What the trees have taught us about growth

One of the most interesting wartime stories I ever turfed up during my time on the TV show QI was how trees helped British people during the Blitz. 

As you'll know, many of Britain's cities and larger towns became the focus of bombing raids during WW2. Therefore, whenever the warning came that the Luftwaffe had been sighted, these places went into total blackout. Any light of any kind would help the German bomb crews to find their target. But what if you need to drive a delivery vehicle around? Or an ambulance? How do you negotiate a city in almost total darkness? 

That 'almost' is the answer. 

Human eyes are pretty good in the dark - not as good as many nocturnal animals of course, but good enough if there's a glimmer of light, such as moonlight. And so the government ordered that certain obstacles should be painted white; things like kerbstones and trees and the corners of buildings on sharp corners. The white could be seen from ground level but not from the air. And the system worked. Women were mostly employed to do the painting work and very few accidents occurred as the result (plus vehicles drove more slowly as they had no lights on).

But that isn't the interesting part of the story. 

Amazingly, some of the painted lines on the trees are still faintly visible, such as on this tree in  London. And they have taught us something we didn't know ...

Look at the height of the paint lines. 

A scientist working at nearby Kew Gardens noticed them and wondered why they weren't higher up the tree. After all, decades had passed since the end of the war and the tree had undoubtedly grown much taller. So how was it that the white lines were still visible to people at ground level? 

This question led to the discovery that, until this point, (here's the good bit of the story) we hadn't properly understood how trees grow. 

Up until relatively recently (the 1990s), it had been assumed that trees grow proportionally like animals do. As humans or dogs or birds grow, every part of them - bones, organs, muscles etc. all gradually increase in size. But it turns out that trees don't do this. Instead of expanding in all directions, they actually only produce new cells in a very limited number of places called meristems. Most of these (called apical meristems) are concentrated in the tips of branches and roots. The trunk of a tree doesn't stretch upwards - new height is generated at the top. If an animal grew the same way it would start as a pair of feet and gradually grow the rest of its body upwards from the ankles.

Meanwhile, the diameter of the truck increases due to a different type of meristem called the vascular cambium. This produces new xylem and phloem each year and, as a result the trunk gets thicker and will engulf anything in its way - like this wire fence buried inside a tree near where I live. When the tree was a young sapling it grew up through one of the holes but then it gradually got wider and wider and the wire became embedded.

So there you go. A tree doesn't grow from the base - it gets wider by growing 'fatter' while, at the top, the branches get higher and longer and fatter. 

All of which is why the white lines painted in wartime are still at eye-level, and why the declaration of undying love for Sharon or Trevor that you carved into a tree in your teens is still at the same height now that you're both old and grey.

Every day is a school day, eh?