$A$. Heartwood is durable,dark,and central in position.
$B$. Tyloses are balloon-like structures of xylem parenchyma in vessel lumen.
$C$. Late wood is formed during spring season.

Open vascular bundles lack .......... .

Secondary growth is absent in

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Late wood has fewer xylary elements with narrow vessels.
Reason $R$: Cambium is less active in winters.
In the light of the above statements,choose the correct answer from the options given below:

$A$: Complementary cells are cut-off by phellogen towards the outside.
$R$: Phellogen is partially secondary in origin.

Cambium is essential for grafting in plants because