Short answers, pulled from the story.
The vascular cambium is the main growth tissue in the stems and roots of plants that exhibit secondary growth, including dicots such as oak trees and gymnosperms such as pine trees. It produces secondary xylem inward toward the pith and secondary phloem outward toward the bark. Generally, more secondary xylem is produced than secondary phloem.
The vascular cambium consists of two types of cells: fusiform initials, which are tall and axially oriented, and ray initials, which are smaller and round to angular in shape. These two cell types maintain the connection and communication between xylem and phloem.
Five angiosperm lineages have independently lost the vascular cambium: Nymphaeales, Ceratophyllum, Nelumbo, Podostemaceae, and monocots. Because monocots lack the vascular cambium, plants such as grasses do not undergo secondary growth and cannot develop thick woody trunks.
The phytohormones involved in vascular cambial activity include auxins, ethylene, gibberellins, cytokinins, abscisic acid, and likely additional hormones yet to be identified. Auxin stimulates mitosis and cell production; gibberellin stimulates cambial cell division and regulates xylem differentiation; cytokinin regulates the rate of cell division.
For successful grafting, the vascular cambia of the rootstock and scion must be precisely aligned so they can grow together. If the cambial layers do not make contact, the two plant parts cannot merge and the graft fails.
The cambium of most trees is edible. In Scandinavia, it was historically used as a flour to make bark bread.