The lateral meristem of a plant from which are differentiated the primary vascular tissues and bundles. The procambium arises in the form of meristematic strands in the growing point of a shoot when leaf traces appear, or it develops in the form of a solid cylinder (or ring) in the growing point of the root in the middle of the primary meristem. The procambium is characterized by intensive longitudinal cell growth and by the elongated (prosenchymal) shape of the cells. Cell division occurs in various directions, with vascular tissues— primary xylem and phloem—arising from the cells. After the formation of vascular tissues in the bundles of gymnospermous and dicotyledonous plants, the procambium functions as cambium. In monocotyledonous plants, the procambium is exhausted completely during the differentiation of closed vascular bundles in the tissue.
Primary Growth in the Root: Procambium is innermost primary meristem which produces cells which will become vascular tissue.
Primary Growth of Shoots: Apical Meristem gives rise to three primary mersitems: protoderm, ground meristem, and procambium just as root apical meristem
Secondary growth begins with the initiation of the vascular cambium, a cylinder of meristematic tissue that produces additional xylic and phloic tissues. The cells that eventually form the vascular cambium come from two sources, the procambium in the vascular bundles and the interfascicular parenchyma cells between vascular bundles. The diagram below shows the positions of these two populations of cells in a stem with only primary growth.