derived from the vascular cambium in plants exhibiting secondary growth. As in the *secondary xylem
, the secondary phloem consists of two systems: the axial (vertical) system, derived from the fusiform initials of the vascular cambium and consisting mainly of *sieve elements
, their associated *companion cells
, and some phloem fibres; and the ray (horizontal) system, derived from the ray initials of the vascular cambium and consisting mostly of ray parenchyma.