Explain the process of secondary growth in stems of woody angiosperm with help of schematic diagrams. What is the significance?

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

In which of the following secondary growth takes place

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

$I.$ It is resistant to microorganisms

$II.$ It comprises dead elements

$III.$ It comprises highly lignified cell wall

$IV.$ It is the peripheral part

$V.$ It is dark and tough

Which of the above property doesn't belongs to heart wood?