 Okay, so last time we concluded talking about much of the angiosperm diversity, but I didn't get a chance to get through the last of the differences that easily distinguish the two major groups of angiosperms, the eudaicots and the monocots. And this is important because in talking about sea germination afterwards there are some major differences there. And also with regard to the structure and growth of angiosperms, the eudaicots and the monocots differ pretty dramatically in really interesting ways. So this isn't just a completely esoteric discussion about the distinction between these two major groups of flowering plants. And so today we're going to move from talking about diversity in general and start talking about the structure and function of plants and really the elegant anatomy and morphology of plants and the ways in which they grow which are pretty conservative across the groups of flowering plants as well as in other cases other vascular plants. And they follow some basic rules which are really elegant and fairly easy to understand although you have to keep sort of a three-dimensional perspective as we go through this. Okay, so last time I went through these few characters here that distinguish monocots on the left from eudaicots on the right. The single cotyledon, of course, the basis of the name here in monocots which is the derived feature, this single embryonic leaf. And I'll talk more about this in a minute. And then two cotyledons, there are two embryonic leaves in eudaicots which is ancestral in flowering plants, the monocot condition being derived. As far as leafination goes, the leaves have the veins oriented in this parallel pattern typically in monocots which is a derived feature versus in eudaicots. We have more of a net-like openly branching sort of phonation pattern which is something ancestral in angiosperms. And we're going to get into the stem anatomy a lot more but I'll just mention again the vascular tissue, the conducting tissue that transports water and sugars through the plant. In angiosperms and in general the xylem and phloem which we're going to talk about is in these little bundles that run through the stems but in monocots the bundles are scattered through the stem. This is a cross-section. You can see how the bundles run down the stem here. In eudaicots though the vascular bundles are arranged in a ring and this is really important when we get to talking about how wood and bark are formed in plants, in woody plants that undergo secondary growth but more on that in a minute. The last features here that we didn't have a chance to get through entirely monocots have a fibrous root system typically and that's because their tap root which is the root that forms from the embryonic root the sole root in the embryo it doesn't persist it grows for a while and then it's replaced by adventitious roots and these are just roots that are formed from the base of the stem anything that's adventitious in plants is basically a structure that's formed from some place other than its normal origin so roots don't normally form from the sides of stems but that's what happens in monocots and you end up with a pretty diffuse root system like you get in grasses often pretty shallow whereas in eudaicots like in other seed plants you get a tap root that is the embryonic root that persists through the life of the plant and it's the dominant root from which lateral roots are formed and these can have very deep root systems. pollen differs fundamentally between monocots and eudaicots in the monocots we have the ancestral condition of just one opening in the pollen grain that allows the pollen tube to emerge and that's what we also see in the gymnasperms but in the eudaicots we have three openings which is a derived condition they can either be slits or pores and sometimes you'll get pollen that has additional openings too in the eudaicots but at least three and that helps to recognize eudaicots in the fossil record because no other plant produces pollen like that except the eudaicots and then in terms of just the flower organization in monocots the flower parts are usually in multiples of three and that actually is what we also see in some of the magnolias and some of these early diverging lineages of flowering plants that are outside of both of these groups so this is the ancestral condition in flowering plants or close to it whereas in the eudaicots the flower parts like the petals and the sepals, stamens, carpals they're in multiples of four or five so you typically have four or five petals, four or five sepals that's something you don't see typically outside the eudaicots so there are a lot of ways you can tell these groups apart that are pretty reliable so the question of why angiosperm diversity is so high you're far outstripping that of all the other seed plants and really the land plants taken together is sort of an open question we've just reviewed all of these different features that differentiate angiosperms from other seed plants they're reduced gametophytes, the double fertilization the flower with its ovary, the fruit there's a bunch of features there to distinguish them but one intriguing possibility that might explain a lot of the diversity is the fact that they've undergone so much co-evolutionary change with other organisms in regards to pollination and dispersal that we talked about as well as with herbivores that has led to really rich secondary chemistry there's some evidence now from looking at groups there's so many groups of angiosperms that you can do interesting comparisons across traits and some of the floral specialization that I mentioned last time for example bilateral symmetry of flowers traits like that seem to be associated with bursts of diversification so it really does appear that to some extent the co-evolution of flowering plants with animals and other organisms has in part led to this increased specialization and increased diversification and in fact of course there's so many flowering plants that we don't really know how many there are they're being discovered and described faster at a slower pace unfortunately than they're going extinct they're still being discovered at a very fast pace but especially down the tropics the rate of deforestation and loss of flowering plant diversity is so high that there's a real race against time for botanists to try to describe and we'll discover and describe and try to protect some of this diversity and characterize it in terms of its ecological importance and its potential economic importance because most of our food a lot of our medicines are wood products and in some cultures their fuel is really crucially dependent on angiosperm diversity and so this is something that we really have to take seriously and try to invest more in and actually some of those same challenges actually exist in the temperate parts of the world too like here in California where the herbarium that I curate downstairs we're discovering and describing new plant species endemic to California all the time and some of these are found in proximity to highways things have never been collected before this is a biodiversity hotspot right here that's in Grave Jeopardy in terms of the rate at which it's being altered and with climate change and all we've got our work cut out for us even in the nearby vicinity here okay so seed morphology is something that I want to introduce now because we're going to get into the growth and development of plants and of course as far as the sporophyte goes this is where it starts and this segues right from what we're talking about with the development of the seed and some of the distinctions between eudaicots and monocots are seen in regards to seed germination so getting back to the angiosperm life cycle the end of it here with regard to the formation of the seed remember that the endosperm is the nutritive tissue formed from the fusion of one of the sperm with the polar nuclei of the central cell of the female gametophyte and the important thing as I mentioned is that the angiosperms wait to invest in that nutritive tissue for the embryo until after fertilization is assured rather than in the gymnasperms where they make the investment before fertilization which can potentially result in wastage of resources and actually the endosperm begins to develop really before we have the embryo start to develop from the zygote so the endosperm gets a head start developmentally even though the fertilization and the double fertilization events occur more or less simultaneously and we eventually get this embryo forming that has these embryonic leaves, the cotyledons as well as an embryonic chute tip and an embryonic root so the two major embryonic regions of the flowering plant the chute apex and the root apex are already present in the embryo at that stage and so the plant has really everything it needs for development as we'll see shortly as the seed reaches maturity it undergoes massive dehydration down to 5-15% water by weight and as that happens it enters what's called dormancy which is a situation where the metabolism of the seed almost ceases so it's almost indetectable the amount of metabolism going on here and of course this is a feature that allows seed plants to potentially persist in a seed bank for decades, maybe hundreds of years some of our fire followers here in California that only germinate after fires have gone well over 100 years between fires, between germination events so seeds have a great way of being able to persist keep an angiosperm basically alive during periods that are not very conducive to survival okay so here's a major distinction between the seeds of some eudicots at least and some monocots these are sort of extremes that demonstrate some of the variation we see in the seed and some eudicots, the example here being a bean the endosperm doesn't persist the embryo absorbs the endosperm and stores the nutrients of the endosperm inside the cotyledons those embryonic leaves so when you eat a bean or a pea the bulk of the tissue there that you're eating are the two cotyledons and if you carefully pull these apart in the right orientation you can see the two cotyledons here the two halves of the bean and they're attached to a little miniature plant axis here which is above the point of attachment of the cotyledons the cotyledons is the chute apex what we call the epicottle in the seed which is basically just means over the cotyledons the part above the point of attachment of the cotyledons where we have, here you can see the first two foliage leaves not the embryonic leaves but the two first foliage leaves which in a bean are already formed in the seed but in some eudicots they only form during germination and then below the point of attachment of the cotyledons we have what's called the hypocottle which is just means below the cotyledons the part of the stem the embryonic stem that's below that point and then the radical is just the embryonic root the embryonic tap root which attaches to the hypocottle and you can't see it very well there but we have a little miniature plant axis here with these gargantuan cotyledons attached to it okay so in the monocots like the grasses and this includes all of our grains like wheat and rye, rice but this example is corn this is a corn kernel here corn fruit the grasses retain most of the endosperm inside the seed until germination and then they absorb the endosperm through the cotyledon which is the shield-like structure right here so the cotyledon are just one of them in monocots, remember it basically is a shield shaped object that absorbs the endosperm making it available to the rest of the embryo which is on the other side of the cotyledon right here and so with this partitioning of the endosperm from the embryo it's easy to mill grain and remove the endosperm from the rest of the grass fruit and this isn't just the seed actually this is the entire fruit of a grass because the fruit wall is fused to the seed here so this yellow part, this thick part here is actually the fruit wall, the ovary wall in a grain and that's what we call commercially the bran you've heard of the bran, the roughage or fiber content of whole grain that's milled away from the endosperm and so is the embryo which is referred to as the germ the germ in commercial terms so refined grain is just endosperms something like white rice or white wheat flour is just endosperm the endosperm is also the white fluffy substance of popcorn and so the endosperm is relatively lacking in nutrition compared to the embryo where there's a lot more investment of resources and so that's in part why in addition to the roughage provided by the bran or the fruit wall that whole grain is more healthy than refined grain in any case that's the general layout there in terms of a eudaicot and a monocot seed so seed plants have a number of innovations that promote optimal timing for germination and this is really important because the establishment of a seedling at the seed stage when it's undergoing germination that's the period of highest mortality that's the most vulnerable stage in the life of a plant so if you look at a mortality curve for plants the mortality peak is right during the establishment of the seedling when it has to get a root system down in contact with moisture so it can draw water up into the plant which is exposed to most of the plant the shoot systems exposed to the atmosphere we also have to get the plant up into the light so the shoot up into the light so it can actually start producing food from light and carbon dioxide so it's that period of time that plants are in really major jeopardy to get established in this land environment and so the timing, the cues that stimulate germination vary widely across plants even really closely related wild plants can have really different cues for germination and they're often complex but they can involve things like light some small seeded plants might need light to germinate that makes sense because they need to be close to the surface in order to successfully germinate they might need a cold period that simulates winter so that they don't accidentally germinate too soon and are killed by frost or potentially by frost or in some parts of the world insufficient light to be successful and they might need chemical cues like are required from passing through the gut of an animal or from smoke or charcoal in the case of fire followers and there might be combinations of those cues that are necessary in addition to the presence of abundant moisture so moisture alone doesn't necessarily result in germination our crop plants have been selected to germinate readily and so they're not a very good gauge of how diverse plants are in terms of their adaptations to different kinds of cues for germination so in the process of germination what basically happens is that the seed starts after it's been stimulated to germinate which can be a hormonal response we have water starts to be absorbed by the seed it starts to resume metabolic activity and as it swells it ruptures the seed coat and then we get the emergence of the seedling and this happens in two very different ways in the eudaicots and the monocots that I just mentioned so in the eudaicots, like the beans what we see is first of all in both the eudaicots and the monocots we have the emergence of the root so the roots emerge first and they have positive geotropism which means that basically they grow with gravity and this is a hormonally induced response that results in the root growing down into the soil the taproot grows quickly in both the monocots and the eudaicots they start producing root hairs which increase the surface area of the root you'll see that in the video in a second but these root hairs are just epidermal outgrows that increase the surface area from which they can absorb water and minerals and in the case of the monocots as I mentioned the taproot doesn't survive very long but we start getting these adventitious roots forming from the base of the chute that create the fibrous root system so after the root system is actually pretty well established only then does the chute start to emerge from the seed or start to emerge from the ground I should say and the first thing that we see happening in the eudaicots that are similar to the bean here is that the hypocautal, that region below the attachment of the caudalidons, the lower part of the embryonic stem it starts to grow up with negative geotropism growing against gravity and as it emerges it forms this germination hook that you can see here and so by doing that, by having the hypocautal rather than the other parts of the chute respond to be the leading edge of the growth this pulls the caudalidons and the protected chute apex through the soil chute apex is shielded between these caudalidons and so we don't have the abrasion potentially damaging the caudalidons which are going to be important for initial photosynthesis food production and the chute apex, which is a critical thing to protect because that's where your embryonic tissue is that's going to result in the further growth of the stem so you pull the caudalidons and the chute apex up this way after full emergence the whole thing straightens out and we get the greening up of the caudalidons the expansion of the first foliage leaves and elongation of the stem that we'll talk about in a minute okay now contrast this with what goes on in the monocots where you have, we already talked about the root system getting established but here there's a sheath called the coleoptile that doesn't have a homolog in the eudaicots it's basically a hollow sheath that surrounds the chute apex the chute apex of the embryo and it grows straight up there's no germination hook here it grows straight up and as soon as it emerges from the soil the chute emerges through that sheath and so it's been protected from the abrasion of the soil from emergence here and then it undergoes expansion of the leaves and elongation of the chute so in this case the seed and actually the whole fruit remains underground whereas here you can see that the seed emerges from the ground although the seed coat is sloughed off as it's pulled through or in the course of the breaking open of the spreading of the cotyledons the seed coat is further ruptured whereas here the seed and the fruit remain underground as does the cotyledon it has a really different sort of development in the case of eudaicots and monocots so now we're going to look at this process in video and so you'll see these steps we're going to first look at a bean emerging so you can see here the roots starting to form and these are the root hairs on the tap root so we have rapid growth and now that happened quickly but the germination hook it formed, the thing straightened out here are the cotyledons, you can see the two cotyledons expanded and now the chute apex here the epicottle will start to grow over here you can see it better where we have the first foliage leaves now that have emerged and well on its way you can see here that the seed coat is left underground or in some cases it's left clinging to the plant whereas in the monocot, in the corn we also get the root system established quickly and here we have the coleoptile going straight up there's no germination hook here and as soon as it emerges the chute comes out of the coleoptile and we get the development of the seedling here you can see here the adventitious roots starting to form from the base of the stem and so the seeds I don't know if you noticed it but the corn fruits remained underground there they didn't emerge okay so that's the germination process and now we're going to start to look at are there questions about this? yeah it does have a taproot initially right yeah it does and it lives for a while it was living through that whole part of the video we saw there and you could start to see the development of those adventitious roots but the taproot wouldn't persist indefinitely yeah good point oh the cotyledons don't persist for very long as soon as the first foliage leaves get established they can hang on for quite a while but eventually they'll drop off yeah after the reserves are exhausted in them they also function in photosynthesis as you saw they green up as well but they don't persist too long okay so we're going to start now looking at the growth and development of the plant body and before we can really do that we need to talk about cell and tissue types and plants and there's some really distinct cell and tissue types that we see repeated over and over again in different plant organs so the first type of cell that we're going to talk about is parenchyma and parenchyma is a cell type that's good to start out with because these are the most metabolically active cells in the plant these are the cells that undergo photosynthesis responsible for storage of food for example and they have also the capability of continuing to divide and differentiate into other cell types if there's wounding of the plant but at maturity they're typically not dividing and they have very thin walls you can see here the thin walls of a parenchyma cell that allows them some flexibility so these cells can be fully mature in parts of the plant that are still growing and they're flexible enough to conform to that so that's a major cell type within the plant body in general another type is calenchyma and calenchyma like parenchyma these are cells that are living at functional maturity which may sound a little bizarre because what isn't everything living when it's mature but actually some of these cell types we're going to be talking about right away are not living at functional maturity that is when they actually serve a function for the plant but calenchyma cells have unevenly thickened walls you can see all this dark staining area these are cross sections by the way but you can see that the walls are thickened unevenly here which gives them some rigidity and strength so this type of cell can impart some structural support but these cell walls are just primary walls we'll get to secondary walls in a minute but primary walls retain some flexibility so even though these have some structural can provide some structural support they also have flexibility and can occur in growing parts of the plant so they can provide support in growing areas and we often find these kinds of cells just inside the epidermis the outermost layer of cells on a plant especially in the primary plant body which we'll get to that in a minute so an example let's see, first of all for perenchyma I should mention that these are cells that are widely found throughout the plant body they're much of the soft tissue in a plant so if you eat a fruit for example a lot of that tissue is perenchyma whereas calenchyma, what you might be familiar with there is if you bite into a stalk of celery the strings that can get caught between your teeth in a stem, in a stalk of celery which is a petiole of a celery leaf that's calenchyma so if you pulled those out of your teeth you'll notice that they're green they're living cells and they do provide some structural support as you can appreciate, they're pretty strong so a third tissue type our third cell type is sclarenchyma and sclarenchyma cells are dead at functional maturity so when they're fully formed the protoplast dies and they become hollowed out and before they die they lay down secondary wall between the primary wall and the cell membrane and these secondary walls are not only adding thickness to the cell wall but they also are impregnated with lignin which is a compound that imparts rigidity to plant parts and so examples of sclarenchyma are things like well there are a couple of different types of sclarenchyma there are sclerids which are not elongated cells but provide structural support for example the seed coat rigid seed coats are made of these really tough sclerids as is say the outer fruit wall of a nut like a walnut shell something like that those are sclerids that impart that hardness and if you bite into a pear I don't know if you've experienced that sort of grittiness of a pear fruit those are sclerids inside the fruit of a pear which is otherwise mostly parenchyma tissue but there are sclerids in there that give it that gritty texture another type of sclarenchyma are fibers fibers are elongated cells that form thread-like they're often in a thread-like organization and you might be familiar with fibers for like hemp fiber which is used traditionally to make rope or flax fiber it's used for weaving fibers are really tough and impart rigidity to tissue but these sclarenchyma cells are only formed in parts of the plant that have quit growing because sclarenchyma is rigid and it can't elongate it's inflexible okay so only in the parts of the plant that are no longer growing is sclarenchyma fully differentiated there are questions about cell types there are a couple of other cell types in the vascular tissue the conducting tissue we're going to get to those in a minute but these are cell types that are found throughout all the tissues of the plant not just the vascular tissue okay so there are tissue systems in plants and just like those well there are more than those three cell types but there are three tissue types there are only three tissue types in plants and tissues are just a functional unit that connects all the plant organs and there are three types of plant organs regardless of whether we're talking about reproductive parts of the plant or vegetative parts of the plant plants are made up of leaves, stems and roots and remember the flower is basically a simple shoot that's a stem, the axis of it with modified leaves both sterile and fertile and stems and roots and these all have characteristic attributes we'll talk about in a moment and across the leaves, the stem and the root we have three types of tissue systems that are known as dermal, ground and vascular tissue so the dermal tissue is the tissue that forms the outer protective covering of the plant so the outer most layer in the case of the primary plant body just the epidermis, a single cell layer in thickness we get to secondary growth it can be more than one layer in thickness and then the vascular tissue in purple here is the conducting tissue conducting water and food sugars and other organic nutrients and the ground tissue in yellow here is basically everything else mostly parenchyma that is the most metabolically active tissue in the plant and you can see how these three different types of tissue with the exception of the epidermis actually the vascular tissue and the ground tissue are organized in different ways in the three different organs so there's a very different organization for example the vascular tissue in the stem compared to the root and the ground tissue as well and the same applies to the leaves but it's all interconnected the unified whole these tissue systems are all interconnected across these three organs three types of organs but we'll talk about the shoot system and the root system separately in terms of development because they are so different anatomically and developmentally the same applies to the leaves actually so the dermal tissue as I mentioned this is just the outer protective covering of the plant body which in the primary plant body whatever is in a monocot in most almost all monocots and in all non-woody eudaicots the dermal system is just the epidermis a single cell layer and thickness around the outside of the entire plant and it's typically covered with this waxy substance called the cuticle or waxy substance that constitutes the cuticle which you can see is typically thicker on the outer most walls the ones that are most exposed to the atmosphere than on the inner walls and this cuticle is critical of course to provide a barrier so that we don't have water loss from the plant out into the atmosphere but the plants have to be able to take in gases especially in particular CO2 in order for photosynthesis to occur so there has to be a way for CO2 to get in during photosynthesis, oxygen is generated so some of the respiratory needs can be met by that oxygen but sometimes oxygen needs to be taken in through the epidermis as well and that's done through what's called a stoma which the plural is stomata adding a TA to that just referring to multiple stoma and here you can see individual stoma here and these are the pores right here that allow air exchange outside of the plant and there's two cells here on either side here which are called guard cells that regulate the airflow they regulate the size of that pore they can completely close that pore if the plant's experiencing drought stress for example or they can open it widely allowing maximum entry of carbon dioxide but the problem with having this open of course is that it is a way that water can leave the plant and so it's a trade-off that the plant's always juggling is getting enough CO2 into the plant for photosynthesis to occur and in some cases sufficient oxygen for respiration but also not letting out too much water and causing the plant to wilt so these are structures that we see on the epidermis of the plant and especially on the leaves and especially on the underside of the leaves of plants that have bifacial or flattened leaves where they're not in direct exposure to the sun and there's less potential for evaporative water loss and also the epidermis can produce some extensions so even though it's only a cell layer in thickness around the plant body there can be epidermal cells that extend outside this layer and even multiple epidermal cells that can make up things like these hairs here you see and we can have hairs on roots we can have hairs on stems we can have hairs on leaves and they serve a variety of function those root hairs that I mentioned earlier that were coming off the tap root that you could see in that video those are epidermal outgrows they're just extensions of the epidermal cells of the root and they allow for an increased surface area for absorption to occur and some of the epidermal hairs on the outside of the chute system on the stems and the leaves those hairs can function in a variety of ways they can deter herbivores by producing toxic glandular exudates maybe you've felt sticky plant parts before that are producing glandular substances that are distasteful to herbivores or maybe even poisonous they can also form a reflective layer if they're densely packed on the surface you've seen leaves that are covered with white hairs that allow them to reflect light so if they grow in a high-light environment reflecting light could be beneficial to keep the leaf from overheating so there are things like that that the epidermists can serve to protect the plant body there are questions about the dermal system we're going to talk more about it soon okay, so now the vascular tissue involves some additional cell types in addition to parenchyma, chalchyma, and sclerchyma that I mentioned before yeah, unfortunate names but we're sort of stuck with those names vascular tissues of two types remember, these are the conducting tissues that transport water and nutrients in the plant so there are two discrete types of vascular tissue there's the xylem which conducts water and minerals through the plant and of course from the roots where the water is absorbed to the other parts and the xylem has a couple of types of cells that we haven't seen yet namely tracheids on the right here and vessel elements here on the left and like sclerchyma these are cells that are dead at functional maturity so the protoplast dies and disappears and these are basically hollow tubes that conduct water and they're arranged together in a conducting system basically a plumbing system through the plant and tracheids are the type of conducting element in the xylem that's found across all vascular plants and in fact that's why the vascular plants are technically known as tracheophytes which just means tracheid bearing plants because even though some masses have conducting tissue it's not homologous to tracheids now the tracheids and the vessel elements like the sclerchyma are not only dead at functional maturity but they have secondary walls that are laid down before the cell dies on top of the primary wall so these thicken the walls make them stronger and again like in sclerchyma the walls are impregnated with lignin the strengthening substance that imparts a lot of strength to these particular types of cells and that's really important for them to withstand the kinds of pressures negative pressures that they're under during water conduction the water moves between these tracheids not by actually big openings between them but by places in the tracheid where their secondary wall wasn't laid down and where there's just primary wall that's very thin and that's what we call pits you can see pits along the side of the tracheid there are also pits here along the side of the vessel element and so water can move through these primary walls there are small holes very tiny holes in the primary wall but you might think well that doesn't sound very efficient to have the water have to cross these membranes or try to cross through these very small openings but one of the functions of having one of the benefits of having some restriction to movement here is that it prevents air bubbles from moving through the conduction system so if an air bubble forms inside of a tracheid that's going to prevent conduction from continuing and it's much more difficult for an air bubble to move through these small openings than it would be if there were large openings so an embolism in vascular tissue can have dire consequences for conduction and tracheids have means of preventing these embolisms from spreading through the conducting tissue with regard to these pits now vessel elements have more potential for embolisms to spread but they also have more efficient movement of water because they actually have perforation plates in the ends you can see one right here in a bleak view so that's actually... there are actually openings these perforation plates have openings that are not covered by primary wall there's no wall there at all and so the ends of vessel elements have pretty free movement between them and together they make up what's called the vessels so these things are hooked together into end and provide for very efficient water movement but under severe drought stress these are more prone to forming air bubbles to really move through the system and cause a problem so we can get both vessel elements and tracheids within angiosperm wood or angiosperm xylem yeah well, the tracheids both serve a structural function as well as a conduction function in general they're narrower they're more tightly packed in the tissue and so they impart some more rigidity to the conducting tissue than the vessels do the vessels are usually found in association with fibers the type of sclerenca I mentioned and so we have, in the case of plants that have vessel elements they often have a mixture of fibers and vessels the fibers serving a more of a structural function and the vessels serving more of a conduction function within the vascular tissue so more of a separation of function than we see in some plants that just have tracheids and so vessel elements are only found in angiosperms and in the needophytes there are weird things like the platypus of the plant kingdom I mentioned, the well witchia and it used to be the reason that people thought the needophytes in the angiosperms one of the reasons that they were thought to be closely related but they've independently evolved vessels vessels are not found very widely outside of those two groups the flowering plants and the needophytes although they're continuing to be discovered in some other groups but for example conifers lack them altogether and just have tracheids okay the other type of conducting tissue is the phloem and the phloem conducts the food through the plant the sugar and organic nutrients and we'll talk about exactly how that happens soon as well as how the water moves through the plant but in terms of the cell types that make up the phloem there are two major cell types that you need to realize one is the sieve tube element which is the type of element that actually conducts the sugar solution through the plant and these are living at maturity but the bulk of this elongate cell is open in the center because the cytoplasm is just restricted to a little layer around the inside, just inside the wall of the cell and it's lost the nuclei, it lost the ribosomes, it's lost the vacuoles it's lost all the organelles including the nuclei which allows the cytoplasm to take up a very small amount of space just around the periphery of the inside of this sieve tube element and so it allows the center of this element to mostly be open for conduction and these sieve tube elements are joined end to end with one another into sieve tubes and between the sieve tubes we have what's called a sieve plate that has openings in it that allow the sugar water to move through the plant this is a longitudinal section here and as I mentioned, the fact that there's no nucleus, there's very little in the way of the ability of a sieve tube element to be able to regulate its metabolism it's in association, each sieve tube element has a companion cell which is a type of parenchyma cell that has nucleus and all the other organelles of the plant cell and it regulates, it helps to regulate the metabolism of the adjoining sieve tube element which is alive but lacking in some really critical parts of its cell and actually the companion cell and the sieve tube element are formed from the same initial cell there's a cell division that results in a companion cell and a sieve tube element that they're developmentally conjoined, they actually come from a common ancestral cell that divides and so the interesting thing about the sieve plate, we'll talk about it later is if these are really precious substances that the plants moving through here, the sugars so if there's any wounding of this, if there's a breach of the sieve tube it behooves the plant to seal this up immediately and it does, in fact it seals it up so quickly that it's really hard to study the anatomy of these cells because this substance called callos is formed instantaneously that seals up the sieve plate holes here and prevents any further conduction so these have really good wound response to keep from losing sugars are there questions about xylem or phloem at this point, we're going to get into it more later okay so we're out of time and we'll just talk about the way these are organized and get to the other system shortly