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

Physiologically,the active part of the wood is ..........

The light-colored outer region of the wood is known as ..........

Knots in wood are formed due to ...........

The periderm consists of ............ .

$I.$ Annual rings are formed as a result of seasonal environmental conditions.
$II.$ Tracheids/vessel elements are larger during periods when water is abundant.
$III.$ Tracheids/vessel elements have thicker walls during periods of water deprivation.
$IV.$ Wood formed in previous years is always darker than newer wood.
Select the combination of correct statements from the options given below.