Introduction

Vascular cambium is the main form of tissue development in angiosperms and gymnosperms like Pine. The vascular system is composed of the xylem and phloem. It has two types, primary and secondary. The primary xylem and phloem construct the primary tissues. The vascular cambium gives rise to the secondary xylem and phloem. This is called secondary growth.

The basic primary vascular tissues of the stem and root only last a limited time in woody dicotyledons and gymnosperms before being replaced by secondary vascular tissues (secondary xylem and phloem). The cambium is lacking in the majority of lower vascular plants and many herbaceous angiosperms.

Origin of Vascular Cambium

The vascular cambium is developed from the procambium. Procambium is formed from apical meristem of stem. The first formed meristem is called pro-meristem. Apical meristem is divided into three parts. These are Protoderm, Procambium and Ground meristem.

The procambium can have the shape of strips, cylinders that are solid or hollow, or both. Procambium development is acropetal (from base to the apex or from below to upward). At various phases of procambial development, procambial cells differentiate into vascular tissue cells in various plants. Procambium gives rise to vascular cambium which is a lateral meristem (1).

Structure of Vascular Cambium

The cells of the vascular cambium differ from typical meristematic cells. They are isodiametric, have a prominent nucleus, and have thick cytoplasm. Vacuolated cambial cells are present. Compared to the active cambial cells, the resting cells have fewer vacuoles.

Vascular cambiums are two types. These are ray initials and fusiform initials.

Ray initials

These are virtually isodiametric in shape, much smaller than fusiform initials, and slightly extended. They cause the cambial zone to generate a radial system.

Fusiform initials

The description “fusiform” suggests that the cell resembles a spindle. However, a fusiform beginning has a prismatic cell in the center and wedge-shaped ends. These cells are numerous to many times wider than they are long (1).

Location of Vascular Cambium

The vascular cambium is primarily located within the vascular bundle, between the primary xylem and phloem. Further division of vascular cambium gives rise to secondary xylem and phloem. The secondary xylem forms the inner part of the cell with tracheid and xylem vessels. Secondary phloem, which is composed of companion cells and sieve elements on the outside. Lignin, the main component of wood and what gives it its stiff structure, is found in the cells of the secondary xylem components.

Function and Role of Vascular Cambium

The primary function of the vascular cambium is to form the xylem and phloem. It also helps in the growth of the plant. It also helps the plant to become thicker and woody. 

Primary growth and Secondary growth

Plant growth can be categorized into two types. One is primary growth where the plant increases in length. The other one is secondary growth. The plant increases in strength. Primary growth takes place due to division in the shoot apical meristem. Lateral meristem divides to form secondary growth.

In the apices or root and shoot tips, primary and secondary growth mostly occurs. The shoot tip and root tip’s apical meristems, where cells divide quickly, are the source of primary growth. The process of continuous cell elongation also aids in initial growth. Plants can continuously seek water (through roots) or sunlight (via shoots) thanks to the expansion of their shoots and roots during basic growth.

Difference between primary growth and secondary growth  

Production of Xylem & Phloem 

Vascular cambium is of two types. These are fascicular and interfascicular cambium. While interfascicular cambium develops from cells of medullary rays, fascicular vascular cambium is a primary meristem that appears as strips in vascular bundles. The meristematic fascicular cambium can divide to create a new xylem on the inner surface and a new phloem near the exterior. These are the secondary xylem and secondary phloem.

The xylem is constituted of four elements. These are tracheid, trachea, xylem parenchyma, and xylem vessel. Except for xylem parenchyma, the other three are dead cells. Xylem helps in water transportation. On the other hand, phloem helps in the transportation of food and minerals. Phloem also consists of four elements. These are sieve tubes, companion cells, phloem parenchyma, and phloem fibers.

Difference between Xylem and Phloem 

Role in plant vascular system 

The vascular cambium is in charge of developing woody tissue and expanding the diameter of stems and roots. The cambium divides its cells to create secondary xylem and phloem. With this, the stem becomes thicker and develops wood and bark.

Tracheary elements or sieve elements rise from derivatives of the fusiform initials. Ray parenchyma is produced from derivatives of the ray initials. Water from the xylem can be transported into the cambium and phloem tissues through the ray parenchyma, and photosynthate can be transported from the phloem into the cambium and xylem live cells.

Process of cambial activity 

A tissue layer that aids in plant growth is the cambium layer. The cambium cells contribute an undifferentiated mass of cells that aid in the plant’s secondary growth. The xylem and phloem regions are where the cambial action is seen.

Cell division and cell differentiation

The vascular cambium has two zones, a division zone and a differentiation zone. The cambial initials divide their cells in the division zone. This division can happen either perpendicularly (periclinal cell division) or parallelly (anticlinal cell division).

After that Cambial initials reach the differentiation zone. They then produce either xylem or phloem mother cells. These mother cells give rise xylem or phloem. (2).

Annual ring

Due to the vascular cambium’s uneven activity, annual rings occur. The cambium’s activity changes with time. The entire year can change it. Environmental and physiological factors have an impact on the activity of the vascular cambium. An annual ring is a band of wood that can be seen in the cross-section of a temperate plant’s stem or root. It symbolizes a plant’s growth throughout the year.

 The growth period is often one year in temperate regions, in which case the growth ring may be referred to as an “annual ring.” Growth rings may not be visible or not yearly in tropical areas. Growth rings are infrequently absent. The dating of ancient wooden structures is done using annual rings.

 Environmental factors affecting vascular cambium

Temperature is one environmental component that affects how trees grow and develop. The physiological mechanisms that are involved in the beginning of cambial cell division and xylem differentiation in trees are influenced by temperatures from late winter to early spring. The cambium remains active from spring to fall but gets inactive in winter. A longer growth phase and quicker cambial reactivation and xylem differentiation in tree stems are caused by cumulatively higher temperatures from late winter to early spring. 

Relation with bark and wood

The outermost coat of woody plants’ stems and roots is called bark. The tree is shielded from the outside world by its outer bark. It protects against heat and cold and keeps off pest insects. Food is transported to the remainder of the tree through a pipeline called “phloem,” which is the inner bark.

The portion of the trunk that is growing is the cambium cell layer. In reaction to hormones, it annually creates new bark and wood. The “auxins” in these hormones promote cell development. As soon as spring growth begins, leaf buds at the ends of branches begin to release auxins (3).

 Importance in the forestry and timber industry

Cambial activity plays an important part in the timber industry. As the timber industry primarily requires hard woody structures, plants with secondary growth are necessary. Vascular cambium is the tissue that causes secondary growth. We all know that secondary growth provides structural strength in plants. Annual rings are formed due to cambial activity. The number of annual rings helps to understand the age of the plant.

Vascular cambium is found in Gymnosperms and woody angiosperms. So, it can easily be said it is the feature of trees. For any forest, canopy(upper layer of habitat formed by mature tree crowns) formation is necessary. It protects the forest floor from some predators and abiotic (non-living) factors like hot temperatures and rain.

Q&A

1. What is vascular cambium?

The primary meristem in the stem is the vascular cambium, which produces undifferentiated wood cells on the inside and bark cells on the outside. Secondary vascular tissues, secondary xylem, and secondary phloem are produced by the vascular cambium.

2. Where is the vascular cambium located?

Vascular cambium first appears in the middle of the primary xylem and phloem in the young stem or root. Gymnosperms or dicot plants typically have cambium tissue.

3. What is the origin of vascular cambium?

The procambium gives rise to the vascular cambium. The apical meristems’ descendant cells give rise to the procambium. 

4. What are the differences between cork cambium and vascular cambium?

The main difference between cork cambium and vascular cambium is cork cambium gives rise to bark and secondary cortex as vascular cambium gives rise to a secondary vascular bundle. Cork cambium develops from lateral meristem while vascular cambium develops from apical meristem.

Summary

• Vascular cambium is the main tissue that gives rise to a secondary vascular bundle that comprises of secondary xylem and secondary phloem.
• The xylem is constituted of the tracheid, trachea, xylem parenchyma, and xylem fibers. Among the four, only xylem parenchyma is the living cell.
• Phloem has four components. These are sieve tubes, companion cells, phloem parenchyma, and phloem fibers. Except for phloem fiber, all are living cells.
• Vascular cambium is developed from procambium while procambium is developed from apical meristem.
• The cells of the vascular cambium are isodiametric, have a prominent nucleus, and have thick cytoplasm.
• Vascular cambium is composed of two types of cells, ray initials and fusiform initials. Ray’s initials are smaller and isodiametric. Fusiform initials are larger and spindle-shaped.
• The vascular cambium is located in between the primary xylem and primary phloem. It helps in the secondary growth of the plant. Secondary growth causes the plant the plant to expand its diameter.
• The uneven cambial activity causes the formation of annual rings. An annual ring is a band of wood that can be seen in the cross-section of a temperate plant’s stem or root. It symbolizes a plant’s growth throughout the year.

Written By: Pritha Chowdhury