cross section: Tilia two years magnification: 100x

In older Tilia the underlying activities of the cork cambium have replaced the epidermal and outer cortical tissues with a protective layer of cork rich periderm. 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 cork cambium or phellogen and just beneath that layers of cork parenchyma or phelloderm. Many cells in the periderm contain dark staining tannins.

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 lenticles, permit gas exchange with tissues under the periderm.

Vascular cambium remains active, producing an annual growth of secondary phloem towards the outside and secondary xylem towards center of the stem. Wedges of secondary phloem are interrupted with bands of parenchymatous phloem rays that serve in the lateral conduction of water and nutrients. Rays taper to a few cells wide as they dip into the secondary xylem. Narrow single cell wide rays also radiate from pith to phloem.

Phloem consists of sieve tubes, companion cells interspaced with thick walled sclerenchyma cells. A narrow band of brick shaped cambium cells separated the phloem and xylem.

Greater annual production of xylem results in a vascular cylinder dominated by well-defined annual rings of secondary xylem interspaced with narrow medullary rays of parenchyma cells. Each annual ring consists of large thin walled spring vessels that taper into progressively smaller, thick walled vessels and tracheids characteristic of later season wood.