Now this bending either towards gravity or away from gravity is caused by changes on different sides of the plant. So here's an experiment where we could see what happens to negative gravitropism in the chute. We see that the plant grows up towards gravity. And if we look at the cells on the side of the bottom of the chute. On the side on the top of the chute, there's actually a difference in their size. The cells on the bottom are longer relative to the cells on the upper part of the chute. And this change in the size of the cells is what causes the bending. This of course, begs the question, how is the gravity sensed? Where is the gravisensor? It might not surprise you to learn that one of the first people to ask this question was Darwin in the same book, The Power of Movement in Plants, in 1880. But before we go into the experiments that Darwin carried out to identify the gravity sensor we need to go a little bit more into plant anatomy. What we see in this cartoon is a picture of the very tip of a root of an onion but it could be the tip of any root, basically, of most plants. And I'm going to dissect this root, both from the inside out, and from the bottom up. So from the inside out, we have a tissue which is called the epidermis. The purpose of the epidermis is to isolate the root from the soil of course, and it's the first tissue which is involved in the absorption of water and minerals from the soil. The absorption of water and minerals is facilitated through an increase in the area of the root, of the epidermis, by these projections that come out of some of the epidermis cells, which are called root hairs. Not every cell has a root hair, but some of them do. And these just increase the area of contact between the root and the soil allowing for the efficient absorption of water and minerals. You sometimes see these root hairs if you're pulling out weeds, you can see sort of like a hairy substance on the roots. Inside of the epidermis is a tissue which is called the cortex. The cortex is the main meat of the roots, such that for example when you're eating a potato, or a carrot, what you're eating is cortex. The cortex cells are a place where materials such as starch accumulate. And they separate between the epidermis and the vascular tissue which brings the water and minerals up to the green parts of the plant. The endodermis encircles the vascular tissue and is basically the gate keeper which determines which minerals go into the vascular tissue and which are kept out. There's actually an active process going on here such as the root regulate what's going up to the green parts of the plant. And what are remaining in the roots excluded from the green parts of the plant. In the middle of the root we have the vascular tissue. In different books this has different names, sometimes it's called the steal. If you cut a carrot vertically you can see the vascular tissue in the middle of the root. And this is where we have the xylem vessels, which bring water up from the root up to the green part. And what are called the phloem tubes which bring sugars from the green parts of the plant down to the root. Or actually sometimes take sugars from the starch from the root up to the green part of the plant. So this is from the outside in. Now we're going to take a look at the root from the bottom to the top. We can divide the root from this part into four different sections. At the very tip of the root, we have a section which is called the area of cell division. This is also called the meristem, and this is the only part of the root where the cells actively divide. At this part of the plant there's no difference between different type of cells. These are very similar to human stem cells, and these are the cells that are actively dividing. Above this we have an area which is called the area or the zone of elongation. This is where the cells are increasing in size up to 10 to 20 times difference in cell size, from this area of cell division, up to the area of elongation. So first the cells divide, then they elongate and once they've elongated, then they can mature. They can assume their final role, for example, the epidermal cells can then develop root hairs. This is the zone of maturation. So we have these three zones. I mentioned that the meristem cells are the embryonic cells. These are the only part of the root that are actively dividing to renew the roots. So as these are the embryonic cells, the plant has to protect them because if they're damaged then there's no way for the root to keep growing. The way that the plant protects it's embryonic cells, it's meristerm cells is what the structure called the root cap. The root cap is a group of cells that surround the meristem at the bottom and from it's side. They are stronger cells. They're more resistant to friction. Thus, protecting the meristem. Now as the root cap grows, as the root grows pushing the root cap through the ground, the root cap secretes a type of mucus, a lubricant, that's called mucilage. And you also see this sometimes that the bottoms of the roots are sort of sticky, it's this lubricant which allows the roots to actually go into the soil. So just to review this, from the bottom up we have the root cap, we have the meristem which are the dividing cells. We have an area of cells that elongate and then the area where the cells mature. Now let's go back to Darwin's experiment. Darwin was interested in understanding where is the gravity sensor of roots. To do this, he did the following experiment. He placed the root on its side and saw that the roots bend down with gravity. Then he started cutting off different parts of the root and so he saw, for example, that if he cut off the tip of the root before he placed it on its side, you could see that the root doesn't bend, it remains straight. On the other hand, if you place the root on its side for 90 minutes and then cut off the tip, the root still would continue bending. So what's the conclusion of this experiment, that it's the tip of the root that has the gravity sensor. Now you might say well that makes sense because that's where the cells are dividing. But actually it was only if he damaged the very tip of the root that he got this response. So not only did he chop off the tip, he then just damaged it by burning the tip. And this is actually a picture from his own book, what he has here is seeds that are place through up a needle on their side and you can see times where their root tip continues growing down and times where it continue straight. You could see that this root tip has not been cut for example here on the bottom, but what he did is he burned just the cells right at the end. And when he burned the cells just at the end, what we call the root cap, then the roots lost their ability to sense gravity. This research has been carried on now in more modern studies that have further shown that it's the root cap itself which is sensing gravity, and we're just going to talk about two such studies. In the first one, what they've done is, genetically, they've gotten rid, the scientists who did these experiments, caused the root cap to stop developing. What you can see here is a vertical section through two roots. This one here on top is a normal root and I think where you could hopefully see four levels of the root cap and behind it, small cells, which are the meristem, these are the cells that are dividing. Below it is our root that's been genetically modified, such that the root cap no longer develops. You could see that there's only about one level of root cap cells and beyond that, the meristem. In the control in the upper root, of course, there's normal gravitropism. Whereas in the bottom one, that's missing the root cap, these roots have lost the ability to respond to gravity. Now I want you to look once second now at this cartoon. In Arabidopsis, there are four levels of cells in the root cap. From this dark green, light green, light gray and dark gray. What was done in the following experiment, is different cells were killed using a very fine laser. This is called laser ablation. If the first level, the most bottom level of cells was killed with the laser, it had no effect on gravitropism. If the second level was killed, it also had very little effect on gravitropism. However, if only the cells in the third and the fourth level were killed by the laser then the root, while it could still grow, lost its ability to respond to gravity. What these experiments show, first starting with Darwin, and then through the genetic experiment and the laser ablation experiment is that the gravity is sensed in the root cap and primarily, at least in Arabidopsis, in specific cells in levels three and four.
