Now, I want to go back to our experiments on gravitropism. What we had seen earlier was that the gravitropism is sensed in specific cells in the root cap, but we still didn't. Come to the conclusion what's sensing the gravity or how the gravity is being sensed. I want to look a little bit more into the structure of the cells in the root cap. And if you look at it with a microscope, what we see is that these cells contain specific organelles, specific subcellular structures, which are called amyloplasts. Amyloplasts are similar to chloroplasts, except that amyloplasts contain starch. They don't do photosynthesis, they accumulate starch and what we can see is that these amyloplasts are found on the bottom side of the cell. Such that if you take a root and turn it onto its side, the amyloplasts then fall to the bottom of the cell and are defining the downside of the cells. This is somewhat similar to what we described in the beginning of the lecture of the otoliths, of the crystals in our ear organs which respond to gravity. They're always found on the bottom of the vestibule. Now, these crystals in our ears and the amyloplast roots are not the same structure. They're completely different structurally, but it seems that their function may be similar to detect gravity. So once the scientists discovered, once they saw that these amyloplast are always falling to the bottom of the cells, this led to the hypothesis that maybe these amyloplasts are the gravity receptor themselves, but does this prove that they're the gravity receptors? What experiments could you do to test the hypothesis that the falling of the amyloplasts to the bottom side of the cell is what is sensing gravity? I want to describe two or three sets of experiments that have been done to test this hypothesis. The first one utilized the very strong magnetic field. If you put a plant in a very strong magnetic field, you can induce the amyloplasts to move to one side of the cell. As you can see here in this picture, when they put a seedling in between two huge magnets, you can get the amyloplasts to go to one side of the cell and what ended up happening is that the shoot would then bend according to where the amyloplasts are. If the imunoplasts were on the right side of the cell, the whole seeding bent to the right. If the magnet was reversed and the amyloplasts were on the left side of the cell, the seedling bent to the left. In other words, the direction of the bending correlated with the position of the amyloplasts. So, does this prove unequivocally that the amyloplasts are what's sensing the gravity? Not unequivocally, but it gives quite strong support, I think. Let's talk about a few other experiments that were done. If you wanted to test gravitropism and the effect on amylopast, maybe the best place to test such a hypothesis would be in conditions where there's no gravity. Now, where can we find conditions with no gravity to carry out an experiment? Well actually, we do have a place that we could do experiments like that and it's called the International Space Station. Actually, since the 1980s, plants have been flown into space, even with the space shuttle to test the effect of gravity on plant growth and I'm going to show you here one experiment that was done in 2009 that was used with rapeseed. And as you can see here, these are rapeseeds which were germinated in space for two days. It's actually not in the absence in gravity, we call it microgravity. Because even in space, there's still very, very small gravitational pull. We could see that their roots, don't know where to go. They're going to the side. They're going up. They're going down. There's no gravitational signal for them to know where down is. Now in the same experiment up on the space station, they put these rapeseed seeds on a centrifuge similar to what Knight did back in the 19th century. Of course, without a water wheel, this is on an electric centrifuge. When these seedlings are put on the centrifuge, the centrifugical force emulates a gravitational force and then the seedlings, the roots grow down. So this is clearly showing, I think that the seedlings need gravity to know where the root is down and where it's not down. So, whats happening here in space to their amyloplast? When you look at the amyloplasts in the absence of gravity, the amyloplasts are distributed evenly throughout the cytoplasm. They don't fall to the bottom. But when you put them in the centrifuge, the amyloplasts go to what would be the bottom side of the cell and that helps the plant know which way to grow. The last approach I want to talk about is genetics. We cited in a few of the earlier experiments in our earlier lectures that we can utilize mutants in arabidopsis to understand plant senses and it ends up just like there are plants that are blind to light, scientists have isolated mutants that are blind. That's actually not the right word. That are agravitropic, mutants that have lost the ability to respond to gravity. I just want to show you one mutant here, which I find fascinating. Because this mutant, when you put it on its side, it stems do know how to grow up. But if you look at its roots, its roots still know how to grow down. So, what we learn from this is that some of the mechanisms of gravitropism may be different in the stems than they are in the roots. Now, I'm not going to go into all of the biology. Soon, you'll find some links to the articles that are explaining what's going on, but the take home messages that we can find mutants that are agravitropic for both roots and for shoots. Mutants that are agravitropic for just shoots or mutants that are agravitropic for just roots.