Secondary Growth

by Jason Amores Sumpter
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primary growth is all about extending the roots and the shoots. But secondary growth, actually, Onley occurs in places that no longer grow in length and secondary growth is all about expanding the circumference of the plant. Usually, we only see this in woody plants. There are some other types of plants that experience secondary growth. But we're just gonna focus on woody plants is our example now, uh, the merest M responsible for secondary growth is called a lateral mary stem, and we're actually gonna look at two types of lateral mary stems, both of which are derived from the pro Camby in which you might remember is one of those primary marry stems that gives rise to the vascular tissue system. Now, vascular Camby, um, is one of those lateral mayor stems and its job is to produce xylem and flow. Um, no, you might recall the pro Camby, um, also produces island flow. Um, we call those asylum and flown produced by the pro Camby, um, primaries. I'll, um, and primary flow him. What does thieve ask Euler Campion produces is known as secondary asylum and flow. Um, now, the growth pattern here is kind of interesting you see the vascular Camby in produces xylem and foam both to the outside and inside of where it's located. I'm highlighting the vascular Camby, Um, as this layer of cells here in blue, that's our vascular Camby, um, cells that it produces towards the outside of the plant. Here is the outside. Here's the inside cells that are produced towards the outside our secondary flow. Um, whereas these cells produce towards the inside our secondary xylem. Now, what this results in is an actually pretty interesting growth pattern. Uh, you've probably seen a tree cut in half, and it has those growth rings, right? Those rings that those concentric circles that continue all the way from the outside of the tree to the interior Well, what's actually happening is the vascular Camby, um, stays on, uh, the basically like the outer layer of the tree. The vascular can be, um, is going to stay towards the outside. It's going to leave behind lots of older asylum on the inside, right? So these are all from the previous year's growth, as you can see, whereas this layer right here is the current year's growth, and again, these air older years seen in here. Well, these Xilin that move, or rather due to the fact that the court court Camby, um continually produces thes cells but keeps moving towards the outside or rather keeps its position on the periphery of the plant. We get this build up of old xylem on the inter portions of the rings, we'll get back to those in a 2nd. 1st, I want to quickly mention the cork Camby, um, that other lateral mayor stem. And this one is actually located even mawr towards the outside than the vascular Camby. Um, we actually highlight the cork Camby. Um, here in red, it's this lateral mirror. Step there. That's our cork, Camby. Um and it's going to produce cells towards the outside. That air, called cork, sells these air non living cells that are highly impermeable to gas and water and actually will form the structure known as bark. Now I want to get back to those interiors Island. See those interiors? I'll, um, have a name that's much more common. That you've probably heard of it's called would write. Would is that poorest structural tissue that's derived from those secondary xylem. Uh, it's it should be noted that it's on Lee the outer layers of asylum that actually transport water. The inner layers accumulate various gums and resins to resist decay. And we actually have two names for these different regions of asylum. We have the heartwood, which I'm gonna jump out of the image. Here is this darker internal area. So this is our heartwood. Then we also have the SEC would which is this lighter colored external air? That's the sap wood. So three heartwood is the layer that's accumulated all these gums and resins in order to resist decay. And it's no longer actively transporting anything. You know, effectively. It is just structural support. At that point, the sap with these lighter outer regions, those will have actively that layer will have actively transporting xylem in it. Now, we also said that that bark is formed from those cork sells. It actually is also made in part from the secondary flow. Um, remember that the vascular Camby, um produces island to the inside and flow into the outside. Well, some of those flow um, the non active ones will become bark in addition with cork cells. So this layer this park because it has these cork cells in it. It's super impermeable to water and gas. But trees have evolved or say woody plants have evolved a way of dealing with that impermeable ity. They have these things called lent ical zin their bark. Basically, it's ah, poorest tissue that creates little openings to allow for gas exchange. And in this birch bark that you see right here we have these horizontal lines that are very characteristic of birch bark. You've ever seen it in the wild. Those are lent ical. So these horizontal lines are our Len tickles. All right, That's all I have for this video. I'll see you guys next time.