The phenomenon by which the undividing parenchyma cells start to divide mitotically during plant tissue culture is called 

Dedifferentiation refers to the process by which specialised cells, such as parenchyma cells, lose their specialised characteristics and revert to a less specialised or more embryonic state, often with the ability to divide mitotically.

(1) Differentiation is the process by which cells become specialised and acquire specific structures and functions to
perform particular roles in the organism. 

(3) Redifferentiation is the process by which dedifferentiated cells regain their specialised characteristics and re-establish their specific structures and functions to form mature tissues or organs.

(4) Secondary growth is the increase in girth or thickness of stems and roots in woody plants. It occurs due to the
activity of lateral meristems, such as vascular cambium and cork cambium, leading to the production of secondary
tissues, like secondary xylem and secondary phloem.