The procambium is a meristematic tissue concerned with providing the primary tissues of the vascular system; the cambium proper is the continuous cylinder of meristematic cells responsible for producing the new vascular tissues in mature stems and roots.
Protoxylem is the first part of the primary xylem that matures and these cells are usually smaller than the metaxylem. Metaxylem is the part of the primary xylem that differentiates after the protoxylem and these cells are usually larger than the protoxylem.