File:Woody Dicot Stem Four Year Liriodendron (36677091156).jpg

cross section: Liriodendron stem magnification: 100x

During the first year of growth the epidermis is stretched laterally by the expansion of secondary xylem, phloem and cambium. The epidermis is replaced by a protective secondary zone of cork rich periderm. Continual growth of the periderm keeps up with that of underling tissues allowing it to replace the functions of the degrading epidermis.

The outermost layer of periderm consists of layers of cork cells, the phellem, which produce the waterproofing substance suberin. Cork cells are dead at maturity. Deep to the phellem is a layer of living green stained cork cambium or phellogen and just beneath that layers of cork parenchyma or phelloderm.

In certain areas the cork cambium over produces cork cells, resulting in the formation of ridges and deep cracks in the periderm. These deep fissures, or lenticels, permit gas exchange with tissues under the periderm.

Deep to the periderm is an outer cortex of tightly packed lamellar collenchyma.

The vascular cylinder consists of a wide outer ring of primary and secondary phloem, a middle ring of vascular cambium and a deeper larger rings of primary and secondary xylem. Phloem bands of sieve tubes and companion cells are layered and interspaced with parenchyma cells masses and occasional small bundles of sclerenchyma cells. Small masses of calcium oxalate crystals are present. Xylem is separated from the pith by a starch sheath of dark staining parenchyma cells.