procambium

Procambium is a primary meristem in plants that gives rise to the primary vascular tissues—xylem and phloem—during primary (apical) growth. It originates from initials produced by the apical meristem and lies between the protoderm (which forms the epidermis) and the ground meristem (which forms cortical and pith tissues). The procambial tissue differentiates into the vascular tissue of the developing organ, establishing the first vascular conduit system.

In shoots, procambium may occur as discrete strands or as a continuous cylinder that connects the protoderm and ground meristem. In stems, these procambial strands organize into vascular bundles, forming the primary xylem and primary phloem. In roots, the procambium generally forms a central vascular cylinder with xylem and phloem arranged in characteristic patterns depending on species. Across angiosperms, the arrangement differs: dicots typically produce a ring of vascular bundles in the stem, while monocots often have scattered bundles.

Procambium activity is largely limited to primary growth; after primary tissues are established, many plants transition to secondary growth driven by lateral meristems. The vascular cambium and, in woody plants, the cork cambium produce secondary xylem and secondary phloem, while procambial tissues may remain as remnants contributing to vascular organization before being incorporated into mature vascular tissue. In herbaceous species, procambial tissues may persist only briefly before differentiating.