Secondary Growth - Vascular Cork Cambium

Secondary growth in plants can be referred to as the increase of stem and root thickness due to the activity of the lateral meristems which are not observed in herbaceous plants. In simple terms, it’s the outward growth of the plant body. Secondary growth is important in woody plants as they grow much taller than other plants and they need more support in their roots and stems. Lateral meristems which play a primary role in secondary growth of plants are composed of the vascular cambium and the cork cambium. Growth of these tissues directly results in secondary growth of the plant. In this topic, we will discuss the formation of secondary vascular tissues and their structure.

Structure of Vascular Cambium

The vascular cambium is situated between the primary xylem and the primary phloem within the vascular bundle. The cells of the vascular cambium divide and form the secondary xylem which consists of tracheids and vessel elements to the inside and secondary phloem which consists of sieve elements and companion cells to the outside. The cells of the secondary xylem elements contain lignin which is the primary constituent of wood and provides the rigid structure of the material. 

The vascular cambium is produced by two types of meristems 

1. Fascicular cambium or intra-fascicular cambium

2. Interfascicular cambium

Fascicular vascular cambium is a primary meristem which occurs as strips in vascular bundles whereas interfascicular cambium arises from the cells of medullary rays which occur at the level of intra-fascicular strips.

These two types of meristematic tissues connect together to form the vascular cambium. Vascular cambium has only one layer but it appears to have a few layers due to the presence of intermediate derivatives. The cell of the vascular cambium system divides periclinal both on the inner and the outer sides (bipolar divisions) to form secondary permanent tissues.

There are two types of cells noticed in the vascular cambium, 

1. Elongated spindle-shaped fusiform initials

2. Shorter isodiametric ray initials

Fusiform initials cells divide to form secondary phloem on the outer side and secondary xylem on the inner side. When the secondary xylem forms on the inner side, the vascular cambium moves gradually to the outside by adding new cells. The phenomenon is called dilation.

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Vascular Cambium Function

The major function of the vascular cambium is the formation of xylem and phloem cells. 

• The cambium forms secondary xylem internally and secondary phloem externally. 

• It plays a role in the growth of roots and shoots

• The cambium also forms the inner bark of the tree and the wood of a tree and is responsible for its thickness. 

What is Cork Cambium?

Cork cambium is the outermost lateral meristem and it produces cork cells that contain a waxy substance as suberin (this has water-repelling characteristics). The bark protects the plant's body against physical damage and helps in reducing water loss. 

The cork cambium also produces a fresh layer of cells called the phelloderm which grows inward from the cambium. The cork cambium, cork cells and the phelloderm are collectively known as the periderm. The periderm is a substitute for the epiderm in mature plants. It has many openings which are known as lenticels. These openings allow the interior cells to exchange gasses with the atmosphere outside and supply oxygen to the metabolically active cells of the xylem, phloem and the cortex.

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Secondary Growth in Stem

The vascular cambium and cork cambium play a primary role in increasing the thickness of the stem for woody plants. The cells of vascular cambium divide into xylem and phloem cells and the increase in thickness is due to the formation of secondary xylem and secondary phloem cells. The cork cambium tissue forms the bark of the plant. It also contains a water-repelling substance called the suberin which makes the bark withstand various environmental factors. A layer of cells called the periderm in the vascular cambium system also plays a role in the growth of the stem as it assists in the exchange of gases in between the atmosphere and the interior cells.

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Secondary Growth in Roots

Roots produce secondary tissues and branch roots at the expense of the primary tissues. Cells in the primary tissue are discarded as secondary growth proceeds. New lateral roots form from within the root system and push outward from the pericycle, destroying cortex and epidermal tissues on their way to the soil.

Initiation of secondary growth in the root system takes place in the zone of maturation soon after the cells stop elongating there. The vascular cambium differentiates between the primary xylem cells and the primary phloem cells in this zone and the pericycle cells divide simultaneously with the procambium initials. This leads to the formation of a cylinder of cambium encircling the primary xylem.

The vascular cambium system almost immediately begins producing xylem cells inward and phloem cells toward the outside of the root. This results in the flattening of the primary phloem against the more resistant endodermis. 

Concomitant differentiation of cork cambium in the pericycle adds other areas of cell division in the stele. The combination of vascular tissue and periderm production breaks the remaining cells of the cortex and epidermis and the lignified and suberised new cell walls are laid down by the cambia isolates the outer tissues as well from their source of supplies in the interior of the root. 

At the end of the first year, secondary growth destroys all but the central core of primary xylem cells and a few fibres of primary xylem pushed against the periderm. The zones at this time, therefore, from outside to inside are periderm, pericycle, primary and secondary phloem, vascular cambium, secondary and primary xylem.

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Abnormal Secondary Growth

This type of secondary growth does not follow the pattern of a single vascular cambium produces xylem towards the inside and phloem towards the outside and is often observed in some dicots such as Bougainvillea, Dracaena etc, where a series of cambia arise outside the oldest phloem. 

Abnormal secondary growth is also observed in arborescent monocot stem where a secondary cambium grows in the hypodermal region and the latter forms conjunctive tissue and patches of meristematic cells. These patches of cells grow into secondary vascular bundles. The bundles are observed in the cortex and the pith regions.

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