Secondary Growth

SECONDARY GROWTH

  • Secondary growth is the process in woody plants where stems and roots grow thicker over time. 
  • This growth occurs through the activity of tissues called vascular cambium (for adding wood) and cork cambium (for outer protection). 
  • It makes trees stronger, forms annual rings, and provides resources like wood and cork.

Vascular cambium

- The vascular cambium is a critical tissue found in the stems and roots of woody plants, responsible for secondary growth or increase in girth.

- It plays a crucial role in the formation of secondary xylem (wood) and secondary phloem (inner bark). 

  • Location:

- Vascular cambium is found between the primary xylem and primary phloem in the vascular bundles of stems and roots.

- In dicot stems, it forms a complete ring, while in dicot roots, it's present as patches. 

  • Structure:

- The vascular cambium is a meristematic tissue composed of small, actively dividing cells.

- It consists of two types of cells: Fusiform initials and Ray initials.

- Fusiform initials are elongated cells responsible for the longitudinal growth (height) of the plant.

- Ray initials are smaller cells responsible for the radial growth (girth) of the plant. 

  • Function:

- Secondary Xylem Formation:

The fusiform initials give rise to xylem cells, contributing to the growth of wood. These cells become tracheids and vessel elements.

- Secondary Phloem Formation:

The ray initials produce secondary phloem cells, which contribute to the inner bark.

- Increase in Girth:

As the vascular cambium divides, it adds new layers of xylem to the inside and phloem to the outside, leading to the growth in girth or thickness of the stem or root.

 Annual Rings:

- In temperate regions, the activity of the vascular cambium varies with seasons, forming annual rings in the wood.

- In spring, the cells produced are larger and have thin cell walls, forming the light-colored earlywood.

- In late summer, the cells are smaller and have thicker cell walls, forming the dark-colored latewood.

- Counting these rings can determine the age of a tree. 

Environmental Factors:

- The activity of the vascular cambium can be influenced by environmental factors such as temperature and moisture.

- Optimal conditions lead to wider rings, while stress or unfavorable conditions can result in narrower rings.

 

 

Springwood and Autumnwood     

- Springwood and autumnwood are two distinct types of wood found in the annual rings of woody plants, particularly in temperate regions. 

Springwood (Earlywood):

- Formation:

Springwood is the wood formed during the early part of the growing season, typically in spring. 

- Characteristics:

It has several distinct features: 

- Large Cells:

The cells in Springwood are larger in diameter. 

- Thin Cell Walls:

The cell walls are relatively thin. 

- Lighter Color:

It appears lighter in color compared to autumn wood. 

- Function:

Springwood is responsible for rapid water conduction and nutrient transport at the beginning of the growing season when the plant's demand for resources is high.

 Autumnwood (Latewood):

- Formation:

Autumnwood is the wood formed during the latter part of the growing season, usually in late summer or early autumn. 

- Characteristics:

It has different characteristics from springwood:

  • Smaller Cells: The cells in autumnwood are smaller in diameter.
  • Thicker Cell Walls: The cell walls are thicker and contain more lignin.
  • Darker Color: It appears darker in color compared to springwood. 

- Function:

Autumnwood provides structural support and durability to the tree. Its denser, compact nature makes the wood stronger and more resistant to environmental stresses. 

Annual Ring Formation:

- The alternation between springwood and autumnwood creates the characteristic pattern of annual rings in temperate trees.

- Each pair of springwood and autumnwood represents one year's growth.

- Counting these rings can determine the age of a tree.

 

 

Environmental Factors:

- The appearance and characteristics of springwood and autumn wood can vary depending on environmental factors:

- Favorable conditions like adequate moisture and temperature lead to wider rings.

- Stress or unfavorable conditions can result in narrower rings.

 

 

 

Heartwood and Sapwood      

- Heartwood and sapwood are two distinct regions found in the cross-section of a tree's trunk and branches, each with specific characteristics and functions. 

Sapwood:

- Sapwood is the outer, younger, and living portion of wood located just beneath the bark. 

Characteristics:

Color-

It appears lighter in color compared to heartwood, often pale or cream-colored. 

Function-

Sapwood is responsible for transporting water and nutrients from the roots to the leaves through a network of living vessels and cells. 

Moisture Content-

Sapwood typically has a higher moisture content due to its active role in water transport. 

Vulnerability-

It is more susceptible to decay and insect infestations due to its moisture content and lower levels of protective chemicals. 

Heartwood:

Heartwood is the inner, older, and nonliving portion of wood, located towards the center of the tree. 

Characteristics:

Color-

 It is darker in color than sapwood, often ranging from reddish-brown to black. 

Function-

Heartwood serves a structural and protective role, providing strength and rigidity to the tree. It no longer actively conducts water or nutrients. 

Chemical Changes-

Over time, the cells in heartwood undergo chemical changes, depositing substances like resins and tannins, which make it less appealing to pests and more resistant to decay. 

Durability-

Heartwood is highly durable and can withstand decay and natural elements for extended periods. 

Transition Zone-

- Between sapwood and heartwood, there is often a transition zone known as the sapwood-heartwood boundary, which contains partially functioning cells.

- This transition zone may appear different from both sapwood and heartwood in terms of color and properties. 

Use in Woodworking-

- In woodworking, sapwood is often less desirable due to its vulnerability to decay and insects.

- Heartwood is preferred for its durability and appearance, making it a choice for high-quality wood products.

 

Cork cambium

- The cork cambium, also known as phellogen, is a meristematic tissue found in the outermost layers of woody stems and roots of plants.

- It plays a crucial role in the formation of cork, which is the protective outer layer in woody plants. 

Location:

- Cork cambium is typically located in the outer cortex of stems and roots, just beneath the epidermis. 

Structure:

- The cork cambium consists of a layer of small, actively dividing cells.

- It forms parallel layers of cork cells toward the outer surface and secondary cortex cells toward the inner side.

- The cork cells produced by the cork cambium have a unique structure with suberin deposits, which make them impermeable to water and gases. 

Function:

Formation of Cork-

The primary function of cork cambium is to produce cork cells. These cells are dead at maturity and serve as a protective barrier for the plant.

Protection-

Cork cells are impervious to water and provide protection against physical damage, pathogens, and extreme environmental conditions. 

Bark Formation:

- The combination of cork cambium, cork cells, and other tissues like secondary phloem forms the bark of woody plants.

- The bark has several layers, with the cork cambium contributing to the outermost layer.