.......... permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.

Assertion: Cambium is a lateral meristem and causes growth in width.
Reason: Cambium is made up of fusiform and ray initials in the stem.

Tyloses thickenings are seen in

$I.$ 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.

What is the study of $Dendrochronology$?

Some vascular bundles are described as open. This is because they: