The Marvel of Wood

I'm showing you here what's called stress plotted against strain. Stress is force divided by the surface area upon which it acts. Strain, we all know that. It's Friday afternoon when the boss tells you everything is due, strain is deformation. What we're looking at is stress plotted against strain for wood under increasing force. What you see here is a change in the slope. That's important because the higher the slope, the stiffer the material. What this diagram os saying is that after a certain amount of force is applied, the wood begins to deform and all of a sudden it gets stiffer. It does that by condensing due to crushing. If you look at wood, at the base of an extremely old tree, most of the wood has been compressed by the weight of the tree and has actually densified and become stiffer. This is unusual in most building materials. Building materials normally don't change their mechanical properties for the better as they get older. But organisms like trees have evolved tissues that can do this.

Now, I'm going to skip ahead because I'm talking too slowly. But, I want to show you a different way of building a tree. Here again is the height of trees and other plants plotted against stem diameter. Everything I talked about so far were for plants that did this. You notice the shape of that curve? It's convex. The reason it's convex is because as plants in this category grow, their stems telescope in length and then broaden in girth later on. That makes sense. Put a seed in a ground, a little seedling for a birch tree. It's first stem comes up very long and slender and then over the years, as it gets older, at the base it starts accumulating wood and it gets stiffer and broader. The data you collect from such a seedling would look like this.

But, look at these data, which have a concave relationship. Just the opposite to the first set of data. This pattern is saying that a plant when it's young is very broad and then later on as it's growing, it gets taller. There are plants that do this. In their juvenile condition they increase in girth and then only subsequently do they elongate and grow in height. They essentially establish a foundation before they grow in height at risk as they start climbing up into the air and the sky.

There are three plants that I'd like to show you that do this. Palm trees, which don't have wood by the way, but are extremely stiff and strong. They in their juvenile condition get extremely broad and then they start elongating.

Cycads, which I'm happy to say are planted all around here. You have a marvelous climate. I wish I could have my cycads out of doors. I do it for a few days every summer.Cycads also have very poorly developed wood but they manage to get really broad stems first and then increase in height.

Finally, tree sized fern like this fossil called Psaronius. We have evidence that this thing got very broad first before it elongated in height. Now what is happening here is essentially all three plants either lack wood or have poorly developed, or very weak wood. Wood remember, is the stiffest tissue for its density, the best thing to build trees. But an alternative to building a tree with wood, is to have a large safety factor, essentially, a very broad base first that's accumulated in the juvenile condition before you start growing upward in height. It turns out that after a while this design doesn't work. You get to a certain height where your slenderness ratio, your height divided by your diameter is so precarious that things fall over and die. But at that point, you've already reproduced and left your progeny. It doesn't really matter then if you die.