Which of the following is the function of heartwood?

The amount of secondary xylem produced is $8-10$ times more than that of secondary phloem every year. Choose the correct option.

Growth rings $(annual\ rings)$ are formed by the activity 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.

Give an explanation about cork cambium.

Dendrochronology is