Cork

What bottle of wine would be complete without its cork? The same is true of most trees.

Everyone knows that trees and woody shrubs are made of wood, surrounded by bark. But there’s a lot more going on in those outer layers than meets the eye.

Cork oak (Sallyofmayflower) CC BY 3.0

The bark you see protecting the living wood of a tree is made up of dead plant cells. This layer is called the rhytidome. The reason these cells are dead is because the cork layer cuts them off from the tree’s resources.

Components of bark

Bark is made up of three basic layers. The inner layer, or phloem, is a living part of a tree’s vascular system. Manufactured sugars ‘flow’ down the phloem to feed the rest of the plant. The middle tissue, or cortex, is made up of porous tissue that stores and transports carbohydrates, tannins, resins, and latex. The outermost layer of bark is called its periderm.

Tree cross section diagram (Brer Lappin) Public Domain

Periderm

The periderm is also made up of three layers: the cork, cork cambium, and phelloderm. Cork (phellem) is produced by a specialized layer of cambium tissue, known as the cork cambium, or phellogen. This cork cambium layer is only one cell thick and the cells divide in parallel (or periclinally) toward the outside of the tree. In some trees, the cork cambium layer also produces cells towards the inside of the tree. These inner cells are the phelloderm layer.

Function of cork

Cork keeps wine safe from the elements because it is impermeable to gases and water. Because of the cork, your wine stays where it is and (as long as the cork remains intact) will only grow better with time. The cork of a tree also blocks air and water. Cork is able to keep trees and wine safe from the elements, along with insects, bacteria, and fungal disease because it contains suberin. Surberin is a waxy material that creates a protective barrier. This barrier also blocks water and gas exchanges between the outermost layers of the tree killing the epidermis, cortex, and secondary phloem. This is the bark you see.

Trees and shrubs also use cork to cut off an unwanted body part (leaf, diseased twigs, mature fruit) from the rest of the plant. This is called abscission.