for initiating the secondary growth process.
• The activity of the cambium ring is central to secondary growth. As the cambium ring becomes active, it produces secondary vascular tissues. These tissues include secondary xylem (wood) and secondary phloem, which are essential for the plant’s structural support and nutrient transport.
• Additionally, secondary growth leads to the formation of the periderm. The periderm replaces the epidermis in older stems and roots, providing protection to the plant. It consists of the cork cambium (phellogen), cork (phellem), and phelloderm. The cork cells, produced by the phellogen, form the outer protective layer, while the phelloderm cells contribute to the inner layers.
• Overall, secondary growth plays a crucial role in the development of woody plants, enabling them to increase in thickness and survive for many years by forming durable, supportive tissues.

Steps of Secondary growth in Plant

Secondary Growth takes place by the following steps:

1. Formation of cambial ring
2. Activity of cambium ring
3. Secondary vascular tissue
4. Formation of Periderm

1. Formation of Cambial Ring:
   • The fascicular cambium, or vascular cambium, originates in the primary vascular bundles between the xylem and phloem.
   • Before secondary growth begins, the parenchymatous cells of the medullary rays, aligned with the fascicular cambium, become meristematic, forming the interfascicular cambium.
   • These interfascicular cambium cells extend laterally and merge with the fascicular cambium, creating a continuous cambial ring.
   • Within this cambial ring, two types of cells exist: fusiform initials, which generate secondary tissues, and ray initials, responsible for forming the ray cells of the xylem and phloem.
2. Activity of Cambium Ring:
   • The cambial ring actively produces vascular tissues on both sides. Cells formed outward become secondary phloem, while those produced inward differentiate into secondary xylem.
   • The formation of secondary xylem pushes the primary xylem towards the center of the stem.
   • Simultaneously, the development of secondary phloem forces the primary phloem outward, causing it to eventually be crushed, while the primary xylem remains intact at the core.
   • Additionally, the ray initials of the cambial ring generate narrow secondary medullary rays, extending into both the secondary xylem and phloem.
3. Secondary Vascular Tissue:
   • The secondary vascular tissues include the secondary xylem (wood) and secondary phloem, both formed by the fusiform initials of the vascular cambium.
   • As the cambium ring produces new cells on its inner side, they gradually transform into the xylary elements that make up the secondary xylem.
   • This secondary xylem is essential for conducting water and nutrients and providing mechanical support to the plant.
   • The cambial cells also divide tangentially to produce secondary phloem elements on the outer side.
   • Secondary phloem, also known as bast, consists of sieve tubes, companion cells, phloem fibers, or bast fibers, and phloem parenchyma.
   • Typically, secondary phloem is less abundant than secondary xylem. The primary phloem usually becomes nonfunctional over time, with the secondary phloem taking over the physiological activities.
4. Formation of Periderm:
   • With the continuous formation of secondary tissues, the older stem’s epidermis stretches, eventually rupturing, leading to the death of epidermal cells and outer tissues. A new protective layer called the periderm is formed to replace it.
   • The formation of periderm is common in gymnosperm stems that increase in thickness due to secondary growth.
   The periderm consists of three components:
   1. Phellogen (Cork Cambium): This new lateral meristem is responsible for producing the periderm. The cells of phellogen are living, thin-walled, and tightly packed.
   2. Phellem (Cork): The outer layer of cells produced by the phellogen, which form a protective barrier on the plant’s surface.
   3. Phelloderm (Secondary Cortex): The inner layer of cells generated by the phellogen, contributing to the secondary cortex.
   • The periderm appears on the surface of plant parts that undergo continuous secondary growth, providing a protective function as the plant increases in girth.

Annual Rings or Growth Rings

Annual rings, also known as growth rings, are a fundamental aspect of secondary xylem development in perennial plants. These rings are indicative of the plant’s growth over successive seasons and provide valuable insights into both the age of the plant and the environmental conditions experienced during each growing period.

Formation of Annual Rings

1. Secondary Xylem Development
   • Structure: In perennial plants, the secondary xylem is produced by the vascular cambium. This tissue accumulates in concentric layers, each representing a seasonal growth increment.
   • Visual Representation: In a transverse section of the stem, these layers appear as distinct rings. Each ring corresponds to one year of growth, thereby reflecting annual variations in growth conditions.
2. Spring Wood and Autumn Wood
   • Spring Wood (Early Wood): During the spring and early summer, the plant experiences rapid growth due to favorable conditions such as increased water availability and warmer temperatures. The secondary xylem produced during this period is known as spring wood or early wood. It is characterized by larger, more widely spaced vessels.
   • Autumn Wood (Late Wood): As the growing season progresses into late summer and autumn, the growth rate slows. The secondary xylem formed during this time is termed autumn wood or late wood. This wood has smaller, more densely packed vessels compared to the early wood, reflecting reduced growth rates and harsher environmental conditions.
3. Ring Boundaries
   • Demarcation: The boundary between the early wood of one year and the late wood of the subsequent year is usually quite distinct. This clear line of demarcation marks the transition from one annual growth cycle to the next.
   • Ring Composition: An annual ring is therefore composed of two primary layers: the inner layer (early wood) and the outer layer (late wood). The early wood is formed at the beginning of the growing season, while the late wood is formed towards the end of the season.

Functions and Applications

1. Determination of Age
   • Counting Rings: By counting the number of annual rings in a cross-section of wood, one can determine the age of the plant. Each ring corresponds to a year of growth.
2. Environmental Indicators
   • Growth Conditions: The width and characteristics of each ring can provide information about past environmental conditions. For instance, wider rings may indicate favorable growing conditions, while narrower rings could suggest periods of drought or other stress factors.
3. Wood Quality and Use
   • Wood Properties: The quality and properties of wood can be assessed based on the growth rings. For example, the density and structure of the wood can influence its suitability for various applications, such as construction or furniture making.

Dendrochronology, Tyloses

Dendrochronology

Dendrochronology is the scientific method used to determine the age of trees by analyzing the pattern of their annual growth rings. Each annual ring in a tree’s cross-section represents one year of growth, which allows researchers to calculate the tree’s age with precision.

1. Annual Rings and Age Determination
   • Ring Formation: As trees grow, they form concentric rings of secondary xylem, or wood, each representing one year’s growth. By counting these rings in a transverse section of the tree, the age of the tree can be accurately determined.
   • Applications: Dendrochronology is widely used in various fields, including ecology, archaeology, and climate science. It helps researchers understand past climate conditions, date historical wooden artifacts, and study ecological changes over time.
2. Anomalies in Ring Formation
   • Multiple Rings: Occasionally, trees may form more than one ring in a single growing season. This can occur due to environmental stressors such as drought or unusual weather patterns. When multiple rings are present, counting them can lead to inaccuracies in age determination.
   • Impact of Stress: The formation of additional rings can be a response to environmental stress, which complicates the interpretation of growth patterns and requires careful analysis to avoid misestimations of the tree’s age.

Tyloses

Tyloses are balloon-like outgrowths that form within the lumen of xylem vessels in many plants. These structures are primarily observed in secondary xylem but can also develop in primary xylem vessels.

1. Formation of Tyloses
   • Structural Development: Tyloses arise from the enlargement of pit membranes, specifically in the half-bordered pits between parenchyma cells and xylem vessels or tracheids. These outgrowths extend into the vessel lumen, effectively blocking it.
   • Presence: Tyloses are commonly found in the heartwood of many angiosperms. Their formation is an adaptive response to environmental conditions, contributing to the durability of the wood.
2. Functions of Tyloses
   • Wood Durability: By occluding the lumen of xylem vessels, tyloses enhance the mechanical strength of the wood. This blockage prevents the rapid movement of water, air, and pathogens, thereby adding to the wood’s longevity and resistance to decay.
   • Preventing Pathogen Entry: Tyloses play a protective role by reducing the entry of fungi and other microorganisms into the vascular system of the plant. This defense mechanism is particularly important for maintaining the integrity of the wood over time.

Secondary growth in Sapwood and Heartwood

In the anatomy of trees, the xylem is divided into two distinct regions: sapwood and heartwood. Each of these regions plays a specific role in the structure and function of the tree.

Sapwood (Alburnum)

1. Composition and Appearance
   • Structure: Sapwood constitutes the outer, newer layers of xylem in a tree. It is characterized by its lighter color compared to the inner heartwood. The sapwood contains living cells and is associated with functional xylem elements such as vessels and fibers.
   • Function: This region is actively involved in the physiological processes of the tree. It is responsible for the conduction of water and nutrients from the roots to the leaves and other parts of the plant. Additionally, sapwood serves as a storage site for food reserves, which can be mobilized as needed.
2. Physiological Role
   • Water Conduction: The vessels within the sapwood are essential for the transport of water and dissolved minerals throughout the plant.
   • Nutrient Storage: It stores carbohydrates and other nutrients, which support the tree’s growth and recovery during periods of stress or dormancy.

Heartwood (Duramen)

1. Composition and Appearance
   • Structure: Heartwood is the central region of older xylem that has undergone significant changes over time. It is filled with substances such as tannins, resins, and gums, which contribute to its darker color and increased durability. The heartwood often appears black or dark brown due to these accumulated compounds.
   • Vessel Occlusion: In heartwood, vessels are frequently occluded with tyloses, balloon-like outgrowths that block the lumen of the vessels. This occlusion further prevents the movement of water and contributes to the wood’s resistance to decay.
2. Functional Role
   • Mechanical Support: Unlike sapwood, heartwood is no longer involved in the conduction of water. Instead, its primary function is to provide structural support and rigidity to the tree. The accumulation of durable substances enhances the strength and stability of the wood.
   • Durability: The chemical compounds within heartwood, such as tannins, make it more resistant to fungal decay and insect damage. This increased durability contributes to the longevity and overall resilience of the tree.

Transition and Variation

• Transformation: The process of sapwood gradually converting into heartwood occurs over many years. As sapwood ages, it accumulates more of the durable substances, transitioning to the darker and more resistant heartwood.
• Species Variation: The proportion of sapwood to heartwood varies significantly among different species of trees. In some species, heartwood constitutes a large portion of the xylem, while in others, the ratio may be less pronounced. This variation influences the mechanical properties and ecological functions of the wood.

Bark, Lenticels

Bark

1. Definition and Composition
   • General Overview: The term “bark” encompasses all tissues located outside the vascular cambium of a stem, whether in a primary or secondary growth stage. This means that bark includes both primary and secondary tissues, depending on the age and development of the plant.
   • Primary Bark: In plants with only primary growth, bark consists of primary phloem and cortex. These tissues are formed during the initial stages of growth and include the outermost layers of the stem.
   • Secondary Bark: In plants exhibiting secondary growth, bark includes primary and secondary phloem, cortex, and periderm. The secondary phloem is produced by the vascular cambium, while the periderm, which includes the cork cambium, cork cells, and phelloderm, replaces the primary epidermis.
2. Functions
   • Protection: Bark serves as a protective layer for the stem, shielding it from physical damage, pathogens, and pests.
   • Water Regulation: It also plays a role in reducing water loss by minimizing transpiration from the stem surface.
   • Support and Insulation: The bark contributes to the structural support of the tree and provides insulation against extreme temperatures.

Lenticels

1. Definition and Structure
   • Description: Lenticels are specialized structures found within the periderm of woody plants. They appear as raised, corky spots or areas on the stem surface where the underlying tissues breach the epidermis.
   • Formation: According to Wutz (1955), lenticels are small portions of the periderm where the activity of the phellogen, or cork cambium, is heightened. The cork cells produced in these areas are loosely arranged, creating numerous intercellular spaces.
2. Function
   • Gas Exchange: Lenticels facilitate the exchange of gases between the atmosphere and the internal tissues of the plant. This function is particularly important during periods when stomata are closed, such as at night or under drought conditions. The profusion of intercellular spaces in lenticels allows for efficient gas diffusion.
3. Development and Dynamics
   • Origin: Lenticels typically originate beneath the stomata and form either just before or concurrently with the initiation of the first layer of the periderm. In many plants, lenticel formation begins during the first growing season or slightly before the cessation of elongation growth.
   • Seasonal Changes: During the spring season, lenticels are filled with complementary cells that have thin, rounded walls and are loosely packed. These cells are not suberized, which contributes to the functionality of lenticels in gas exchange. By the end of the spring, the lenticel may become sealed by a formation of a closing layer, reducing the size of the intercellular spaces.

What is Cork Cambium?

Cork cambium is a specialized lateral meristem located in the outermost region of plant stems and roots. It plays a crucial role in the secondary growth of plants by producing protective tissues.

Functions of Cork Cambium

1. Production of Cork Cells
   • Composition: Cork cambium generates cork cells, which are characterized by their content of suberin, a waxy substance.
   • Properties: Suberin imparts water-repellent properties to the cork cells, enhancing their role in protecting the plant.
2. Formation of Phelloderm
   • Cell Growth: In addition to cork cells, cork cambium produces a layer of cells known as the phelloderm.
   • Direction of Growth: The phelloderm grows inward from the cork cambium, contributing to the formation of the periderm.
3. Development of the Periderm
   • Structure: The periderm comprises the cork cambium, cork cells, and phelloderm.
   • Function: It replaces the epidermis in mature plants, serving as a protective outer layer.
4. Lenticels and Gas Exchange
   • Openings: The periderm features lenticels, which are specialized openings.
   • Purpose: Lenticels facilitate gas exchange between the plant’s interior cells and the external environment, allowing for the supply of oxygen to the xylem, phloem, and cortex.

Secondary Growth in Stem

Secondary growth in stems is a fundamental process that increases the girth of woody plants, enabling them to support larger structures and adapt to environmental changes. This growth is driven by the activity of two key cambial tissues: the vascular cambium and the cork cambium.

Role of Vascular Cambium

1. Formation of Secondary Xylem and Phloem
   • Cell Division: The vascular cambium, a lateral meristem, divides to produce secondary xylem and secondary phloem.
   • Increase in Thickness: The accumulation of these tissues contributes to the overall increase in stem thickness. Secondary xylem, or wood, forms towards the interior, while secondary phloem forms towards the exterior.
2. Production of Secondary Xylem
   • Structure: Secondary xylem consists primarily of cells that conduct water and minerals from the roots to other parts of the plant.
   • Function: The formation of secondary xylem adds structural support and enhances the plant’s ability to transport water and nutrients.
3. Formation of Secondary Phloem
   • Structure: Secondary phloem is responsible for the transport of organic nutrients, such as sugars, from the leaves to other plant parts.
   • Function: It supports the plant’s metabolic needs by distributing food produced in the leaves.

Role of Cork Cambium

1. Formation of Bark
   • Cell Production: The cork cambium produces cork cells that form the outer protective layer of the stem.
   • Suberin Content: Cork cells contain suberin, a water-repelling substance that enhances the bark’s resistance to environmental factors such as moisture loss and physical damage.
2. Formation of the Periderm
   • Components: The periderm includes the cork cambium, cork cells, and the phelloderm, which collectively replace the epidermis in mature stems.
   • Function: The periderm protects the stem and helps in the exchange of gases between the plant’s interior and the external environment.
3. Gas Exchange
   • Lenticels: The periderm features lenticels, which are openings that facilitate gas exchange. They allow oxygen to enter and carbon dioxide to exit, maintaining metabolic processes within the plant’s inner tissues.

Secondary Growth in Roots

Secondary growth in roots is a critical process that contributes to the increased girth and structural support of the root system in many plants. This process involves the formation of secondary tissues and the development of new lateral roots, which expand the root’s capacity for anchorage and nutrient uptake.

Initiation and Development

1. Initiation of Secondary Growth
   • Location: Secondary growth begins in the zone of maturation, where cells cease elongation.
   • Formation of Vascular Cambium: In this zone, the vascular cambium forms between the primary xylem and primary phloem. Pericycle cells also begin dividing in conjunction with procambium initials.
   • Cambium Cylinder: This results in the formation of a continuous cylinder of cambium encircling the primary xylem.
2. Activity of Vascular Cambium
   • Xylem and Phloem Production: The vascular cambium starts producing secondary xylem cells inward and secondary phloem cells outward.
   • Impact on Primary Tissues: This production leads to the compression of primary phloem against the endodermis, which becomes more resistant to deformation.

Role of Cork Cambium

1. Differentiation of Cork Cambium
   • Formation: The cork cambium differentiates in the pericycle, adding another layer of cell division within the stele.
   • Function: The cork cambium produces cork cells that form the periderm, a protective layer for the root.
2. Development of Periderm
   • Periderm Composition: The periderm consists of cork cells, the cork cambium, and the phelloderm. It replaces the epidermis as the root thickens.
   • Cellular Impact: As the periderm forms, it isolates the outer tissues of the root from their internal supply of nutrients and water.

Changes During Secondary Growth

1. Destruction of Primary Tissues
   • First-Year Growth: By the end of the first year, secondary growth results in the destruction of most primary xylem cells and primary phloem fibers, leaving only a central core of primary xylem.
   • Tissue Arrangement: The final arrangement of tissues from outside to inside consists of the periderm, pericycle, primary and secondary phloem, vascular cambium, and primary and secondary xylem.
2. Structural Adaptations
   • Root Expansion: The formation of secondary xylem and phloem increases the root’s diameter and provides additional support and nutrient transport capabilities.
   • Protection: The development of the periderm offers enhanced protection against environmental factors and reduces water loss.

Abnormal Secondary Growth

Abnormal secondary growth refers to deviations from the typical pattern of secondary growth observed in most dicotyledonous plants. Unlike standard secondary growth, where a single vascular cambium produces secondary xylem inward and secondary phloem outward, abnormal secondary growth exhibits atypical patterns and structures. This phenomenon is particularly evident in certain dicots and arborescent monocots.

Abnormal Secondary Growth in Dicots

1. Multiple Cambia Formation
   • Occurrence: In some dicotyledonous plants, such as Bougainvillea and Dracaena, abnormal secondary growth is characterized by the development of multiple cambial layers.
   • Process: In these cases, several cambia arise outside the oldest phloem. Each cambium contributes to the production of additional vascular tissues, leading to a complex arrangement of secondary xylem and phloem.
2. Layering and Growth Patterns
   • Cambia Arrangement: Multiple cambia result in a layered structure where each cambium produces secondary vascular tissues in distinct zones.
   • Effects: This abnormal layering can lead to irregular growth patterns and variations in the distribution of xylem and phloem within the stem.

Abnormal Secondary Growth in Monocots

1. Secondary Cambium in Arborescent Monocots
   • Formation: In arborescent monocots, secondary cambium develops in the hypodermal region, which is located beneath the epidermis.
   • Tissue Formation: The secondary cambium forms conjunctive tissues and patches of meristematic cells that contribute to secondary growth.
2. Development of Secondary Vascular Bundles
   • Vascular Bundles Formation: The meristematic patches of cells differentiate into secondary vascular bundles, which are observed in the cortex and pith regions of the stem.
   • Distribution: These secondary bundles are interspersed among the primary vascular tissues, leading to a more complex internal structure compared to typical dicot secondary growth.

Functional Implications

1. Structural Variability
   • In Dicots: Multiple cambia and layered structures result in increased variability in the distribution and organization of vascular tissues.
   • In Monocots: The formation of secondary vascular bundles in unusual locations affects the overall structural integrity and function of the stem.
2. Growth Patterns
   • Impact: Abnormal secondary growth can lead to irregularities in stem thickness, vascular tissue distribution, and overall plant form.
   • Adaptation: While these variations can be considered abnormal, they are often adaptations to specific environmental conditions or evolutionary strategies.

Quiz on Vascular cambium

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FAQ

References

• https://cmpcollege.ac.in/wp-content/uploads/2017/07/wood-anatomy-vascular-cambium.pdf
• https://www.sciencefacts.net/vascular-cambium.html
• https://nickrentlab.siu.edu/PlantAnatomyWeb/LecturesDLN/Lecture12_VascCambium.html
• https://egyankosh.ac.in/bitstream/123456789/68181/1/Unit-6.pdf
• https://www.jsscacs.edu.in/sites/default/files/Department%20Files/SECONDARY%20GROWTH%20new.pdf
• https://milnepublishing.geneseo.edu/botany/chapter/secondary-growth/
• https://study.com/academy/lesson/vascular-cambium-function-definition-quiz.html
• https://www.biologydiscussion.com/plant-anatomy/vascular-cambium/vascular-combium-plants/69346
• https://www.biologydiscussion.com/plant-anatomy/vascular-cambium/vascular-cambium-origin-and-activities-plants/69334