2019-03-01 Biology Times

(singke) #1
Biology Times March 19

Extrastelar secondary growth
The increase in thickness of stellar region exerts pressure on the endodermis and cortex. It results in rupture
of endodermal cells. The pressure also results in the dedifferentiation of hypodermal (collenchyma) cells to
give rise to a secondary meristem, cork cambium or phellogen.
It is one layered and the cells show cell division and produce cells towards the innerside and outerside.
The cells formed towards innerside differentiate into large parenchyma cells with chloroplasts. It is the
secondary cortex or phelloderm, sometimes these cells carry on photosynthesis and store food materials.

The cells formed outerside differentiate into cork or phellem. The cellwalls of cork cells become highly
suberised and finally the cells become dead. The number of cork cell is always greater than the cells of the
All these cells are arranged in radial rows and cells have no intercellular spaces. These cells are impervious
to water and air. Addition of these tissues results in the rupture of epidermis at many points. To overcome
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