Secondary Growth in Plants: Tissues and Tissue Systems

Secondary growth is a crucial process in the life of many plants, particularly dicots and gymnosperms. Unlike primary growth, which leads to an increase in length (height and root depth), secondary growth results in an increase in girth or diameter of stems and roots. This thickening is essential for providing structural support, enabling efficient transport of water and nutrients over longer distances, and protecting the plant from mechanical stress and environmental factors. This growth is primarily driven by the activity of two lateral meristems: the vascular cambium and the cork cambium.

Vascular Cambium and Secondary Growth

The vascular cambium is a cylindrical sheath of meristematic cells located between the xylem and phloem in the vascular bundles of dicotyledonous stems and roots. In primary growth, vascular bundles contain procambium, which differentiates into primary xylem and primary phloem. In secondary growth, the vascular cambium arises from the interfascicular cambium (formed from dedifferentiated parenchyma cells in the medullary rays) and the intrafascicular cambium (remnants of the procambium). Together, these form a continuous ring.

The vascular cambium is responsible for producing secondary xylem (wood) towards the inside and secondary phloem towards the outside. The cells of the vascular cambium divide anticlinally (perpendicular to the cambial ring) and periclinally (parallel to the cambial ring). Periclinal divisions are more significant for secondary growth, as they produce new cells.

When a cambial cell divides periclinally, one daughter cell typically remains meristematic as part of the cambium, while the other differentiates into either secondary xylem or secondary phloem. The cells differentiating into secondary xylem are usually more numerous than those differentiating into secondary phloem. This unequal production contributes to the significant increase in the diameter of the stem or root.

Formation of Secondary Xylem (Wood)

The secondary xylem, commonly known as wood, consists of tracheary elements (tracheids and vessel elements), xylem parenchyma, and xylem fibres. As the vascular cambium produces more secondary xylem than secondary phloem, the wood accumulates on the inner side of the cambium. The older, inner layers of secondary xylem eventually become clogged with resins and tannins, losing their conducting function and forming heartwood, which provides mechanical support. The younger, outer layers, known as sapwood, are lighter in colour and actively involved in water and mineral transport.

Formation of Secondary Phloem

The secondary phloem is produced on the outer side of the vascular cambium and consists of sieve elements (sieve cells and sieve tube elements), companion cells, phloem parenchyma, and phloem fibres. Unlike the secondary xylem, the secondary phloem is produced in smaller quantities and is gradually crushed or replaced by the continuous formation of new secondary phloem and the outward expansion of the stem or root. The phloem tissue is responsible for the translocation of sugars from the leaves to other parts of the plant.

Growth Rings

In temperate regions, the activity of the vascular cambium is seasonal, influenced by temperature and water availability. During spring, when conditions are favourable (plentiful water and moderate temperature), the cambium produces larger, thin-walled cells with wider vessels, forming 'springwood' or 'earlywood'. As summer progresses and conditions become less favourable (less water, higher temperature), the cambium produces smaller, thick-walled cells with narrower vessels, forming 'summerwood' or 'latewood'. The difference in the appearance and density of springwood and latewood creates distinct annual growth rings, which are visible in a cross-section of the stem or root. These rings are invaluable for determining the age of a tree and studying past climatic conditions.

Cork Cambium and Periderm Formation

As the stem or root increases in diameter due to secondary growth from the vascular cambium, the epidermis and the outer layers of the cortex rupture. To protect the underlying tissues, a new lateral meristem, the cork cambium (or phellogen), arises in the outer part of the cortex or the pericycle. The cork cambium is also a cylindrical layer of meristematic cells.

The cork cambium divides periclinally, producing secondary cortical cells (phelloderm) towards the inside and cork cells (phellem) towards the outside. The phelloderm cells are parenchymatous and are living. The phellem cells, or cork, are dead at maturity and have cell walls impregnated with suberin, a waxy substance that makes them impermeable to water and gases. This waterproof nature prevents desiccation and protects the stem from mechanical injury and pathogen invasion.

The cork cambium, along with the cork and the phelloderm, collectively form the periderm. The periderm replaces the epidermis as the protective outer covering of the stem and root during secondary growth. In older stems and roots, multiple layers of periderm may form, creating a thick bark.

Bark Formation

Bark is a general term referring to all tissues outside the vascular cambium. It includes the secondary phloem and all the tissues of the periderm (cork cambium, cork, and phelloderm). As secondary growth continues, the periderm may form successively deeper layers. The outermost layers of the periderm are often shed periodically, while new periderms form inwards. The pattern of bark formation varies greatly among species, leading to different textures and appearances, such as fissured, scaly, or smooth bark.

Lenticels

While the periderm is largely impermeable, gas exchange is still necessary for the living cells beneath it. To facilitate this, small pores called lenticels are formed in the periderm. Lenticels are areas where the cork cambium is more active, producing loosely packed, unsuberized cells that allow for the passage of air into and out of the stem or root. They often appear as raised, corky spots on the surface of bark.

Secondary Growth in Roots

Secondary growth in dicotyledonous roots follows a pattern similar to that in stems. The vascular cambium originates from the pericycle and cells of the xylem and phloem rays. Initially, the cambium is wavy because it forms first in patches opposite the phloem, then extends to form a continuous ring.

The vascular cambium produces secondary xylem towards the inside and secondary phloem towards the outside. The secondary xylem in roots contains vessels, tracheids, xylem parenchyma, and xylem fibres, similar to stems. The secondary phloem is located outside the vascular cambium.

As secondary growth progresses, the pericycle becomes the origin of the cork cambium. The cork cambium produces cork (phellem) outwards and phelloderm inwards, forming the periderm. The epidermis and the outer cortex are soon replaced by the periderm, which becomes the protective outer layer of the root. The periderm in roots also contains lenticels, though they are less prominent than in stems.

Differences in Secondary Growth: Stem vs. Root

While the fundamental processes are similar, there are some key differences in secondary growth between stems and roots:

• Origin of Vascular Cambium: In stems, the vascular cambium is partly derived from interfascicular parenchyma and partly from the procambium within vascular bundles. In roots, it originates from the pericycle and cells of the xylem and phloem rays.
• Shape of Cambium: The vascular cambium in stems is typically circular from the beginning of secondary growth. In roots, it initially forms a wavy or polygonal ring and later becomes circular as it expands.
• Presence of Pith Rays: Pith rays, which are regions of parenchyma between vascular bundles, are prominent in dicot stems and contribute to the interfascicular cambium. In roots, the vascular tissue is arranged radially, with xylem and phloem alternating, so the concept of distinct pith rays in the same way as stems is less pronounced, although medullary rays do contribute to cambium formation.
• Secondary Phloem Position: In both stems and roots, secondary phloem is produced externally to the vascular cambium.
• Secondary Xylem Position: In both stems and roots, secondary xylem is produced internally to the vascular cambium.

Tissues Involved in Secondary Growth

Secondary growth involves the coordinated activity of several specialized tissues:

1. Lateral Meristems:

These are the primary drivers of secondary growth, producing new cells in a radial direction.

• Vascular Cambium: Produces secondary xylem and secondary phloem.
• Cork Cambium (Phellogen): Produces cork (phellem) and phelloderm.

2. Derivatives of Lateral Meristems:

These are the tissues produced by the activity of the lateral meristems.

• Secondary Xylem (Wood): Composed of tracheids, vessel elements, xylem parenchyma, and xylem fibres.
• Secondary Phloem: Composed of sieve elements, companion cells, phloem parenchyma, and phloem fibres.
• Cork (Phellem): Dead, suberized cells forming the outer protective layer.
• Phelloderm: Living parenchyma cells formed inwards by the cork cambium.

3. Associated Tissues:

These tissues are part of the plant body and are affected by or contribute to secondary growth.

• Parenchyma: Found in both primary and secondary xylem and phloem, and as phelloderm. It plays roles in storage, wound healing, and formation of interfascicular cambium.
• Collenchyma: While primarily a supporting tissue in primary growth, it may be present in the cortex before it is replaced by the periderm.
• Sclerenchyma (Fibers and Sclereids): Provide mechanical support. Fibres are abundant in secondary xylem and phloem. Sclereids may be found in bark.
• Epidermis: The outermost protective layer in primary growth, which is eventually replaced by the periderm.
• Cortex: The tissue between the epidermis and the vascular cylinder, which may be involved in the formation of interfascicular cambium or be partially incorporated into the periderm.

Tissue Systems Involved

Secondary growth primarily affects and involves the vascular tissue system and the dermal tissue system.

1. Vascular Tissue System:

This system, comprising xylem and phloem, is fundamentally altered and expanded by secondary growth.

• Secondary Xylem: Forms the bulk of the wood, providing structural support and water transport.
• Secondary Phloem: Forms part of the bark, responsible for sugar translocation.
• Vascular Cambium: As a lateral meristem, it is a crucial component of the vascular tissue system during secondary growth, ensuring continuous production of secondary vascular tissues.

2. Dermal Tissue System:

This system, which provides outer protection, is significantly modified.

• Epidermis: Initially present, it is eventually ruptured and replaced.
• Periderm: The secondary protective tissue system, composed of cork cambium, cork, and phelloderm, which takes over the protective role of the epidermis.

3. Ground Tissue System:

While not the primary focus of secondary growth, the ground tissue system is involved.

• Parenchyma: Cells of the ground tissue system (in the cortex and pith) dedifferentiate to form interfascicular cambium. Parenchyma also forms the phelloderm and is present in secondary xylem and phloem.

Significance of Secondary Growth

Secondary growth is vital for the survival and ecological success of many plant species. The increased diameter provides robust structural support, allowing plants to grow taller and withstand wind and other environmental stresses. The extensive development of wood (secondary xylem) ensures efficient long-distance transport of water and minerals, essential for large plant bodies. Furthermore, the formation of bark (periderm) offers protection against dehydration, temperature fluctuations, fire, and pathogen attack. In economic terms, wood is a critical resource for timber, fuel, and paper production, all derived from secondary xylem.