what is the utility of the bark of a tree ?

The bark is the external part of the trunk, which is made up of three essential parts. In the centre of the trunk is the wood, which is formed by successive layers every year. It is the hard wood searched by industry and trade. Its role is to support the crown.

The wood is enveloped by the cambium, slim, fragile and vital layer, itself divided into two layers, the sap-wood, towards the centre of the trunk, and the bark, outside.

The sap-wood includes channels (the xylem) which will drive the going up raw sap towards leaves, during some years, before lignifying (it becomes waterproof), by annual layers at the end of summer (it is the '"lignification"). The successive layers form the tree-rings wich allows to determine the age of the tree by counting them (this technology is called Dendrochronology). The meteorologists analyse the variations of thickness of rings to deduct from it climatic variations.

The bast (or phloem) piles, as the pages of a book (hence its latin name "liber"), spare layers (amino acids, carbohydrates to feed during winter), layers containing the channels which transport the downward sap, worked out by leaves, and other layers of hard fibres which constitute the bark. This one thickens (as the heart of the trunk), every year, and often cracks by thickening.

The bark serves for protecting the cambium, which is vital for growth: a tree can survive with a decomposing heart, since the cambium is undamaged. A torn off bark, it is a wound atrophicans.

With age, the branches of the bottom die, because, in the shade of upper branches, they receive not enough light. By falling, they leave a scar on their base which can gather rain water, drawing away seepage and rot. The scar is normally stopped by the bark (these are the knots of the wood). Momentarily, if it is on the side of the trunk, it will shelter birds, as the nuthatches which will make their nest there by shrinking the mouth to their size. Other birds appreciate the tree holes: creepers, peaks, starlings, tits coal. But the hole used by birds has little chance to stop, and risk to degrade inside. The rot of the trunk is not easily seen outside: nature undertakes to eliminate ill subjects (the first violent storm will get rid of ill trees, which is not without risk for the passers-by).

The aspect of barks is also various as foliage or blossoming:
smooth barks,
smooth slightly cracked ,
smooth, fluted
harsh shedding in strips,
shedding in flakes,
shedding in scales,
cracked (fissures are vertical),
deeply cracked and some other particular (cut up in squares, tormenteds).

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