Organization of Vascular Cambium

Once the vascular cambial ring is complete, its cells resolve into two functionally distinct kinds of initials, distinguishable by their shape and orientation. Fusiform initials are long, vertically (axially) elongated cells; dividing repeatedly, they build up the entire longitudinal (axial) system of both secondary xylem - its tracheary elements, fibres and axial parenchyma - and secondary phloem - its sieve elements, fibres and axial parenchyma. Ray initials, by contrast, are shorter and horizontally elongated; they build the radial system of both tissues, giving rise to the ray cells that run sideways across the wood and bast, at right angles to the main direction of water/food flow. How the fusiform initials themselves are arranged, seen in a tangential longitudinal section of the cambium, further divides vascular cambium into two types: storied (stratified) cambium, in which comparatively short fusiform initials line up so that the ends of cells in one horizontal tier fall a …