Dicot root at the onset of secondary growth

Vascular cylinder, vascular cambium initiation

Thin-walled endodermal cells are "passage cells"

A detail of the previous light micrograph of a buttercup root (Ranunculus sp.), showing a fully differentiated vascular cylinder. As is typical of roots, the primary xylem (stained red), as well as the primary phloem (stained green) followed a centripetal, exarch order of differentiation in which the oldest and smallest elements are farthest from the center of the vascular cylinder. The vascular cambium is initiated by periclinal division of cells located between the xylem and the phloem.

Also seen are the one-layered pericycle (situated inward and adjacent to the endodermis), and cortical cells with amyloplasts (stained violet due to the presence of starch).

Notice complete secondary cell walls (stained red due to lignification and suberization) in the endodermis cells lying opposite the phloem, and thin cellulosic endodermal walls--readily permeable for water and solutes--located opposite water-conducting xylem "arms." The thin-walled endodermal cells are often designated passage cells since they are more likely to transport materials symplastically than the thick-walled endodermal cells. They still possess the Casparian strip characteristic of all endodermal cells.