Here is a piece of bark, please talk about it (Biological Sciences, Oxford)

Every kind of tree has its own unique bark pattern that an expert can identify as easily as leaves. The papery white of silver birch is unmistakable. But from the grey colour and rough texture, with those deep ski-run fissures and oblong scales, I’d guess this bark is some species of oak.

Bark is a tree’s guard against the world – its shield against the weather and its defence from natural predators that would damage the vulnerable tissues inside. Life is a wonderful, precious thing – a tiny realm of order in the vast chaos of the unliving universe around – and all living things need protection. A cell has its cytoplasm. Fish have their scales. Humans have skin. Bark is a tree’s.

Without bark, trees could never grow so tall nor endure so long. Many trees can live for centuries, surviving winter chill or summer heat, drought, flood and fire, as well as the unwanted attention of insects, fungi and many other pests. While herbaceous plants – green plants without bark – must forever be growing anew and spreading, trees can send up a single trunk that lives on year after year, protected by bark.

Thick bark like oak bark is particularly effective protection. Its toughness protects the softer, living part of the tree from animals like deer that might gnaw on it, while tannins and other chemicals in the bark deter destructive insects. The bark’s air-filled cells and moisture content insulate the inner wood against warm and cold, while its ridges and scales not only trap insulating air, but also become radiator fins, and so reduce temperature variations. In dry, fire-prone regions, thick bark shields a tree against the heat of all but the most intense blazes. Smooth bark, like beech, protects in a different way. It offers far less insulation and armour, but its smoothness makes it much harder for insects and plants such as epiphytes to get a grip, which is why smooth bark is the norm in tropical regions.

Yet bark is much more than just a tree’s armour. Beneath the dry, dark and dead corky material of the outer bark, there is a multi-layered sandwich of paler, living tissues, known collectively as the periderm. Innermost there’s the softish ‘active phloem’ through which are piped the fluids that carry the sugars and nutrients the tree needs to grow. Outside the phloem there’s another couple of layers: the cork skin and the cork cambium.

The cork cambium is where the cork cells grow that in time die and form the outer cork shell of the bark. The cork skin is the greenness you see when bark is just stripped and still living – green because it contains the pigment chlorophyll. It’s not just a tree’s leaves that catch the sun’s energy in photosynthesis; these green bark cells do a little photosynthesising too – the thinner the bark, the more they can do. When leaves fall in winter, bark cells add energy to help the tree through.

Each year a tree grows by adding phloem cells just under the bark. That’s what creates the yearly growth rings. By the end of the year, some phloem cells have been squeezed out to form the cork cambium, then die and become the outside of the bark. As the tree’s girth swells, thick bark like oak cracks into wonderfully gnarled ridges and furrows to accommodate the growth. Smooth bark like beech grows slower and expands without cracking, so beech trees may sometimes still bear witness centuries later to a name carved into the bark on romantic impulse.

Humans aren’t the only ones to make their mark on bark. For beavers, of course, bark is breakfast. Quite a few voles dine on it too, while woodpeckers cling on and hammer into it to get at bark beetles, termites, spiders and ants, and treecreepers shimmy up and down searching for insects in its nooks and crannies. In fact, bark is a little wildlife habitat in its own right, mottled green with variegated mosses and lichens; crawling with and burrowed by myriad insects and other tiny creatures. Even when dead and lying on the forest floor, bark is a rich habitat for fungi and insects and other bugs.

It has a range of uses for us too. Long ago, native North Americans made canoes out of paper birch bark, native Australians made shelters and natives in South America made clothes. Today, of course, cork bark gives us cork, and the latex fluid in rubber tree bark gives us rubber. And bark can be medicinal, too. Aspirin, for instance, came originally from the bark of the willow tree, and the phenolics in Scots pine may help treat arthritis.

Unlike the showy green of leaves, bark is mostly browns and greys, and blends into the soft, intense dun-ness of woods and forests with an unobtrusiveness that only emphasises the deep and subtle richness of its hues and textures, and provides a ground for all the other colours of the wood to glow. It’s easy to forget the bark for the trees, but when you look at bark closely, it reveals itself as a substance of breathtaking beauty – not just in its appearance and feel, but in the perfection with which it performs its allotted natural function of protecting the tree throughout its life.