Knots in stems are formed due to

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:

Define the following terms:
$(i)$ Conjoint vascular bundles
$(ii)$ Secondary growth

Secondary xylem and phloem in dicot stem are produced by

Grafting is not possible in monocots because...

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