 We'll hopefully give you a sense of the kinds of questions that I'm likely to ask during the final. And we'll also have a review session during the last week of lecture. And we'll have to decide whether to do that on Wednesday or Friday, the 1st of December or the 3rd of December. So give that some thought and we'll vote on that coming up at some point here. Okay, so is that better? Sure. There, maybe that's better. Yeah, so we'll have a review session the last week of class and we'll just figure out when we're going to do that a little bit later. And the questions I'll either put up on B-Space or they'll be posted on the Bio1B site. Haven't decided on that, but there'll be an email about that. Alright, so last time we started to get into looking at cell types and tissue systems, which is really a prelude to looking at getting a holistic sense of plant structure and later function and the way that plants grow, which is pretty elegant and we'll talk about that today. So today we're going to focus on the shoot system and then later the root system, looking at these two major systems in detail in terms of structure and growth and hope you'll find this really interesting. The last time we talked about the dermal system, basically the outer protective covering of the plant and we talked about some cell types in the vascular system and the major vascular tissue, the xylem and phloem and I didn't have a chance to mention that the xylem and phloem are discrete tissues within the vascular tissue but they're adjacent to one another regardless of where we find them. The leaf, the stem and the root, the xylem and the phloem are found adjacent to one another in the vascular tissue. So they're discrete but right next to one another and their exact arrangement differs between the different types of organs. It's different in leaves, stems and roots and that's one way in which you can distinguish leaves, stems and roots from one another if other characteristics are lacking. So the ground tissue is the third major tissue system and this basically takes in everything inside of the dermal system, the outer protective covering, the blue here in the different organs that isn't vascular tissue shown in purple. So this yellow tissue here is the dermal tissue and in stems of eudaicots as I mentioned earlier there's a ring of vascular bundles which you can see here forming this ring in a cross-section of the stem and we have ground tissue both internal to the vascular bundles which we call pith and ground tissue that's external to the vascular bundles which we call cortex. In the roots as we'll get to later there is no pith in eudaicots but there is cortex to the outside of the vascular tissue. So these terms cortex and pith are just to orient you with regard to the position of the ground tissue that's being discussed. They're not completely separate tissues. You can see they're connected by ground tissue between the vascular bundles which doesn't really fall into the cortex or pith category. So this is just a sort of topological orientation that these terms provide and there can be different cell types in these different regions depending on the stem. Okay so the ground tissue includes some of the most metabolically active cells in a plant and it includes those that are actively involved in photosynthesis and support as well as tissues that are involved in structure like calencoma that I mentioned earlier that cell type that's found just inside the epidermis typically provides structural support during elongating growth and sclarencoma which is tissue that's dead at maturity that provides support in the older parts of the plant body that have already finished primary growth. Okay so that's the ground tissue. And so now you've been introduced to the three major types of tissue and the major cell types so we're going to move on to talking about the major kind of holistic systems of the plant, the chute system and the root system and an archetypal angi... And these are systems that are actually already in place in the embryo in the seed so we have a chute apex and a root apex already in the seed that are going to give rise to these two systems so that's an early distinction that occurs during development and it persists through the life of the plant. So the chute system is, unlike the root, the root system is made up of one organ, the root which can include the tapped root and lateral roots or adventitious roots as well in monocots and in some dicots, eudicots. But the chute system is made up of two discreet organ types, stems and leaves. And the stem basically in an archetypal plant sort of a stereotypical plant represents, I mean is a main axis that the main stem beginning at about ground level and you can tell stems from leaves which may seem like an obvious distinction but it's sometimes not by the presence of nodes. So stems have nodes, roots don't have nodes, leaves don't have nodes and a node is basically a point along a stem where you have a leaf originating, one or more leaves. Okay, so that's something that we see in all kinds of stems, even highly modified stems that don't look at all like this. And there are also inner nodes on stems which are basically, inner nodes just means between nodes. It's a region between two successive leaves on a stem, a leafless region where there's never been a leaf that formed and dropped off for example. It's a region between the two most successive leaves along a stem. And leaves then are organs that are typically comprised of a blade and a petiole. The blade is the expanded part of the leaf where photosynthesis occurs most to the greatest extent. Often bifacial that is having two surfaces, an upper and lower surface. And an extensive venation system. And the petiole is the stock of the blade. It's part of the leaf, it's not a stem. You can tell it's part of the leaf if you look at its anatomy. We'll get into leaf anatomy in a moment. But it's the stock of the leaf and it attaches at the node on the stem. It has a direct vascular connection with the vasculature of the stem. And at the upper base of the leaf there's what's called an axillary bud. And that also is found at nodes as well. So nodes are places where leaves originate. And also where we find axillary buds at the upper base of leaves. And these axillary buds have the potential to become activated and to develop into shoots. So into branches. So plants often branch and these branches have their origin in these axillary buds. So branches form from the axles of leaves, from the upper bases of leaves, from these axillary buds which are actually attached to the stem. The axillary bud is attached directly to the stem at the base of the leaf. And it forms a new shoot. So plants have this what we call modular growth where you can have new shoots arising from old shoots. And here you can see additional axillary buds that can also produce a new branch. So you can have branches from branches from branches from branches. And depending on the pattern of activation of these axillary buds, it can really affect the architecture of the plant. So plants have different architectures, patterns of branching, depending on this bud activation in part. And I should mention also that some leaves are actually sessile and they lack a petiole. The blade is actually directly attached at the base to the stem. That's just another point. Not every leaf is petiole later, has a petiole. It's attached directly to the stem. Yeah, it's not attached to the leaf. It doesn't attach them through the leaf. But it's found in a node too and helps to define the node. They're found right at the base of the leaf. Sometimes the petiole will wrap around the axillary bud and it'll be hidden inside the petiole. But it's still part of the stem. It's basically a side shoot of the main shoot here. But it hasn't been activated at this stage. So the apical bud is really important to talk about because this is a bud that's actually been activated and is actively undergoing growth. And this is the site where we have primary growth of the plant concentrated, where it's happening. We'll talk about primary growth now, which is elongating growth. I want to make a distinction here, though, between what's happening with regard to the elongating growth in an apical bud and what's happening in a reproductive shoot, a flower. We talked about flowers already. Flowers, remember, have a growth pattern that terminates after we have the production of the carpal. So we have these four sets of appendages, and after the production of the carpals, this shoot terminates and doesn't have any further potential for growth. The shoot that terminates in this apical bud, the vegetative apical bud, as opposed to a reproductive one, is indeterminate. And this is a major distinction between plants and animals is determinate versus indeterminate growth. So like us, we have determinate growth. Animals have a determinate growth where we go through a juvenile stage where we have tissues that are still differentiating, and eventually we get to an adult stage and we don't continue to grow. We're completely fully formed individuals, whereas plants never reach that point. Plants continue to have the potential to grow. They retain embryonic cells in their plant body that allow them to keep growing. And these are what we call meristems. So these meristems are regions of growth, and in primary growth they're what we call apical meristems. So apical meristems are sites of active cell division which can continue growing indefinitely during the life of the plant. And here you can see the shoot apical meristem. This is terminating the tip of a shoot. So apex is just the tip, and this is the very tip of the whole shoot, the main axis of it. And here we have a root apical meristem terminating a root. We'll talk about those next time. Today we're going to focus in on the shoot and its growth, both primary and secondary. So these shoot and root tips terminate in these embryonic regions, these apical meristems, retain the capacity for elongating growth, adding to the length of the plant. So okay, now we're going to just zoom in on the shoot system and focus on that for the rest of the lecture today. And here we again, we have the shoot apical meristem. It's a dome-like region of cells. It's very tiny. This is highly magnified. This is a longitudinal section. So we're looking at this sideways. And so this region is the one, the small, small region of the plant where we have active cell division on an individual shoot. And at its margins, it's producing new leaves. So all leaves of a plant are produced initially at the edges of these apical meristems. And here you can see leaf primordia, two leaf primordia here. Here are some larger leaves here that have developed earlier. And so the leaf primordia will develop into mature leaves eventually. And at this stage, they actually help to protect, they serve in part to protect this delicate and extremely precious apical meristem from which the growth, primary growth of the plant is going to be occurring. So from this region, we have cell divisions occurring. We already have the distinction between the dermal tissue system and the other tissue systems. The divisions that are going on in this outermost layer are perpendicular to the surface of this meristem. And they're producing new cells off to the side that increases the surface area, the surface layer. But most of the divisions are happening parallel to the surface of this meristem and are producing cells in this plane. So it's like you're standing on top of a brick wall and you're adding bricks from the top and you're moving up, adding those bricks as you go. So you're building a brick wall from the top. You know, think about it that way or a pillar from the top. And you're leaving behind these cells that still enlarge and will differentiate into tissues as we get into different cell types and tissues as this meristem continues to grow, add cells to this region. But in a sense that's what's happening is that this meristem is moving away from the tissue that it produced earlier. So that's the way in which primary growth is occurring is that for the most part we're having cell division in this direction and that's causing this meristem to move away, move up and leave behind these cells that it formed earlier which can continue for a while to enlarge and of course differentiate. But there's not too, when this meristem moves away to a certain extent those cells are more or less locked into place in terms of their height on the stem. Does that make sense? Okay, so the growth is occurring basically from the top down. There are some cells that retain the potential for cell division and remain meristematic in the regions below the shoot apex, the apical meristem that were produced earlier and these are bud primordia and these will become those axillary buds that are in the upper bases of the leaves. Remember these are leaves here, this is the main stem. So these regions, these darkly staining regions are still meristematic and these will have the potential then to form those branches, the side shoots of the stem. And the bud primordia, I should have mentioned the axillary buds, I mentioned that they need to be activated for them to elongate or to become new branches and that's something that's under the control, that's under hormonal control and basically buds are inhibited from developing by hormones produced by the apical meristem. So the apical meristem has what we call apical dominance and that's what helps to control the architecture of the plant, otherwise you just have this crazy pattern of branching going on. Plant would just be one massive condensed system of branches and the inhibition of branching by the apical meristem can be broken in a number of ways. One way would be just for the meristem here, the apical meristem, to move away from these buds, grow away from them and with increasing distance we have decreasing inhibition. So we often see branching further away from the apex and also if this gets damaged or destroyed that can release these axillary buds or at least one of them will assume apical dominance, there'll be some release of inhibition and one of these will then ultimately become the new dominant apex. So that's how plants can overcome damage to this system. So now I'm going to show you a little video that basically shows you in a very brief sort of shot, it's not a very extended one, some growth of some stems of a eudicot and what I'd like you to pay attention to is watch the lower leaves after the lower leaves have formed. Those that are close to the apex, the tip, they'll continue to rise briefly because there's a region of cell enlargement just below the apex but at a certain point they'll lock in at a particular height along the stem and they won't continue to rise up. And also the leaves, notice that the leaves will enlarge to a certain point, they get larger in width and length and then they stop. So one thing I haven't mentioned is leaves have determinate growth. So unlike the stems which are indeterminate, the shoot system in general is indeterminate, individual leaves generally, except with very few exceptions, they have marginal meristems around their periphery that add cells and cause them to get wider and longer, but then eventually they become completely formed and don't continue to grow. So you'll see that the leaves will expand for a while and then stop. Okay, so here we go. You can see some of those leaves, they were rising for a moment and then they stopped rising as the rest of the shoot continued past them. Look at that one more time if you want. Okay, so that gives you just a little sense of how that process works or how it looks in fast motion. So there are questions about the way that the shoot enlarges in primary growth. Okay, so next I just want to mention that these tissue systems, those three systems, the dermal tissue, the ground tissue, the vascular tissue, those systems already are, they developed from distinct cell lineages at the shoot apex. I mentioned already that the dermal tissue, the epidermis, which is what the dermal tissue consists of during primary growth, that's already established right at the tip of the stem at the shoot apex where we have those cell divisions occurring perpendicular to the surface of the shoot apex. Ground meristem forms ground tissue, so that is a specific set of cell lineages that give rise to the ground tissue and here you can see some cells that are beginning to differentiate in the region that will become vascular tissue and this is the procambium which is early vascular tissue, so those cell lineages are established right away as well. So these early tissues are already dictated in the overall structure of the apical meristem. All right, so stems in cross-section we already looked at when I was showing you differences between monocots and eudaicots and we're going to look at this in a little more detail, especially with the eudaicots because eudaicots undergo secondary growth, which I want to talk about now or shortly, not immediately but shortly, whereas monocots don't undergo secondary growth and they have this really different architecture of their stems. This is a cross-section, cutting the stem and looking at the cut-in. So here you can see in monocots the vascular bundles, the xylem and phloem which are together in these bundles are scattered through the stem with the ground tissue all in around them, not really separated into a cortex or a pith. And in the eudaicot, in contrast, we have the vascular bundles in this ring that I showed earlier. Here you can see individual vascular bundles and the xylem is to the inside facing the center of the stem and the phloem is to the outside facing the outside of the stem. That's typical in eudaicots. Here's the pith, that area of ground tissue, and the cortex is just out here. There's not a lot of cortex in this stem in between the epidermis and these vascular bundles and there's a little layer of calencoma just inside the epidermis here, that flexible living cell type that provides some structural support. Okay, so, and again, the ground tissue that connects the pith and the cortex and that will be important here in a moment. So now we're going to move on from stems to leaves briefly and just give you a little brief introduction to leaf morphology and anatomy before we start talking about the secondary growth. So remember nodes are the points of attachment of leaves on a stem and help to define a stem or help us diagnose a stem. And the pattern of attachment of leaves to stems differs in different taxa. So it's something that's taxonomically conserved, helps to identify plants and it's interesting in terms of the ways in which plants have arranged their leaves on stems to in part minimize self-shading. So you don't want to produce leaves on a stem that are going to shade each other out and prevent photosynthesis or inhibit photosynthesis just by virtue of their arrangement. So plants have specific arrangements which we call phyllotaxis. Phyllot just means leaf. The phyll is a leaf. And the taxes refers to an arrangement. You think of taxonomy or classification or arrangement of taxa. So it's the arrangement of leaves on a stem. So alternate leaves are a common pattern where we have one leaf per node. So there's one leaf attached at each one of the nodes. And these are, if you look down the stem of one that's planted as alternate leaves, you can see they either have a spiral arrangement or helical arrangement so that leaves at adjacent or at successive nodes are slightly displaced from one another and don't shade each other. Or they'll be in two ranks typically where that is where they'll be, if you look down the stem from the tip, there'll be leaves coming out on one side and leaves coming out on the other side. They're not opposite. They're not attached at the same point, the two ranks, but they're alternating on either side of the stem. So opposite leaves are a situation where we have two leaves attaching at the same node. And so those two leaves are coming off away from each other. They don't shade each other. And at successive nodes they tend to alternate 90 degrees from one another if you look down the stem. And so they're not shading each other at successive nodes either. So those are just points of information in terms of... Alternate leaves and opposite leaves differ in terms of the leaves per node, but they both have ways of preventing self-shading of leaves by other leaves. And world arrangements are kind of rare, but we do see them in many plants. And this is where we have three or more leaves per node, as you can see here. And these are also slightly displaced from one another from successive nodes. So they don't self-shade too much, but this is a relatively rare pattern. Okay, so that's phyllotaxi. General leaf morphology. I'm not going to really have much to say about this, but we do have a lot of different leaf shapes that are often... that are diagnostic for different groups of plants. This is a eudaicot leaf, and you can see the blade and the petiol are differentiated. The blade, again, is this expanded part where photosynthesis is concentrated. And the petiol is the stalk of the leaf that attaches at its base to the stem. And again, leaves can be sessile. The blade might not have a petiol, but might attach right at the base to the stem. Okay, and another thing that's often seen in leaves is that their margins can be either... they can be entire like this without any notching or lobing at all, or they can be variously serrated, toothed, or they can be completely separated into separate leaflets like this, where the leaf is divided into separate blade segments. And this is what we call compound. So a simple leaf has a continuous blade where we have this expanded tissue all interconnected, whereas in a compound leaf, the blade is separated into separate leaflets. So each one of these units here is a leaflet. And this sort of thing happens, happened independently. This compound leaves have evolved multiple times in plants in response to different environmental pressures. For example, in windy environments, this might be beneficial in that it would keep the leaf from being torn... the blade from being torn apart allows for ready air movement. And air circulation around these segments could also help to keep the leaf from overheating in a dry, hot environment. You often see this kind of leaf in a dry environment, where the leaves are subjected potentially to heat stress. So that's a possible explanation as well. And also, if you have a heavy pathogen load in the environment, this kind of dissection of the leaf into leaflets can help to localize pathogens to particular blade segments so the pathogen doesn't move throughout the leaf readily. So there are a number of possible explanations for the evolution of compound leaves, which are derived evolutionarily from simple leaves. All right, and in grasses and in some other monocots that are grass-like, closely related, the leaf is a little bit different in its morphology. And it's worth mentioning, because they're such ubiquitous plants in our environment, grasses and grass-like monocots. We already talked about how monocots tend to have parallel as opposed to net-like venation patterns. But also instead of having a petiole or even having a blade that attaches directly to the stem at the point that it diverges from the stem, in grasses the base of the leaf actually sheaths the stem. So at the point at which the blade diverges here, below that point the leaf is actually surrounding the stem as a really tight sheath. So this is leaf tissue running down the stem, and at some point it'll actually connect into the stem itself. So in grasses the base of the leaf, which is a sheath tightly surrounding the stem, and is fused to the stem well below the base of the actual leaf blade. So the leaf is providing some additional structural support to the stem wrapping around it. And so typically when you look at a grass you're often not seeing much of any, if any, stem exposed. You're just seeing leaf tissue with the leaf tightly surrounding the stem. Alright, so leaf anatomy is different from stem anatomy, and as we'll get to later root anatomy. And this is a leaf cross-section looking at it from the side. And here you can see the upper epidermis of the leaf with this layer of cuticle, waxy layer on the outside. And here's the lower epidermis with the waxy layer here. So as we already mentioned, you know, leaves are the site of most active photosynthesis. And so this is the place where we have a real need to allow air circulation into the leaf or gas exchange. And so although there's a cuticle around most of the leaf, we already talked about stomata, which is plural for stoma. An individual stoma is a pore that's surrounded by two guard cells. These are cells that by adjustments in their turgor can increase or decrease the size of this pore or close that pore altogether. For example, in response to darkness or water stress. So the trade-off that plants are involved in here is that they have to get carbon dioxide into the leaf in order for photosynthesis to occur. Oxygen, which is a product of photosynthesis, can potentially leave then through the stoma. But the one really negative consequence here is that water can also leave through the stoma in vapor form. And that's a negative for the plant in general. So adjustment of the stomata, the pore of the stomata by the guard cells is crucial to strike a balance between sufficient CO2 entering the leaf and keeping water from leaving too rapidly. And so we typically find these stoma, an individual stoma or the stomata concentrated on the lower surface of the leaf that's not exposed directly to sunlight, typically. So it's going to be less likely to have evaporative loss of water from that surface that's not actually exposed directly to the sun. Okay, now that's the epidermis. The ground tissue or the internal part of the leaf inside the epidermis is like in the case of the stem and the root. We have ground tissue and we have vascular tissue. So the ground tissue is typically comprised of two major types of what we call mesofil, which just means inside the leaf. Meso-referringed inside or in between in the leaf and a fill as a leaf. So how a sage mesofil is, this is the ground tissue where we have the most active photosynthesis. These are really tightly packed elongated cells that are packed with chloroplasts and they form a dense layer here where photosynthesis is active. Spongy mesofil, there's also photosynthesis occurring in this ground tissue but not to the quite extent and these cells are more loosely arranged, which allows for gas movement throughout the leaf. So we often see this distinction between palisade mesofil and spongy mesofil in leaves of angiosperms, monocots and eudaicots. Now the vascular tissue, basically the external manifestation of vascular tissue when you just look at a leaf as the venation system, the veins represent the vascular tissue, the main vein and the side veins of the leaf in a eudaicot. And the vascular tissue, again xylem adjacent to phloem with xylem on top and phloem on the bottom if you're looking at it in this orientation. And they're surrounded by a dense layer of parenchyma, what we call the bundle sheath, the bundle sheath which provides some structural support to the bundle. And also to the leaf in general, the venation system is extensive in leaves and it helps to provide structure to the leaf, to that blade especially. So the vascular tissue is really important in providing that structure and the parenchyma here contributes to that, the bundle sheath. So there's little chloroplasts you can see right here, the little green units right there. There's a nucleus here, these are individual cells. Yeah, good to point out, these are the individual cells of the spongy mesofil, chloroplasts are inside each one of these cells. This is an individual cell of the palisade mesofil, here's the nucleus of each one of these cells here. Yeah, well those are parenchyma cells. So these are the cells of the bundle sheath that surrounds the xylem and phloem, provides some additional structural support to the vein system. And also there's some other functions of the bundle sheath that we may have time to get into later. All right, so stems and leaves can be highly modified for other functions and they look in this case often very different from sort of archetypal stems and leaves we talked about so far. So we'll start out with modified stems and there are a number of different modifications that we can talk about. One of them is a rhizome and we already talked about rhizomes as the main roots or the main stems of most ferns. And a rhizome is basically just a horizontal underground stem. So stems can occur underground and in that way in seed plants or in flowering plants in particular they can help to allow for storage. This is a drawing of a ginger rhizome. You've no doubt seen ginger rhizomes in the produce sections of supermarkets or sold rods oftentimes. And so this is an underground horizontal stem that's in this case also is serving for a storage purpose and that's part of the function of rhizomes. They often store food and water during the time of the year that a plant is dormant in say perennials that die back each year. Many perennials die back to rhizomes. They also can function in vegetative reproduction because these grow out along the ground and can produce new shoots at different points. And then the rhizome can eventually die back and we can have separate genetically identical plants. So we can have vegetative reproduction from rhizomes. And like all stems, rhizomes have nodes. So one way to tell a rhizome from a root even though it's growing underground is that it has nodes along which you can often see scale-like leaves or axillary buds. So you can look for these nodes and know that you're looking at a stem. And here you can see shoots arising from buds on rhizomes. And also adventitious roots form from rhizomes. So adventitious again just means forming from some place other than its normal origin. And that's what's going on in the case of root production here. Okay, bulbs are another type of modified stem. And a bulb is just a vertically oriented underground stem. A highly condensed shoot really, a highly condensed stem. Essentially no inter-nodal separation between the fleshy leaf bases here where storage occurs. So in a bulb the storage function, which is the major function of a bulb, is tied up in the basal parts of the leaves. So this is an onion bulb in longitudinal section and you can see the main shoot axis here with individual leaves coming off. These are the bases, fleshy bases of leaves that will extend beyond this point. But the bases of the leaves are persistent fleshy storage tissue. You might have heard of a quorum too. I'm not going to really talk about quorms, but quorms also form as vertical storage organs and plants. But those are stem tissue rather than leaf tissue that serves as the storage function. All right, so any questions about rhizomes or bulbs? Well, the next type of modified stem is a stolon or a runner. And this is basically another type of horizontal stem that runs along the ground, but above ground, not underground like a rhizome. And this is an example of a strawberry plant. You may have seen these often reddish stolons or runners from strawberry plants that vegetatively reproduce in this way. So they can produce new plantlets genetically identical by asexual reproduction in this way, moving out away from the parent plant. And finally, another modified stem is a tuber. And a great example of a tuber is a potato tuber. And a tuber is just the enlarged storage structure at the tip of a rhizome. So this is an underground enlarged tip of a rhizome. And if you've actually left a potato out in the light and forgotten about it, you probably noticed how the eyes of the potato are axillary buds that eventually will green up and sprout and form a chute. So it's pretty clear that a tuber is a stem. It does have nodes from which new chutes arise. So that's something you might be familiar with. Tubbers are formed in a number of different groups. Okay, there are also a lot of different types of modified leaves that serve different functions. One is a tendril. And here you can see a pea leaf that has tendrils. Basically part of the leaf is modified into these structures that are sensitive to touch. And when they come into contact with something, they'll grow around it. They'll coil around it. So there's a hormonally mediated growth response as they come into contact with other structures and they'll tend to coil around that. And that provides some structural support for climbing, for plants that have a climbing sort of habit. So tendrils can be modified leaf tips. They can be the entire leaf. The whole leaf can be modified into a tendril. There are also stems that can be modified into tendrils. So some plants have stems that serve this function. So tendril is more descriptive of the function of the structure than its actual origin, because stems can be leaves or stems in different taxa. Another type of modification of leaves are for protection. And this is something that we call spines. Spines are just hardened, sharp-tipped leaves or leaf bases oftentimes that provide protection to the plant like in a cactus spine. Not to be confused with a thorn. A thorn is a modified stem that serves the same function. So like tendrils, we can have sharp pointed objects that form from different types of plant organs. Thorns from stems and spines from leaves. And also we can have prickles like a rose prickle on the side of a rose stem. Those are actually just epidermal outgrows of the stem. So different organs can be modified or parts of organs can be modified in different ways to serve the same function. There's been a lot of convergence in evolution. And if you've looked at an ice plant or a jade plant, you've no doubt noticed that some plants produce succulent leaves that serve a water storage function. These are typically plants of desert or saline environments, salty or alkaline environments. And where the leaf is modified to have a water storage function. And stems can be modified in that way too. Think of a cactus, the stem is serving the water storage function. So again, convergent evolution towards similar functions in different organs. And also leaves can be modified into traps like in the Venus fly trap. And again, this has happened many times independently in plants. And especially in plants that grow in environments where nitrogen is limiting. So plants that have trap leaves have evolved different ways of trapping small animals, typically mostly insects, not to harvest their carbohydrates. These plants with trap leaves are green. They're photosynthetically active. They're producing their own food as far as carbohydrates go. But nitrogen is limiting in those environments. For example, a Venus fly trap grows in acidic bog, boggy environments in the coastal plains of the Carolinas. And other insectivorous plants grow in situations like tropical rainforests where the soil is heavily leached of nitrogen. And so what they're doing is they produce enzymes that break down the tissue of the animal. And then they directly absorb the nitrogen. And these traps can be formed in a number of ways, as you'll see and talk about in lab, too. They can be pitcher traps. So the leaf can be modified or part of the leaf can be modified into a pitcher with an enzymatic solution at the base of the pitcher with a slippery surface around its rim and nectaries around the rim that attract insects. So we have a slippery edge downward pointing hairs in the pitcher and the insect falls into the soup and gets digested. We also can have sticky traps, sticky leaves, like in a sundew that admire the insect and sticky goo that also attracts the insect by the presence of nectar, but it's deadly and the insect gets digested. And in the Venus fly trap, we actually have an active closing mechanism that involves... We have a situation where you have two stable states, the open state and the closed state. And it's like a clicker where it goes between those two states. There's no intermediate state that's stable in the way it's conformed. It has a convex and concave state that it alternates between depending on a change in turgor in the hinge cells, the hinge of the trap. And the change in turgor of those cells is dictated when there's actually a couple of trip hairs inside the trap that if they're hit in rapid succession by the insect, it'll cause an action potential to be generated. And that'll result in a change in turgor of those cells in the hinge and the thing will close. It's a very complicated mechanism. It's still being understood. It's a subject of really intensive research, but it does have only two stable states, which helps to explain the speed of the trap. Okay, so secondary growth, we'll just get to for a moment. Just to introduce you to what it is before we get into it in more detail next time. This is thickening growth rather than elongating growth. And this is growth that happens back from the actively lengthening tips. So the secondary growth happens by means of what are called lateral meristems, not apical meristems. These are meristems that run along the length of the parts of the plant that have already finished primary growth, that are already at their full length. And there are two major lateral meristems systems we'll talk about. One's that contribute to the vascular tissue. One that contributes to the vascular tissue, the vascular cambium, and then the cork cambium, which is a secondary, a lateral meristem that adds to the dermal tissue. And we'll just have to get into the details of that next time. And it's a pretty cool way in which plants can grow from seedlings into big trees in this way.