Do the experiments that show how plants respond to ethylene prove, though, that plants communicate with each other through volatile chemicals. To start answering this question, I want to describe some experiments that were carried out in the laboratory of Dr Consuela De Moraes at Penn State University. Among the many projects in her lab, she studies a plant that's called Dodder or Cuscuta. Dodder is a parasitic plant. That means it can't do it's own photosynthesis, and it can't live unless it attaches itself to a neighboring plant and sucks off the nutrients from that plant. One of the questions Dr Moraes is asking is how does the parasitic plant find its prey? So let's take a look at a time-lapse video of germinating Cuscuta. What you see here on the right are young Cuscuta plants that are germinating. And you see they're sort of dancing in a movement, going in circles. But what starts as what looks like random circles starts to become directional, as they somehow or another find their way to the tomato plant, and then encircle the stem of the tomato plant, attaching themselves. And then, they start sucking off the nutrients which allows them to continue to grow. So Dr Moraes constructed the following apparatus to test her hypothesis that the Cuscuta found the tomato by smelling it. In one box, she put a tomato plant. And in the other box, she put the Dodder. And these two boxes are only connected by a tube that allowed the air to transfer from one box to the other. And every time she did this, or almost every time she did this, the Cuscuta grew towards the air pipe coming from the tomato. If she did this where she put an empty pot in the other box, Cuscuta would not grow in that direction. So it wasn't that the Cuscuta was growing towards the smell of soil, or to the pot. It was going towards the tomato plant. So, in order to further test her hypothesis that it's the smell coming off the tomato, she made what we might call a tomato extract. Or, she took leaves and extracted chemicals from it, sort of making a tomato perfume. And she put this tomato perfume then on a stick, put this in the box, and once again, the Cuscuta grew towards the direction of the tomato perfume, towards the tomato. Again, now, the question could come, is this specific for tomato? Or would the Cuscuta then grow towards any plant? So, to test this hypothesis, she then repeated this experiment. But this time, did it with wheat, with wheat seedlings. And surprisingly, when you put the Cuscuta in a box that's connected to the wheat seedlings, the Cuscuta will actually grow in an opposite direction of the wheat. In other words, the Cuscuta thinks the weed is disgusting. It's repelled by it. And again, if you would give, then, the Cuscuta the choice between a wheat smell and a tomato smell, it will always go towards the tomato smell. Now that I've explained the basic idea of her experiments, I want to show you actually in a figure from the actual article where she published her results. What she did was put the Cuscuta in the middle of a paper disk, and then measured in which direction a number of different Cuscuta grew over a number of experiments. So we can see that here, on the first part, if we have moist soil towards the bottom, the Cuscuta sometimes grew towards the moist soil, and sometimes it grew away from the moist soil in a random motion. But here, in the middle, what we could see if she put the tomato towards the bottom, and nothing towards the top, in most cases, the Cuscuta actually did grow towards the tomato. It's not a 100%, but in most cases, very statistically significant, the Cuscuta grow towards the tomato. And the same thing, when it was growing just towards the tomato towards the tomato's smell, towards the tomato perfume. So we can see that this is not a 100%. It's not as simple as I made it out to. But it's still very convincing result. Which begs, though, the question now. What's the difference between tomato and wheat? And to do this, what the scientists did was analyze what is in the exact extract of the tomato leaf and of the wheat leaf. And what they found that for the tomato, and they analyzed seven different chemicals, three of them, each by themselves, was enough to cause the Cuscuta, the Dodder, to grow towards that chemical. One of them being a chemical called Beta-myrcene. When you look at the extract from wheat seedlings, from their leaves, they also found about seven chemicals, and one of them was also beta-myrcene, the same chemical that we find in tomato that attracts the Cuscuta. So, why didn't the wheat attract the Cuscuta, the Dodder? One of the reasons is, it only has been beta-myrcene and not the other ones also. So, we can sort of see here we're talking about the bouquet, the general smell. But more than that, wheat has a chemical that wasn't found in tomato, that's called (Z)-3-hexanyl acetate. Now, I should tell you that will not be on the exam, you do not have to learn that by heart. But what's important about this chemical is that this is the volatile chemical that the Cuscuta despises. This is the one that it finds just repulsive. Tomato doesn't have it. So, in the end, we can say it's the bouquet, it's the combination of volatile chemicals that causes the Cuscuta in order to grow the tomato. And just like we can describe smells as a bouquet of different chemicals, apparently, for at least the Cuscuta, it can also describe it this way.