Weirdly the definition of bark seems to be variable depending on what book or article you read. As my main reference for this post I’m using Romero’s “Bark: Structure and Functional Ecology” accessible via a free account on JSTOR here.

According to Romero, bark is all the tissues on a tree outside the vascular cambium – that is everything from (and including) the phloem outwards. The inner bark is simply the phloem (both the conducting layer and the non-conducting layer). The outer bark collectively is known as the ‘ritidome’. This is where a diagram is needed! This is the best one I could find (from the University of Vigo website).

The ritidome contains another meristem within the tree – the cork cambium. The cork cambium (called the phellogen) works similarly to the vascular cambium – it has a layer of stem cells which create layers of differentiated cells. In the top diagram there is one phellogen, a pale beige line. On its left is the phelloderm – this layer is not always created depending on the species but if present it contains living cells. On its right is the phellem, or cork, this is the thickest layer and these cells become suberised/lignified (impregnated with suberin or lignin) so they become the corky bark texture we are familiar with. All of these ‘phello’ layers together are known as the periderm. A microscope image of these layers in an old Pinus sylvestris are shown in the image below:

What’s interesting is that multiple periderms can develop over the life of the tree. A new periderm will develop on the inside of the old one, pushing that periderm layer to the outside. These aren’t always continuous either, and are affected by the structure of rays and growth rings within the phloem, which is why old bark has more character. Periderms can be shed, or retained, depending on the species. The pattern of a tree’s bark is genetically determined by the structure of the phellem cells which are produced, and by the location of successive periderms. Smooth bark can come from a single periderm and continuous shedding, while rough bark is created when the periderm has structural fractures or constraints – for example due to the development of rays (radial lines of cells in the phloem).

See this image of old bark from ‘The Plant Stem – A Microscopic Aspect’ by Schweingruber & Börner. It shows how the bark splits apart as the xylem and sapwood layers expands from the inside of the tree.

Bark is made up of quite different materials from the wood or foliage of the tree, with considerably more mineral compounds (such as ash). Both the inner and outer bark contain so-called ‘extractives’ (organic substances which can be dissolved in solvents, such as polyphenols, alcohols, resin acids, vitamins, alkaloids, pigments including flavonoids, terpenes, steroids and essential oils) as well as suberin, lignin and cellulose. Bark chemistry in general is poisonous and indigestible, representing a good barrier to herbivores or insects. As the inner bark is living tissue, it can produce its own metabolites as a defence mechanism, whereas outer bark is dead tissue and relies on its physical structure and the substances impregnated into its cells to repel invaders.

Bark helps trees reduce water loss, prevents pathogens entering and provides a protective layer to protect the living tissue underneath from mechanical and heat/cold damage. It provides a flexible covering for the tree which can absorb the stresses of bending and twisting, and prevent cracking of the trunk.

As bonsai enthusiasts, bark is a key part of the look of our trees and we want to encourage interesting bark with good character. Since the cells of bark are renewing from the inside, the only way to modify the appearance and texture of bark in a natural way is to manipulate the periderm – as mentioned above, this causes fractures and divergence of the growth habit of the phellem. Harry Harrington has a video showing exactly this on a young black pine – he wires the tree so that the wire interrupts the shape of the periderm and forces the phellem to grow in a twisted habit.

I’m a bit nervous by the suggestion to leave the wire in as this seems like it would then cut through the phloem and ultimately the xylem. Whilst the twisted shape should leave continuous conducting cells for both, I’d be concerned at how much water and photosynthate conducting would be reduced. If possible to remove the wire I think I would.