Q.Usually, the monocotyledons do not increase their girth, because
a. They possess actively dividing cambium
b. They do not possess actively dividing cambium
c. Ceases activity of cambium
d. All are correct

The vascular cambium is the lateral meristem responsible for producing a dicot or gymnosperm stem or root's secondary vascular tissues - secondary xylem (wood) on its inner face and secondary phloem (bast) on its outer face - and it is one of the two lateral meristems (alongside the cork cambium) that together carry out secondary growth. Its origin differs between organs: in a stem, part of the eventual cambial ring - the intrafascicular (fascicular) cambium - is already present inside each vascular bundle from early development, believed to originate from the procambium, and is therefore primary in nature from the start; the gaps between bundles are closed by interfascicular cambium, which forms later when parenchyma cells of the medullary rays in line with the intrafascicular cambium become meristematic on their own, making it secondary in nature from the beginning. These two components join on both sides to form one continuous vascular cambial ring. In a root, by contrast, the vascular cambium is completely secondary in origin, forming from a combination of conjunctive tissue below the phloem strands and part of the pericycle above the protoxylem, initially as a wavy ring that only later becomes circular.

Structurally, an active vascular cambium does not look like a typical meristem - rather than small, densely cytoplasmic cells, its cells develop a large central vacuole surrounded by a thin layer of dense cytoplasm, letting the ring expand in circumference as the organ thickens. It is built of two kinds of initials: vertically elongated fusiform initials, which produce the axial (longitudinal) system of both secondary xylem and secondary phloem (tracheary elements/sieve elements, fibres, and axial parenchyma), and horizontally elongated ray initials, which produce the radial system - the xylem and phloem rays that run sideways across the wood and bast. Based on how the fusiform initials are arranged, cambium is further classed as storied (stratified), where short fusiform initials line up in regular horizontal tiers, or non-storied (non-stratified), where longer fusiform initials overlap strongly at their ends with no tiered pattern. When active, the cambial ring divides periclinally on both faces, but unevenly - producing many more layers of xylem inward than phloem outward over the same period - while also periodically cutting off horizontal parenchyma bands (rays) that run through both tissues; the accumulating secondary xylem and phloem progressively crush the stem's original primary xylem and phloem.