1. Origin and activity of cambium
Intrafascicular of fascicular cambium is present in dicot stem bundle in between the xylem and pholem of each vascular bundle. During secondary growth the parenchymatous cells of medullary rays become meristematic and form strips of interfascicular cambium from a complete ring of vascular cambium. The vascular cambium consists of two types of cells.

(a) Fusiform initials. These are spindle shaped elongated cells having brick like appearance. They produce secondary xylem elements and secondary pholem elements. These are arranged in vertical rows.

The cambium is active towards inside than outside so secondary xylem increases more rapidly than secondary pholem pushing the cambium ring outwards. In the mean time some cells cambium divide by radial walls to increase circumference of cambium ring.

The ray initials cut off parenchymatous cells towards outside as well as inside to form radial bands of parenchymatous cells passing through secondary pholem and secondary xylem as secondary medullary rays.
Many of natural fibres such as jute, hemp, flax, etc. are bast fibres of secondary pholem. The primary pholem present outside the secondary pholem gets crushed. The secondary xylem forms the bulk of stem called wood. The primary xylem persists as conical projections on inner side of secondary xylem around pith.