Now that we've understood what a plant cell looks like, let's see what happens when we start putting these cells together to make tissues. Now, all the organs of the plant are made up of only three types of tissues. These are the dermal tissue which is on the outside of the plant, what we call the ground tissue, which is the main part inside the plant, and the vascular tissue which is, connects the various organs. While all organs have these same three tissues, it is the structure, it's the organization of these tissues which gives each organ its unique shape. So the ground tissue is the main mass of the plant. For example, when you're eating an apple, you're mainly eating ground tissue. When you're eating a potato, you're mainly eating ground tissue. This is the part that gives us most of the energy in food. The epidermis the dermal tissue which surrounds the plant is main function is in protecting the plant both from foreign objects coming in and from water evaporation going out. Because of this, the structure of the epidermis has cells which fit very very tightly together with no air spaces whatsoever to really isolate the ground tissue from the outside environment. We could see though that within the epidermis there are certain types of specialized structures. One of those would be the trichomes that we saw earlier in today's class. These are the hairlike structures, which extend from various types of leaves, and these trichomes have different functions. For example, in spice plants, for example, such as in basilicome. It is the trichomes which contain the chemicals which give it the taste and the smell. In wild plants themselves, this is for defensive purposes. Trichomes themselves can also have this type of umbrella structure, or an umbrella skeleton structure, which can keep insects off of the surface of the plant. And trichomes can also form a protective environment which makes a microenvironment underneath of them which would have higher humidity which can keep the plant from becoming dried out. So different types of plants have different trichomes, and each type of trichome can have a different type of structure depending on what its exact function is. In the epidermis of roots, we see another structure, which we call root hairs. The function of the root hairs is to increase the volume of the epidermis so that the roots can absorb more water. These are actually elongations coming out of individual cells, each hair coming up from one cell. These are not multicellular structures, which increase, as I said, the volume of the root hair, allowing it to absorb more water from the ground. When we're talking about vascular tissue, we're talking mainly about two types of cells. The first is called the xylem, which is responsible for transporting of water. The second one which are called the phloem. The phloem cells, which are responsible for transporting of sugars. The xylem cells within plants, form long tubes which transport water from the roots up to the shoot. Now there's something very interesting about these tubes, though, these xylem cells, because the xylem cells are actually empty tubes. They're cell walls, which are open on the top and on the bottom, which really fit together just like water pipes used in irrigation. So you can see that there's a contradiction here, I call these cells, but on the other hand these cells are empty. In other words they're dead. Xylem cells, in order to function, have to be hollow tubes in order to let the water to flow. So there's actually an interesting developmental process going on here which I wanna go into a little bit. How can xylem develop? How can a plant develop a dead tissue for its own use? And the way that a plant, actually the way any organism develops a dead tissue is through a function which is called apoptosis. Apoptisis is what we also call programmed cell death. In other words, the killing of the cell, the death of the cell is programmed in it's DNA. So when a xylem cell is born, or it is formed earlier in the developments, it's actually a living cell like any other cell, with a cell wall, with a cell membrane with the cytoplasm, with mitochondria, with a nucleus, and with a vacuole. However at some point during development, this cell gets a signal that it's supposed to become xylem. In order to become xylem, it knows it has to go through a couple processes. The first process is that it has to form a much stronger cell wall around its outside. So the first thing it does is deposits more cell wall on the inside, forming a very strong tube structure. Then what it starts doing, it starts degrading the cell walls on its top and its bottom. Then what happens through a very clear chronologically structured process, it then starts degrading the membrane, the tonoplast, around the vacuole. When this happens, then the vacuole then spills its contents into the cytoplasm. Which is then also a signal for the nucleus to start to degrade. The nucleus is degraded, all the DNA is degraded. At this point, we've lost the cell on the top and the bottom. The vacuole is no longer around. Everything is washed out and we're left with a tube that can then be used for transporting water. This is a genetically programmed process. We know that it's a genetically programmed process because in our modern laboratories we can study this through genetics. And we actually can have mutants, which are missing a gene, where the xylem does not go through apoptosis. So what do you think would happen to a mutant plant which can no longer go through apoptosis to form xylem?
