Secondary Growth In Plants: Vascular Cambium and Cork Cambium

Secondary growth of plants refers to growth of lateral meristem tissues. It means that an increase in stem thickness takes place due to lateral meristem activity.

• Two types of lateral tissues are involved in the secondary growth of plants, namely, vascular cambium and cork cambium.
• When secondary growth occurs, these two tissues produce secondary phloem and secondary xylem.
• As a result, the root and shoot thickens.

Keyterms: Plants, Tissue, Meristem tissues, Phloem, Xylem, Vascular cambium, Cork cambium, Root

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Primary Vs. Secondary Growth in Plants

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When length of the root and shoot increases by cell division in shoot apical meristem, it is termed as the primary growth of plants. On the other hand, increase in thickness of roots and shoots due to activity of lateral meristem is called secondary growth of plants.

• Primary growth occurs in apical meristem.
• It is found at the tips of shoots and roots.
• Secondary growth occurs at lateral meristem that involves vascular and cork cambium.

Pictorial representation of primary and secondary growth in plants is given below.

Primary growth in plant, due to growing apical meristem.

Secondary growth in plant, development of secondary cambium, xylem, and phloem 

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Secondary Growth in Vascular Cambium

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Vascular cambium is a lateral meristem which is responsible for thickness of the root. In the initial growth phase, a layer of meristematic tissues are situated between the vascular tissues- primary xylem and primary phloem. This layer grows as patches and upon maturity, the patch separates the vascular tissues. This tissue is called secondary cambium. An active cambial ring initiates the differentiation of new cells. The newly grown cells in the center region form the xylem while the cells in the periphery develop into phloem. 

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Secondary Growth in Cork Cambium 

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Cork cambium is the outermost lateral meristem in woody plants. It is also termed as phellogen. The cambial ring activity leads to crushing the outer layers such as cortex cells and epidermal cells. At this time, cork cambium develops as a new protective layer and starts to differentiate cells that form outer cork (phellem and inner secondary cortex (phelloderm). Phellogen, phellem and phelloderm together, makes up the periderm. The secondary phloem and periderm collectively called the bark of trees.

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Things to remember

• The secondary growth in plants is due to lateral meristematic activity.
• During secondary growth, roots and shoots get thick.
• Secondary xylem and phloem are developed during the secondary growth phase.
• Two tissues are involved in this phase i.e. vascular cambium and cork cambium.
• The xylem is developed by vascular cambium and the phloem is formed during the growth of cork cambium.
• Increase in the length of root and shoot is called primary growth while increase in thickness is due to the secondary growth.

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