The interfascicular cambium is therefore a secondary meristem. The cells of the cambium are often termed initials , since they initiate the formation of specialized progenitor cells after dividing. They are the mother cells of all xylem and phloem elements, as well as of all other cells that are orientated parallel to the organ's axis. They are flat, elongated with pointed ends and highly vacuolate. It is their spindle-like shape that caused their name.
Ray initials are nearly isodiametric, small cells that occur often in groups.
They develop from fusiform initials or their progenitors. The developing xylem elements grow towards the inner and those of the phloem towards the outer of the cylinder. The progenitors of the initials are organized in radial rows that make it easy to trace back their descent. The increase in girth is compensated by the anticlinal divisions at regular intervals. The activity of the cambium is temperature-dependent and is thus the cause for the existence of annual rings.