Bark is present only on woody plants - herbaceous plants and stems of young plants lack bark. From the outside to the inside of a mature woody stem, the layers include the following: • Bark • Periderm •
Cork (phellem or suber), includes the rhytidome •
Cork cambium (phellogen) • Phelloderm •
Cortex •
Phloem •
Vascular cambium •
Wood (
xylem) •
Sapwood (alburnum) •
Heartwood (duramen) •
Pith (medulla) In young stems, which lack what is commonly called bark, the tissues are, from the outside to the inside: •
Epidermis, which may be replaced by periderm • Cortex • Primary and secondary phloem • Vascular cambium • Secondary and primary xylem. Cork cell walls contain
suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, and prevents infections by
bacteria and fungal spores. The cambium tissues, i.e., the cork cambium and the
vascular cambium, are the only parts of a woody stem where
cell division occurs; undifferentiated cells in the vascular cambium divide rapidly to produce secondary
xylem to the inside and secondary
phloem to the outside. Phloem is a
nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with
parenchyma and fibers. The
cortex is the primary tissue of
stems and roots. In stems the cortex is between the
epidermis layer and the phloem, in roots the inner layer is not phloem but the
pericycle. In woody plants, the epidermis of newly grown stems is replaced by the periderm later in the year. As the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. A limited number of cell layers may form interior to the cork cambium, called the phelloderm. As the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis, cortex and older secondary phloem die. Within the periderm are
lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere. As the bark develops, new lenticels are formed within the cracks of the cork layers.
Rhytidome The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees. It is composed mostly of dead cells and is produced by the formation of multiple layers of
suberized periderm, cortical and phloem tissue. The rhytidome is especially well developed in older stems and roots of trees. In shrubs, older bark is quickly
exfoliated and thick rhytidome accumulates. It is generally thickest and most distinctive at the trunk or
bole (the area from the ground to where the main branching starts) of the tree. ==Chemical composition==