Wood anatomy (5) Juvenile wood and heartwood

[0:09]Are we recording? Okay. Um, so I'm going to say something about uh different types of wood that you get in the tree. I'm going to talk about softwoods in this particular part of the lecture, and I'm going to draw somewhat idealistic tree. We don't really look like that of course, except on Christmas cards, but if we take a section through the trunk of a tree, there's different types of wood that form in the tree. And one of them is called juvenile wood. And this wood is more flexible than what's called mature wood. So that is the juvenile wood. And it has a number of features associated with it. It absorbs moisture in a different way, and it has different mechanical properties. But the main thing about it is it's much more flexible than mature wood. And the reason for that is, of course, the top of a tree gets blown around in the wind, when the tree's young, when it's a sapling. It also gets blown around a lot, and it's important that the tree doesn't break. But as it grows, as it matures, it lays down this much stronger, stiffer wood, which isn't so flexible, and that's referred to as mature wood. So, I can label that in green, mature wood. And also we have, uh as the tree grows, it begins to create something called heartwood. And heartwood does not conduct water. The function of the sapwood in the tree is to supply water from the roots up to the crown of the tree, where the water evaporates. So, we have a conduction pathway through the sapwood, but as the tree grows, uh the crown as a proportion of the trunk is smaller. So, the water demand for the whole diameter of the trunk is not sufficient to keep the sapwood alive. And what happens is that as the sapwood dies off, heartwood is formed. And I should just emphasize, when I say the sapwood is alive, we're not talking about the tracheids, we're not talking about the vertical cells. We're talking about these ray parenchyma cells. They are the only cells in the wood that are still living. The rest of the cells just have a supporting role or a conduction role. So that labeled in pink is the heartwood. So, that does not conduct water, whereas the sapwood does conduct water. Within the sapwood, these ray cells are still living. In the heartwood, the ray cells are dead. The mature wood has got different properties compared with the juvenile wood. So, depending on where you take wood from the tree, you can have a mixture of heartwood and sapwood, juvenile wood, mature wood. If you cut it in a certain place, you'll get all different types of wood. And there's one more complication. In softwoods, if the tree starts growing at an angle like that, there will be a correction mechanism that comes into place to make the tree grow straight. And the way that that happens in softwoods is by laying down what's called a reaction wood. And in softwoods, that wood pushes to put, to make the tree grow upright again. So, it's referred to as compression wood. And that's quite easily identified in softwoods because it has a much higher lignin content. So that shows up quite obviously in in microscopic sections.