The balloon-shaped structures called $tyloses$:

In old trees,the greater part of secondary xylem is dark brown and resistant to insect attack due to:
$(a)$ secretion of secondary metabolites and their deposition in the lumen of vessels.
$(b)$ deposition of organic compounds like tannins and resins in the central layers of stem.
$(c)$ deposition of suberin and aromatic substances in the outer layer of stem.
$(d)$ deposition of tannins,gum,resin and aromatic substances in the peripheral layers of stem.
$(e)$ presence of parenchyma cells,functionally active xylem elements and essential oils.
Choose the correct answer from the options given below:

The waxy substance associated with the cell walls of cork cells that makes them impervious to water is:

In dicot stem,secondary growth is due to the activity of:

The $phellogen$ (cork cambium) develops from $...................$.

Increase in the girth of plant (organ) takes place by