Hey guys, we are going to finish up plant structure with the last special organ, which is leaves. We will take a look at leaf structure as well as some different specialized types of leaves. Leaves are fundamentally going to be flattened, with a flat blade sort of shape to them. This is to provide a greater surface area for sunlight absorption as well as for gas exchange. The blade will contain veins, which are bundles of vascular tissue, xylem and phloem, running throughout the blade. Microphylls will have only one vein in them, whereas megaphylls will have multiple veins. Both of these leaves we are looking at here are megaphylls with multiple veins through them, but you can see they have different arrangements of those veins. So which leaf is a monocot and which leaf is a eudicot? See if you can remember. For eudicots there will be a specialized stalk connecting the leaf to the stem, which is termed the petiole. This is one of those places where you can find lots of collenchyma cells to provide that extra rigidity but still be flexible. The leaf can come in a few different forms. Many plants will have simple leaves, where the blade is one single structure for one single leaf. But a lot of plants may instead have compound leaves, where we're going to take the blade and divide it up into separate leaflets. Looking at a compound leaf can be confusing, because it looks like these are actually separate leaves, but they are in fact all one leaf, just divided up. The way you can tell is because they have a single petiole which connects to the stem at one place, and are derived from one node. They are not connected to hard woody kind of tissue. They're all connected to one flexible petiole. So that tells you that it's all one blade. The advantage potentially of dividing the blade up into compound leaflets is that you can have large surface area, while still allowing wind to pass through. You can have a really big leaf but it has gaps in it, so that it doesn't tear if there are strong winds. It also can help to isolate the spread of disease so that if one leaflet gets a disease of some kind, not the entire leaf necessarily has to catch it. There are also different ways in which leaves can be arranged on the stem. Most commonly in the majority of vascular plants you will see leaf nodes spiraling around the stem. So one leaf will come out at one angle, and then at the next node the leaf will come out at a different angle, and at the next node the leaf will come out at a different angle. Typically they spiral at a consistent angle, which is 137.5 degrees, the angle of the golden ratio or the Fibonacci spiral. So I'd like you to think about why it would be beneficial to have leaves spiralling around the stem, as opposed to all at the same angle on the stem. And why do you think it is so consistent, that it's always kind of at this angle of the Fibonacci spiral. Even if the node is spiraling, they still can have different leaf arrangements. Commonly leaves will have an alternate arrangement, where each node has one leaf on it. So one leaf, and then 137.5 degrees and then another one, and then 137.5 degrees and then another. You could also have an opposite arrangement, where each node has two leaves sticking off instead of one. Usually the nodes will still spiral, but you'll have two leaves from each one, and then an angle and two leaves from the next one. A third possibility is to have a whorled arrangement, where from every node you get multiple leaves coming off. Typically the nodes will still spiral, but it won't be very obvious because there are so many leaves coming from each node. Let's zoom in on the internal structure of a leaf. The leaves are going to have epidermis on either side. If the leaf grows horizontally, you will typically see most of those stomata in the lower epidermis, and most of the trichomes on the upper epidermis. The stomata are usually going to be on the lower surface, where they are a little bit shaded from the sun, whereas the trichomes are usually going to be on the upper surface where there might be things like insects walking around. In between the epidermis is a layer of ground tissue, which is called mesophyll. Mesophyll means middle leaf. It's literally everything in the middle of the leaf, in the sandwich of epidermal tissues. Running throughout the mesophyll will be veins, containing the vascular bundles of xylem and phloem. Surrounding the xylem and phloem will be special ground tissue cells called the bundle sheath. The bundle sheath cells are there to hold the xylem and phloem together into a coherent vein. Just like with stems and roots, we do see differences in the leaves of monocots and eudicots on the inside. This leaf we are looking at here is a eudicot. For a monocot leaf, there won't be as big of a difference between the upper side and the lower side of the leaf, so it might be hard to distinguish between the upper epidermis and the lower epidermis. But you can clearly see the epidermal cells, with guard cells leading to stomata. Underneath the epidermis will be mesophyll. In a monocot leaf, the mesophyll is pretty undifferentiated. It pretty much looks the same all throughout the different parts of the leaf. In the vascular bundles, you will find very noticeable, bubbly, large bundle sheath cells. So an easy way to recognize a monocot leaf is that the bundle sheath is very large and very noticeable. This is because the bundle sheath cells are involved in a special kind of photosynthesis done by monocots, which is called C4 photosynthesis. We'll learn about that next time. In a eudicot leaf, there will be a clear difference between the upper layer and the lower layer. The lower epidermis is going to have a greater number of stomata in comparison to the upper epidermis. The mesophyll will also be distinct. Towards the upper portion of the leaf, you will see palisade mesophyll, which is parenchyma cells that are long and narrow and packed closely together. So palisade parenchyma are long and narrow, towards the upper side of the leaf, and very closely packed together. Towards the underside of the leaf, you will see spongy mesophyll, where the parenchyma cells are a little bit rounder and more bubbly looking and there are a lot of air spaces in between them. So think about why this would be the case. Why do you want tightly packed palisade mesophyll towards the top, and loosely spaced spongy mesophyll towards the bottom in a eudicot leaf? Notice also that the bundle sheath cells are much smaller and less noticeable in the veins of the eudicot than they are in the veins of the monocot. Just like we did with roots and stems, let's take a look at some specialized leaves. First are floral leaves, or bracts. We already know that flowers are made of specialized leaves. Bracts are cases where the appearance of the flower is actually made of leaves rather than any of the floral parts. Common examples are poinsettias and bougainvillea. These colored leaves around the outside are in fact just leaves containing pigment. You can see they really do look like leaves. The actual flower containing all of the four floral organs are these little things in the center. So the flower itself is quite small, but the appearance of the flower is derived from the leaves. Another specialized leaf is spines. Spines are non- photosynthetic leaves that are highly reduced in size, and are used to deter herbivores from gaining access to the plant. Spines are especially useful in dry climate plants because they have trouble growing quickly, and so it is a good idea to protect their valuable water-filled tissues from herbivores if they're going to have trouble replacing them after damage. We've also talked about how leaves can be used for vegetative reproduction, asexual reproduction, such as adventitious plantlets which is what we are seeing here. A highly specialized form of leaves are window leaves. These are found in arid plants in very extremely dry habitats like the Sahara desert. This is an adaptation to prevent evaporation from the leaves. So all of the actual photosynthesis in these plants is going to occur underground. We're going to isolate it in the soil, so that less evaporation occurs of the photosynthetic tissues. The window leaves are going to serve as a skylight, because if you're going to do photosynthesis, you of course need sunlight. So we need some way to get sunlight to the photosynthetic tissues underground. That's what the window leaves do. So you can see that they are clear like a window, to transmit sunlight to the below ground tissues where photosynthesis is actually occurring. This is going to reduce evaporation, since most of the tissues of the plant are underground. Even within one single plant, you can still see different development patterns in the leaves. On a tall tree, for example, the leaves towards the bottom of the plant might look different from the leaves towards the top of the plant. These are shade leaves versus sun leaves. The leaves at the bottom of the plant are shaded most of the time. They're usually going to be broader and darker, and will be more efficient at picking up sunlight when they are shaded. The leaves towards the top of the plant will be smaller and brighter in color, and will max out at higher rates of photosynthesis. So even within one individual plant, you can have leaves that are developmentally different, to help maximize photosynthesis at all heights on the plant. The final kind of special leaf that I wanted to mention is insectivorous leaves. These are leaves that are specialized for trapping animal prey, including sundew plants, Venus fly traps and pitcher plants. This is a very highly specialized leaf which is going to have sticky structures on it or a trap of some kind to actually catch and retain animals, and then digest them. All of these plants that are carnivorous are in fact also photosynthetic. So they are not doing this as a food source. They make their own food in the form of sugars from photosynthesis. If they're not doing it for sugars, what are they doing it for? It's actually nitrogen. Most carnivorous plants live in soil that is very nutrient-poor, especially nitrogen-poor. Nitrogen is a major limiting resource for plants. It is a major component of chlorophyll, as well as a number of other molecules. If you can't get nitrogen from the soil, you have to find some other way to do it. Animal tissues contain quite a bit of nitrogen. So by trapping the animal and digesting their tissues, the plants can supplement the nitrogen available in the soil with this very nitrogen rich source. All right. So that's it for leaves and that finishes it up for plant structure.