The cells of vascular cambium are of two types, elongated spindle-shaped fusiform initials and shorter isodiametric ray initials. Ray initials given rise to vascular rays. Fusiform initial divide to form secondary phloem on the outer side and secondary xylem n the inner side. With the formation of. secondary xylem (tracheary elements) on the inner side, the vascular cambium moves gradually to the outside by adding new cells. The phenomenon is called dilation.