Quercus suber L. has been investigated with special attention to the stem bark and its cork formation layer, but excluding the roots. Roots are the location of infection by pathogens such as Phytophthora cinnamomi responsible for the tree's sudden death. It is widely accepted that suberin establishes boundaries within tissues, serves as a barrier against free water and ion passage, and works as a shield against pathogen attacks. We followed the suberization of young secondary roots of cork oak. The first suberin deposition detectable by transmission electron microscopy (TEM) and neutral red (NR) was in the endoderm Casparian strips. Casparian strips are not detected by Sudan red 7B and Fluorol yellow (FY) that specifically stain lamellae suberin. Reaction to Sudan was verified in the endodermis and later on in phellem cells that resulted from the phellogen. Under TEM, the Sudan and FY-stained cells showed clear suberin lamellae while the newer formed phellem cells displayed a distinct NR signal compared to the outermost phellem cells. We concluded that suberin chemical components are arranged differently in the cell wall according to the physiological role or maturation stage of a given tissue.

Plants have suberized cells that act as protective interfaces with the environment or between different plant tissues. A lamellar structure of alternating dark and light bands has been found upon transmission electron microscopy (TEM) observation of cork cells and considered a typical feature of the suberized secondary wall. We observed cork cells from periderms of Quercus suber, Quercus cerris, Solanum tuberosum, and Calotropis procera by TEM after uranyl acetate and lead citrate staining. A lamellated structure was observed in S. tuberosum and C. procera but not in Q. suber and Q. cerris where the suberized cell wall showed a predominantly hyaline aspect with only a dark dotted staining. Removal of suberin from Q. suber cells left a thinner secondary wall that lost the translucent aspect. We hypothesize that the species' specific chemical composition of suberin will result in different three-dimensional macromolecular development and in a different spatial location of lignin and other aromatics. A lamellated ultrastructure is therefore not a general feature of suberized cells.

The ultrastructure of phellem cells of Quercus suber L. (cork oak) and Calotropis procera (Ait) R. Br. were analyzed using electron transmission microscopy to determine the presence or absence of plasmodesmata (PD). Different types of Q. suber cork samples were studied: one year shoots; virgin cork (first periderm), reproduction cork (traumatic periderm), and wet cork. The channel structures of PD were found in all the samples crossing adjacent cell walls through the suberin layer of the secondary wall. Calotropis phellem also showed PD crossing the cell walls of adjacent cells but in fewer numbers compared to Q. suber. In one year stems of cork oak, it was possible to follow the physiologically active PD with ribosomic accumulation next to the aperture of the channel seen in the phellogen cells to the completely obstructed channels in the dead cells that characterize the phellem tissue.