The position of xylem in secondary xylem is:

What is/are true about heartwood?
$I.$ It does not help in water conduction.
$II.$ It is also called alburnum.
$III.$ It is dark in colour but very soft.
$IV.$ It has tracheary elements,which are filled with tannin,resin,etc.

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:

Diffused porous woods are characteristic of plants growing in

In old trees,the greater part of secondary xylem is dark brown due to the

The vascular cambium produces . . . . . . .