Secondary growth occurs in all gymnosperms, some dicots and few monocots.  The plant body is a product of this diametric growth.  There are two lateral meristems:  the vascular cambium and the cork cambium.  The vascular cambium extends the xylem and phloem as the cork cambium extends the epidermis.  As the vascular cambium moves outward what is inside that ring is mostly dead vascular tissue that only supports the tree.

Vascular Cambium.

The vascular cambium forms from parenchyma that becomes meristematic.  This always occurs between the primary xylem and phloem of each vascular bundle in the stem.  This part of the vascular cambium is called the fasicular cambium.  The cells in this cambium are called fusiform initials and produce xylem on the inner side and phloem on the outer side.  Since the vascular tissue is growing the ground tissue must grow as well or all trees would be hollow!  So between bundles are ground tissue connections between the cork and the pith.  This ground tissue part of the vascular cambium is called the interfasicular ("in between groups of" fasicular cambia) cambium.  The cells of the interfasicular cambium are called ray initials that form tracheids and fibers.  

Note:  Only the youngest phloem helps in sugar transport.  Older phloem protects the stem.

Cork Cambium.

The cork cambium is old epidermis from primary growth.  It splits, dries, and falls off while the cork cambium replaces it.  The outer cortex also creates the phelloderm (inside part of periderm) and cork cells (outside part of periderm).  The periderm is the entire outer part of the cortex including the cork cambium.  The cambium is a fixed size!  When vascular tissue expands cork cambium dies, and eventually the outer phloem will be cork cambium.

Next:  "Invertebrates."