Bark (botany)

In old stems the epidermal layer, cortex, and primary phloem become separated from the inner tissues by thicker formations of cork.

Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack.

[30][31] In woody plants, the epidermis of newly grown stems is replaced by the periderm later in the year.

It is composed mostly of dead cells and is produced by the formation of multiple layers of suberized periderm, cortical and phloem tissue.

[34] 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.

Bark tissues make up by weight between 10 and 20% of woody vascular plants and consists of various biopolymers, tannins, lignin, suberin and polysaccharides.

[35] Up to 40% of the bark tissue is made of lignin, which forms an important part of a plant, providing structural support by crosslinking between different polysaccharides, such as cellulose.

It has been proposed that, in the cork layer (the phellogen), suberin acts as a barrier to microbial degradation and so protects the internal structure of the plant.

[35] Guaiacyl units are less susceptible to degradation as, compared to syringyl, they contain fewer aryl-aryl bonds, can form a condensed lignin structure, and have a lower redox potential.

[37] This could mean that the concentration and type of lignin units could provide additional resistance to fungal decay for plants protected by bark.

In the past, this damage was called bark-galling and was treated by applying clay laid on the galled place and binding it up with hay.

[38] In modern usage, "galling" most typically refers to a type of abnormal growth on a plant caused by insects or pathogens.

The Sami people of far northern Europe use large sheets of Pinus sylvestris bark that are removed in the spring, prepared and stored for use as a staple food resource.

Bark is important to the horticultural industry since in shredded form it is used for plants that do not thrive in ordinary soil, such as epiphytes.

[50] Wood bark contains lignin which when pyrolyzed yields a liquid bio-oil product rich in natural phenol derivatives.

These are used as a replacement for fossil-based phenols in phenol-formaldehyde (PF) resins used in Oriented Strand Board (OSB) and plywood.