Garden Word of the Day
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Bark is far more than just a protective coating. Let’s learn some of the basics about bark.
First, we will take a cross-sectional look at a tree trunk, from the inside out:
How bark is born
Young stems of woody plants do not have bark. Instead, from the outside in, they have an epidermis (skin), cork (periderm), cortex, primary and secondary phloem, vascular cambium, primary and secondary xylem, early wood and late wood (each double ring represents one year of growth), combined with the primary and secondary xylem, and the pith. As the stem grows, the cork gets thicker, pushing the skin away from the wood. Isolated from water- and nutrient-carrying vascular tissues, these cells die and become what we recognize as bark. This tough, outer surface helps keep water in, and pests and diseases out. It also provides protection against temperature extremes and sunburn damage. [The skin of a potato, being a modified stem, is actually the cork.]
What is bark?
Bark is mostly lignin. Lignin is the material that makes trees stand up. Bark also contains tannins. Tannins are believed to inhibit decomposition. Bark is made up of two distinct parts, the living phloem and the dead periderm. Phloem is the vascular tissue responsible for helping sap flow downward throughout the plant. The periderm is made up of cork (phellem), cork cambium (phellogen), phelloderm, and the cortex. Within the periderm are large spaces that allow gases to move from the tree to the atmosphere and vice versa. These spaces are called lenticels. As a tree grows, and inner layers are pushed outward, the lenticels create unique markings that are used in tree identification. For example, silver birch trees (below) have distinct horizontal lines which are the lenticels.
When bark is severely damaged by mechanical injury, insect or bird feeding, or girdling, tree death can occur.
Types of bark
Bark comes in many shapes, colors and thicknesses. Bark is generally described by its texture. It can be smooth (American beech), scaly (black cherry), plated (black birch), warty, shaggy, papery (paper birch), furrowed, or fibrous. They ridges can also be useful when identifying a tree. Bark may form vertical strips (red maple), ridges (white ash), ridges that are broken horizontally (white oak), or it may be uninterrupted ridges (red oak).
Did you know that cinnamon is actually the bark of trees?
And those little plugs that protect your wine are bark from the Quercus suber (cork oak) tree.
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