What Is Secondary Growth In Plants

Secondary growth in plants is the process that leads to an increase in the girth or thickness of the plant, particularly in stems and roots. This contrasts with primary growth, which results in an increase in length.

Understanding Secondary Growth in Plants

Secondary growth is a hallmark of dicotyledonous and gymnosperm plants, enabling them to grow into massive trees or shrubs. While herbaceous plants primarily undergo primary growth, woody plants exhibit both primary and secondary growth. This developmental adaptation allows plants to withstand environmental stresses, support increased height and weight, and ensure the efficient transport of water and nutrients.

Primary Growth vs. Secondary Growth: A Quick Comparison

To fully grasp secondary growth, it's essential to differentiate it from primary growth:

• Primary Growth:
  • Occurs at the apical meristems (tips of roots and shoots).
  • Responsible for increasing plant length.
  • Involves the formation of primary tissues such as epidermis, primary xylem, primary phloem, and ground tissue.
  • Occurs in all vascular plants.
• Secondary Growth:
  • Occurs at the lateral meristems (vascular cambium and cork cambium).
  • Responsible for increasing plant thickness or girth.
  • Involves the formation of secondary xylem (wood) and secondary phloem (inner bark).
  • Occurs predominantly in woody dicots and gymnosperms.

The Key Players: Lateral Meristems

Secondary growth is facilitated by two types of lateral meristems:

1. Vascular Cambium: This cylindrical layer of meristematic cells is located between the primary xylem and primary phloem. It produces secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outside.
2. Cork Cambium (Phellogen): This meristematic layer develops in the cortex and produces cork cells (phellem) to the outside and phelloderm cells to the inside, forming the outer bark.

The Vascular Cambium: Building the Woody Infrastructure

The vascular cambium is the engine of secondary growth, continuously dividing to produce new cells that differentiate into secondary xylem and secondary phloem.

• Formation of the Vascular Cambium: The vascular cambium originates from procambial cells left behind between the primary xylem and primary phloem after primary growth. In the fascicular regions (vascular bundles), these procambial cells become cambial cells. In the interfascicular regions (between vascular bundles), parenchyma cells of the medullary rays (tissues that radiate out from the center of the stem) become meristematic, connecting the fascicular cambium to form a complete ring.
• Activity of the Vascular Cambium: The vascular cambium consists of two types of cells:
  • Fusiform initials: These elongated cells give rise to the cells of the axial system – the secondary xylem and secondary phloem.
  • Ray initials: These shorter, cuboidal cells produce the ray parenchyma cells, which form the vascular rays that radially connect the secondary xylem and secondary phloem.
• Secondary Xylem (Wood) Formation: The vascular cambium divides to produce more cells towards the inside than towards the outside. The cells formed on the inside differentiate into secondary xylem, which is the primary component of wood.
  • Tracheids and Vessel Elements: In gymnosperms, the secondary xylem consists mainly of tracheids, which are elongated cells with tapered ends that conduct water and provide structural support. In angiosperms, the secondary xylem contains both tracheids and vessel elements, with vessel elements being more efficient in water transport due to their wider diameter and perforations.
  • Wood Fibers: These cells provide additional mechanical strength to the wood.
  • Wood Parenchyma: These cells are involved in storage and transport of nutrients within the wood.
• Secondary Phloem (Inner Bark) Formation: The cells formed on the outside of the vascular cambium differentiate into secondary phloem.
  • Sieve Tube Elements and Companion Cells: These are the primary conducting cells of the phloem, responsible for transporting sugars and other organic compounds throughout the plant.
  • Phloem Fibers: These provide structural support to the phloem.
  • Phloem Parenchyma: These cells are involved in storage and transport of nutrients within the phloem.
  • The fate of Secondary Phloem: Unlike secondary xylem, which accumulates over the life of the plant, the secondary phloem is a relatively short-lived tissue. As new layers of secondary phloem are formed, older layers are crushed and sloughed off as part of the bark.

The Cork Cambium: Protecting the Plant

The cork cambium, also known as the phellogen, is another lateral meristem that plays a crucial role in secondary growth by forming the protective outer layers of the bark.

• Formation of the Cork Cambium: The cork cambium typically arises in the cortex, just beneath the epidermis. In some plants, it may originate in the epidermis or even in the phloem. Parenchyma cells dedifferentiate and become meristematic, forming a continuous or overlapping layer of cork cambium.
• Activity of the Cork Cambium: The cork cambium divides to produce:
  • Cork Cells (Phellem): These cells are formed to the outside of the cork cambium. They are dead at maturity and their walls are impregnated with suberin, a waxy substance that makes them impermeable to water and gases. Cork cells provide a protective barrier against desiccation, mechanical damage, and pathogen attack.
  • Phelloderm: These parenchyma cells are formed to the inside of the cork cambium. They are living cells and may function in storage.
• Bark Formation: The term "bark" refers to all the tissues outside the vascular cambium, including the secondary phloem, cork cambium, cork cells, and phelloderm.
  • Outer Bark (Rhytidome): As the stem or root expands due to secondary growth, the epidermis and cortex are eventually ruptured. New layers of cork cambium form deeper in the stem, producing new layers of cork cells. The outer layers of dead cork cells, along with the remnants of the epidermis, cortex, and old secondary phloem, make up the outer bark or rhytidome. The rhytidome is what we typically see as the rough, textured surface of tree trunks.
  • Lenticels: These are small, raised areas on the bark that allow for gas exchange between the interior of the stem and the atmosphere. Lenticels are formed by loosely arranged cells in the cork layers, providing pathways for oxygen and carbon dioxide to diffuse in and out of the plant tissues.

The Significance of Secondary Growth

Secondary growth is an essential adaptation that allows plants to thrive in various environments. Here are some key benefits:

• Increased Structural Support: The production of wood (secondary xylem) provides the necessary strength and stability for plants to grow tall and support their increasing weight. This is especially important for trees, which can reach great heights and live for hundreds or even thousands of years.
• Enhanced Water and Nutrient Transport: The secondary xylem and secondary phloem provide efficient pathways for the transport of water, minerals, and sugars throughout the plant. The increased vascular capacity allows plants to supply their growing tissues with the resources they need.
• Improved Protection: The formation of bark, with its layers of cork cells, provides a protective barrier against environmental stresses such as desiccation, temperature extremes, mechanical damage, and pathogen attack. The bark also helps to insulate the vascular tissues from fire.
• Longevity: Secondary growth enables plants to live longer by continuously replacing damaged or worn-out tissues and providing ongoing structural support and protection.

Factors Affecting Secondary Growth

The rate and pattern of secondary growth can be influenced by a variety of factors:

• Genetics: The genetic makeup of a plant determines its inherent capacity for secondary growth. Some species are genetically programmed to exhibit extensive secondary growth, while others have limited or no secondary growth.
• Environmental Conditions: Environmental factors such as temperature, water availability, light intensity, and nutrient availability can significantly affect secondary growth.
  • Temperature: Warmer temperatures generally promote faster growth rates, while colder temperatures can slow down or even halt secondary growth.
  • Water Availability: Adequate water is essential for cell division and expansion, and water stress can inhibit secondary growth.
  • Light Intensity: Light provides the energy for photosynthesis, which is necessary for producing the sugars that fuel growth.
  • Nutrient Availability: Nutrients such as nitrogen, phosphorus, and potassium are essential for cell growth and development, and nutrient deficiencies can limit secondary growth.
• Hormones: Plant hormones such as auxins, gibberellins, and cytokinins play important roles in regulating secondary growth.
  • Auxins: Promote cell division and differentiation in the vascular cambium.
  • Gibberellins: Stimulate cell elongation and xylem differentiation.
  • Cytokinins: Promote cell division in the cambium.
• Age: As plants age, their rate of secondary growth may change. In some species, growth rates may decline with age, while in others, they may remain relatively constant.
• Mechanical Stress: Wind or other mechanical stresses can stimulate secondary growth, leading to the development of thicker stems and roots that are better able to withstand the forces acting on them. This phenomenon is known as thigmomorphogenesis.

Annual Rings: A Chronicle of Growth

In temperate regions with distinct seasons, the activity of the vascular cambium varies throughout the year. In the spring, when water is plentiful, the cambium produces large, thin-walled xylem cells, forming what is known as early wood or spring wood. In the late summer and fall, as water becomes more limited, the cambium produces smaller, thicker-walled xylem cells, forming late wood or autumn wood. The contrast between the early wood and late wood creates visible annual rings in the wood.

By counting the annual rings, we can determine the age of a tree. Furthermore, the width of the rings can provide valuable information about past environmental conditions. Wide rings indicate years with favorable growing conditions, while narrow rings suggest years with stress, such as drought or competition. This field of study is known as dendrochronology.

Examples of Secondary Growth in Different Plants

Secondary growth can manifest differently in various plant species:

• Trees: Trees exhibit extensive secondary growth, leading to the formation of massive trunks and branches. Examples include oak, maple, pine, and redwood trees.
• Shrubs: Shrubs also undergo secondary growth, but to a lesser extent than trees. Examples include roses, azaleas, and lilacs.
• Vines: Some vines, such as grapevines and wisteria, exhibit secondary growth, allowing them to develop woody stems that can support their climbing habit.
• Woody Lianas: These are climbing plants that start their life on the forest floor and then climb towards the canopy using other trees for support. They exhibit significant secondary growth to develop long, strong stems.

Secondary Growth in Roots

While secondary growth is most conspicuous in stems, it also occurs in roots, providing them with increased strength and vascular capacity. The process is similar to that in stems, with the vascular cambium producing secondary xylem and secondary phloem, and the cork cambium forming the outer layers of the bark. Secondary growth in roots helps to anchor the plant in the soil and transport water and nutrients from the soil to the rest of the plant.

Modified Secondary Growth

While the basic processes of secondary growth are consistent across most plants, some species exhibit modifications to this process.

• Anomalous Secondary Growth: This refers to deviations from the typical pattern of secondary growth. These deviations can involve the formation of multiple cambia, the production of parenchyma cells within the xylem or phloem, or other unusual features. Anomalous secondary growth is often found in vines and other plants that require specialized structural adaptations.
• Successive Cambia: Some plants form successive cambia, where new vascular cambia are initiated outside the existing ones. This can result in the formation of multiple rings of vascular tissue.
• Included Phloem: In some plants, the phloem may be embedded within the xylem, rather than being located outside of it.

Economic Importance of Secondary Growth

Secondary growth has significant economic importance, as it is responsible for the production of wood, which is one of the most widely used natural resources. Wood is used for a variety of purposes, including:

• Construction: Wood is used for building houses, bridges, and other structures.
• Furniture: Wood is used for making furniture, cabinets, and other household items.
• Paper: Wood pulp is used for making paper and other paper products.
• Fuel: Wood is used as a source of fuel for heating and cooking.

In addition to wood, secondary growth also produces other valuable products, such as:

• Cork: Cork is harvested from the bark of the cork oak tree and is used for making bottle stoppers, insulation, and other products.
• Tannins: Tannins are extracted from the bark of various trees and are used for tanning leather.
• Resins: Resins are secreted by certain trees and are used for making varnishes, adhesives, and other products.

Conclusion

Secondary growth is a vital process that enables plants to grow into large, long-lived organisms. It involves the activity of lateral meristems, namely the vascular cambium and cork cambium, which produce secondary xylem (wood) and secondary phloem (inner bark), respectively. Secondary growth provides plants with increased structural support, enhanced water and nutrient transport, improved protection, and longevity. The rate and pattern of secondary growth are influenced by a variety of factors, including genetics, environmental conditions, hormones, and age. Understanding secondary growth is essential for appreciating the diversity and complexity of plant life and for managing forest resources sustainably.