Can’t see the wood for the trees

At the moment, the woodland floor is dotted with seedlings, tiny trees in the first stage of growth, newly emerged from last years seed. Their stems are so thin, so tender, that I could break them at one pinch; there is nothing wooden about these plants. So how is it that these little sprouts become mighty oaks, gnarled yews, great beeches? How do the trunks expand and where does the wood come from? The answer is a little bit genius.

There are cells at the tip of our tiny tree shoots that can give rise to all the different kind of cells it needs throughout its life – leaves, buds, twigs; everything. This group of cells (the meristem) pushes itself ahead of the growing shoot – sort of like climbing a ladder by starting with one rung, then adding new ones, one-at-time, under your feet. Some of the cells that are laid down are elongated – closed-end barrels stacked on top of each other to make modular cylinders. The cells making up the cylinders have walls that are thick and strong, made of cellulose, and in our little tree sprouts, groups of these cellulose tubes are bundled together, destined to become the vascular system of the plant – the plant’s veins and arteries as it were. But when first created, these tubes are just a stack of cells – for a vascular system you need open pipes. And that is what is slowly produced as the cells mature. Gradually, in some of the cell stacks, the walls separating the cells in the tube disappear. The cells’ side walls are further strengthened with lignin and all the bits and pieces that the cells contained are lost. What remains are long straws of dead material which together span the distance from the root to the top of the plant – these are the pipes (the xylem) that move water around the tree. In other stacks of cells, instead of the joining walls being lost completely, the cells become linked through the development of many little perforations in the joining walls which consequently end up resembling colanders. This type of tube loses much of the original cells’ material, but keeps the parts that pass sugars through the leaky connections, again effectively creating a type of pipe (the phloem) from a cell stack. And this is how our little sprout can grow so tall – sugar from the leaves at the top of the tree can be moved to the places it is needed using the phloem, while water can be moved from the roots to where it is needed through the xylem.

However, this doesn’t explain how wood is produced, or how the tree trunk grows in girth – if it were only the meristem at the tip of the shoot laying down the tubes behind it, the tree would be very thin and unable to grow very tall. Fortunately for the tree, evolution solved this problem. As the vascular bundles are created, they are organised with the phloem on the outer side of the bundle, the xylem on the inner side of the bundle, and a layer of cells capable of creating new xylem and phloem splitting each bundle in half. So now we have a stem which, if cut across, would be a circular surface containing within it a ring of vascular-bundle-dots, like the dots marking the hours on a fancy clock that has no numbers. These dots would be split in half with the phloem tubes towards the outside of the stem and the xylem on the inside, divided by a layer of cells. This layer of cells spreads to link with the layer in the bundles either side, so marking out a ring within our cross section of the stem, on which all the dots of vascular tissue would look as though they were threaded like beads. It is because of this ring of cells – the cambium – that the tree grows in girth. On the inside the cambium ring produces more xylem cells and on the outside, more phloem, meaning the stem becomes gradually wider.

This is the part I really like, the part when the biology explains something I see every day. Each spring, the cambium is activated, producing many large xylem cells. With the increasing volume of xylem being laid down inside the cambium ring, the cambium cells are shuffled outwards towards the perimeter, pushing everything beyond it outwards, expanding the trunk. Then as the summer progresses and the water becomes scarce, the new xylem cells become fewer, smaller, more densely packed. The xylem is made of those pipes supported by lignin, the accumulation of lignin fibres is what makes up wood, and this spring activation of the cambium is what I am seeing when I slice open a trunk and see the tree rings; I am seeing how the tree annually laid down the water pipes that kept it alive, large cells in the spring grading down to the smaller ones of late summer. Over time, the older xylem vessels become silted up so creating the heartwood of the tree; the younger vessels that are still open to move water make up the lighter ring of wood round the heartwood – the sapwood.

The phloem is also being added to by the ring of cambium, always being pushed out away from the centre. The older phloem also clog up over time, and, being pushed every further outwards, form the under layer of the bark. The younger phloem beneath the bark transports the sugars. And this explains why a mighty oak tree can rot on the inside, it can have a fire set within it, it can be storm damaged and still survive, but the simple removal of a strip of bark and soft tissue from the circumference of the tree trunk will kill the tree despite all its size and strength. The living part of the tree’s trunk is a thin cylinder of material around its perimeter while the rest, hidden within, is the solid, beautiful wood – the tree’s wooden heart.

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