How do we compute the age of a living tree?

Plants having little or no secondary growth are

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:

Which of the following meristems is responsible for extrastelar secondary growth in dicotyledonous stems?

The collective name for cork cambium and phelloderm is

Distinguish between the following: Interfascicular cambium and intrafascicular cambium.