Secondary Growth, Meristematic Activities in Growth

SECONDARY GROWTH

❝ The growth in which plant girths is increased due to the activity of the vascular cambium (lateral meristem) in the woody plants, is called secondary growth.❞

During primary growth, stems increase in length while somates and sunflower also increase in diameter. The result of secondary growth is most evident in woody perennial plants like tree, shrubs and Vine.
The lateral thickening involves the activity of a secondary meristem, the Vascular cambium. Secondary growth occurs due to cell division in:

(i) Vascular cambium (ii) Cork cambium.

(a) Vascular Cambium as a Cylinder:
Firstly, cambium is restricted to the vascular bundles. It appears as a cylinder.

(b) New Tissue Formation:
Later, it forms a continuous cylinder with in the stem between the cortex and pith.
Its cell divide in such a way as to make new xylem cells on the inside and new phloem cells on the outside.

(c) Increase the Thickness of Stem:
The continuous division of cambium cells add new cell to the stem and it increase in thickness i.e., secondary thickening.
In such woody stems the epidermis is often replaced by dead corky layer, Bark, which itself is made by separate, layer of Cork Cambium.
Cork cambium is found beneath the epidermis.

(d) Right Formation:
Cambium produces layers upon layers of secondary xylem. These layers visible as rings. Since one growth ring is formed in one year, a count of the rings at the base of trunk indicate the age of trees at the time it was cut.

(e) Limitations of Secondary Xylem:
In older trees, only few annual growth rings are active in conduction at one time.

(i) Heart Wood:
After, a tree or branch is several year o;d, the inner part of the stem usually becomes inactive and filled with resins, gums, oils and tannins.

(ii) Sap Wood:
Outside the heartwood is a light-coloured, less dense region of active wood called sapwood.

(f) Callus Formation:
Callus is a mass of parenchymatous cells tht forms at a wounded surface. It produced by cambium. Where a branch has been cut off a tree. The callus tissue forms a ring of thickening (wound wood) around the wood. Evebtually, it may completely cover the exposed wood.