What are $P, Q$ and $R$ in the given anatomical structure?

Old trees obtain $O_2$ from

Cambium is present in between

From the plants given below,select the monocots having secondary growth in their stem.

Read the different components from $(i)$ to $(iv)$ in the list given below and tell the correct order of the components with reference to their arrangement from outer side to inner side in a woody dicot stem.
$(i)$ Secondary cortex $(ii)$ Wood $(iii)$ Secondary phloem $(iv)$ Phellem
The correct order is:

Cambium activity is