General Botany BOTANY 130
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Topic 16 Secondary Growth Introduction To properly understand secondary growth one must first be familiar with primary structure of the stem and the root Specifically you should have an understanding of the organization of the primary tissues in the two stems we have studied Medicago and Coleus and of the Ranunculus root It may be a good idea to review both quotCells and Tissues of the Plant Bodyquot The Root and quotThe Shootquot before proceeding Some Important De nitions Primary tissues Tissues generated from the growth of an apical meristem Cambium A lateral meristem consisting of a sheet of cells Growth of these cells increases the girth of the plant organ involved Secondary tissues Tissues generated from the growth of a cambium Vascular Cambium A cambium that gives rise to secondary xylem to the inside and to secondary phloem to the outside Periderm A structure that consists of a cork cambium phellogen producing cork tissue phellem to the outside and in some cases a layer of cells to the inside called phelloderm Periderm functions to limit dehydration and block pathogens after the epidermis is disrupted by the onset of secondary growth Cork phellem you need know only the term quotcorkquot Tissue dead at maturity generated from a cork cambium The cell walls of the tissue are impregnated with suberin This waterproof s the tissue The cork used to seal wine bottles is cork tissue harvested from a species of oakThe cell theory was first proposed by Robert Hooke in 1665 after microscopic examination of a slice of cork Cork Cambium A cambial layer that functions to produce cork and in some cases phelloderm In roots it is derived initially from pericyle In stems it is first derived from the cortex Unlike the vascular cambium these cambial layers do not persist for the duration of the life of the plant organ Over time one cork cambium will be supplanted by another generated from parenchyma cells further inside Phelloderm In some periderms the layer of living secondary tissue is generated by the cork cambium to the inside We will not consider the phelloderm in the following exercise I The Primary Structure of the Tilia Stem Take the prepared slide of the cross section of a Tilia basswood stem at the end of primary growth and survey the slide with your microscope at 40x Identify the three tissue systems This stem differs from that of Medicago or Coleus In Tilia the pith rays are only one cell layer wide and the primary vascular tissue appears as an almost continuous ring As in both Coleus and Medicago the primary xylem lies inside the phloem As in the stems studied earlier the ground tissue is segregated by the vascular tissue into pith and cortex The dermal tissue consists of an epidermis This structure will be transformed by the growth of two cambiums the vascular cambium which forms between the primary xylem and primary phloem and by the cork cambium that will form in the cortex and will supplant the epidermis View the Figure below and identify all the labels Draw the intact epidermis Draw a primary vessel element II Secondary Tissues of an older Tilia Stem Take the prepared slide of the cross sections of Tilia basswood stems l 2 and 3 year sections on the same slide Survey the section of the 1 year old stem with your microscope at 40x Starting in the middle of the stem identify the following the pith the xylem tissue the phloem tissue the cortex the periderm and the epidermis Now switch to 400x Carefully study the boundary of the xylem with the pith The innermost xylem is primary xylem The outermost is secondary xylem The boundary between the two is subtle the secondary xylem starts where the xylem rays begin Xylem rays are areas of parenchyma of the secondary xylem running laterally through the stem Start at the outside of the cylinder of xylem and locate a ray and follow it towards the pith Where it ends marks the location of the primary xylem Study the boundary between the xylem and the phloem This marks the location of the vascular cambium Note that on either side of the vascular cambium are secondary tissues derived from the vascular cambium Also note that the farther the tissues lie from the vascular cambium the older the tissue This means that as we move outward through the xylem we move progressively from older to younger xylem However as we move outward through the phloem we move progressively from younger to older phloem The outermost phloem is the protophloem and is obvious because of the fiber cells in that tissue These fibers differentiated from cells in the primary phloem that matured with the onset of secondary growth Outside of the phloem is the cortex In the cortex a periderm has formed to the outside The periderm consists of a cork cambium together with the cork tissue derived from that cambium Note the epidermis being sloughed off the stem Label the gure of the oneyear old cross section on the next page FDB A I k u i n quot9 4 x I 939 grovequot g 39 F A I l my Section at the end of Three Years Growth Switch back to 40x and survey the 3 year old Tilia section The obvious changes visible here are the growth rings present in the secondary xylem and the growth of certain rays in the phloem forming wedge shaped regions of parenchyma in that tissue The sequence of tissues outlined before are the same from the center outward pith primary xylem secondary xylem vascular cambium secondary phloem primary phloem cortex and periderm Switch to 400x and carefully study a growth ring of the secondary xylem The growth increments are areas where smaller thick walled vessel members border larger thin walled vessel members The smaller cells make up late summer s growth and the larger cells early spring growth By observing this boundary you should be able to tell in which direction is the pith You will need to make this determination for one question on the lab examThe rays in the xylem are continuous with those in the phloem The enlargement of some of the phloem rays serve to relieve the tension on the phloem created by the expanding cylinder of xylem This stress tends to create longitudinal rips in the phloem which would destroy its integrity The expansion of these rays they are called dilated rays prevents these tears The phloem outside of this ray tissue consists of bands of fibers alternating with areas containing sieve tube members and companion cells Label the following gure Cross Section of a Threeyear Old Tilia Stem Label The Figure from a 3year old Tilia stem Notes Identify the labels here of the periderm of an older Tilia stem Drawings Draw the following of a woody Tilia stem below and on the next page a phloem ber sievetube element With companion cel cork cells adjacent vessel elements across with cork cambium a growth ring Parenchyma in Parenchyma in a a xylem ray non dilated phloem ray III Face View of a Vascular Cambium The vascular cambium is a hollow cylinder of actively dividing cells In cross sections of stems this dimensional aspect of the structure isn t apparent To see a the cambium as a sheet of tissue one must observe a longitudinal section of a woody stem through the cambium This type of section is a tangential section The cut is made following a plane tangential to the cylinder of the cambium Observe the demonstration slide of the tangential section of the cambium of Robinia black locust Note that there are two types of cells each with a different orientation One type of cell is arranged vertically and are termed fusiform initials The initials of these cambium cells go on to form tracheary elements in the wood and other cells oriented vertically in the wood or if incorporated into the phloem sieve tube members companion cells or fibers again all the cells oriented vertically The other cells are arranged horizontally and are called ray initials These go on to form the rays both in the secondary xylem and secondary phloem IV Lenticels Lenticels are tears in the bark Lenticels allow for the diffusion of gases to and from the living tissues in the woody stem Observe the demonstration slide at the front of a section of a lenticil in Sambucus V Gross structure of woody stems Woody stems are mostly secondary xylem wood surrounded by bark The xylem may include heartwood and sapwood Heartwood is darker While its cells are all dead and the tracheary elements are nonfunctional the tissue still provides support The sap wood is lighter In the sap wood the tracheary elements tracheids and vessel members are functional and the tissue includes living parenchyma cells The boundary between the bark and wood marks the location of the vascular cambium The bark itself is divided into two regions by the cork cambium the living area inside the cork cambium is the inner bark and the dead tissue outside is the outer bark Evidence of earlier cork cambia can be easily discerned in some woody stems Take a section of oak wood from the front bench and identify the following pith heart wood sap wood vascular cambium xylem rays inner bark and the outer bark If you can t locate these all of the above ask your TA for help Identify the Labels on the next page UOUJDgt Cross Section of a Woody Stem of Bur Oak Quercus macrocarpa VI Secondary Growth in Roots The onset of secondary growth in roots is somewhat different than that in stems This is well illustrated in your text on page 601 in figure 25 16 The pericycle plays an important role in secondary growth It both forms the periderm and also splices together the pieces of vascular cambium at the protoxylem poles where it is discontinuous at the beginning of secondary growth Because the first periderm is formed by the pericycle all the tissues outside and including the endodermis are sloughed off immediately when secondary growth occurs in the root VIa Secondary Growth in Tilia Locate the prepared slide of Tilia root and observe with your microscope Identify the vascular cambium secondary xylem and secondary phloem Can you identify the star shaped mass of primary xylem inside this root Identify the periderm vascular cambium primary xylem secondary xylem and secondary phloem From what primary tissue is the secondary tissue surrounding the phloem derived VIb The carrot Carrots are roots with lots of secondary growth At their core they are composed of xylem tissue surrounded by phloem tissue Make a cross section of a carrot and observe it with a dissection microscope Note the rays which are indicative of secondary tissues Identify the vascular cambium primary xylem secondary xylem and secondary phloem From what primary tissue is the secondary tissue surrounding the phloem derived Draw a vessel element in your carrot section Oak wood and the inner core of carrots are both examples of xylem tissue Why is one hard and tough while the other is crisp and delicious Can you relate this to the different functions of a roots vs stems Topics for Discussion The wood sections studied in this topic were of Quercus macrocarpa bur oak This tree was common in the fire swept savannahs of Wisconsin Based on what you observed today how is this tree adapted for surviving fire Pinus banksiana trees are sensitive to fire The species however is dependant on fire Without fire the tree would be excluded from the forests of Wisconsin How can trees sensitive to fire persist in an environment dominated by fire Boreal forests are dominated by conifers spruce and fir In what way may tracheids be adaptive for trees subject to extremely low minimum winter temperatures A girdled tree is doomed Exactly how does the tree die Some girdled trees will live for a season or more Account for this delayed death Topic 13 Cells and Tissues of the Plant Body I Identifying Tissue and Cell Types in Cross Sections of Dicot Stems Introduction In this exercise you will make observations of various tissues and cells found in the stem of herbaceous eudicots specifically a freehand section of Coleus stem and the stained cross section of Medicago Alfalfa stem on a prepared slide There are labelled images of these in our computer bank of images which your TAs can use to assist you Further these same images are available on the course page for review Getting Started If someone at you table has the knack for making good sections have that person share with others Making a make freehand section of Coleus Coleus plants are available on each student bench Cut pieces of stem should be placed in the bowls of water at the front of each student bench for others to use Before cutting into the stem rub it between your fingers Notice the square shape of the stem This is a characteristic of all members of the Lamiaceae the mint family Do not use the razor blades in your drawer to make these sections Use the razor blades provided in the petIi dish at the front of each bench These are particularly sharp They should be used up one at a time and everyone at a bench should use the same blade until it becomes dull at which point it should be thrown away and replaced by a fresh blade Please be careful not to mix these blades with the ones in your drawers Preliminary Observations After preparing a wet mount observe the cross section with your 4x objective The stem is made up of three different tissue systems Please identify these in turn The dermal tissue system consists of the outer layer of cells of the stem These cells make up the epidermis The vascular tissue system represented by distinct vascular bundles These are arranged in a circular pattern inside the stem The ground tissue system consists of everything enclosed by the epidermis except the vascular bundles Typically in eudicots the vascular bundles are arranged in a circular pattern embedded in the ground tissue This arrangement of the vascular bundles divides the ground tissue into two regions pith and cortex After identifying these tissue systems with your Coleus slide repeat the process with the prepared slide of Medicago Cross section of MedicagO Stem Identify the Three Tissue Systems la The Epidermis The epidermis functions to control the loss of water from the plant It accomplishes this primarily through the excretion of a waxy layer called the cuticle Like a sheet of wax paper the cuticle limits the passage of water In doing this it also blocks the passage of gasses between the plant and its environment To keep the tissues from suffocating the epidermis must have openings These openings are called stomata singular stoma Structure Stomata Stomata are not easily seen in a freehand section hence look for these using the Medicaga cross section First identify large intercellular spaces directly below an area of epidermis using low power These are substomatal chambers and they are positioned directl below each stoma Switch 39 to 400 and observe the stoma and Stoma Wlth Guard Cells its two guard cells The level of turgor of these two cells serves to open and close the stoma controlling the movement of gasses and water into and Trichomes out of the underlaying tissue Trichomes Another way the epidermis limits the plant s loss of water is through the growth of hairs called trichomes Switch back to your section of Coleus and observe these hairs projecting from the epidermis Can you think of any other possible adaptive function of trichomes Function of the Guard Cells Guard cells must react to environmental conditions It is adaptive for stomata to open in the light when the plant is not water stressed It is generally maladaptive for the stomata to be open at other times The closing of a stoma by its guard cells is ultimately a phenomenon of osmosis though mechanisms exist to translate environmental conditions to the turgor pressure of the guard cells In the following exercise we will simply observe the response of the stomata to a change in the turgor of the guard cells of Setcreasea Work in pairs Set aside your section of Coleus stem and prepared slide of Medicago for use next section 1 Make an epidermal peel from the lower surface of a Setcreasea leaf Mount the peel in distilled water and cover with a coverslip Avoid trapping air bubbles under your wet mount 2 Scan your slide to find an area with few air bubbles and with open stomata 3 Switch to high power In the space below draw an open stoma including guard cells and subsidiary cells 4 While the field of view of your microscope is centered on an open stoma place a large drop of 10 solution of NaCl next to one side of your coverslip On the other side place a piece of tissue to wick up the distilled water to draw the salt solution under the coverslip Observe any changes Make a drawings of the stomata below on the next page Ib Ground Tissue Collenchyma in the Cortex Retrieve your section of Coleus stem place it on the stage of your microscope and observe with your 4x objective Switch to a higher power and obseive the cells types at the corners of the stem in the cortex This region of the ground tissue includes collenchyma tissue Collenchyma has thickened pIimary walls Primary walls are elastic and collenchyma provides static resistance to turgor pressure This balance of forces in herbaceous stems and leaves provides supp01t Switch to your Medicago slide This stem also has collenchyma positioned at the ribs Note however that in this preparation the cells are dehydrated and their walls are uniformly thick Collenchyma is both a cell type and a type of tissue consisting of collenchyma cells Coillenchyma Tissue Medicago Coleus living prepared slide Collenchyma in the Petioles of Celery The strands that can be peeled from celery are made of collenchyma tissue Get a piece of celery at the front You may eat this tissue but While you do so break a piece of it and peel out these stands Note that they are elastic Make a cross section of your celery petiole to observe the collenchyma Draw collenchyma and parenchyma cells at a boundary between the two tissue types Parenchyma of the Pith Using your Coleus section observe the pith the region of the ground tissue inside the ring of vascular bundles In Coleus this region is composed entirely of parenchyma tissue consisting of parenchyma cells Parenchyma is both a cell type and a tissue type made up of parenchyma cells Parenchyma cells have thin primary walls Stain your section with IZKI Based on your observation of the staining What is one obvious function accomplished by this tissue Ic Vascular Tissue Switch to the slide of Medicago stem Vascular tissue includes xylem and phloem both of which are complex tissues that is they each consist of more than one cell type In these two dicot stems the xylem occurs in the innermost area of each vascular bundle facing the pith The most prominent cells here in the xylem are vessel elements These are large dead cells with secondary walls that stain red Vessel elements function as pipes through which water moves by mass flow These are surrounded by parenchyma cells which are also part of the xylem tissue The phloem is positioned to the outside of each vascular bundle The phloem of Medicago includes sievetube elements companion cells parenchyma cells and bers Try to identify as many cell types as possible The cell types in vascular tissue will be more fully explored using Cucurbila slides later Label the Figure on the next page include xylem and phloem use you own arrows to label epidermis pith pith ray cortex and collenchyma II Demonstration of Sclerenchyma Cells Sclerenchyma cells have thick secondary walls IIa Fibers Observe the two IIb views of fiber cells at the front bench One microscope has a prepared slide of a woody Tilia stem Fibers on this slide can be seen in cross section in the bark Note their thick secondary walls Compare this view with that of the second microscope On this microscope is a slide of macerated Tilia bark Here the fibers have been physically disassociated from the bark and can be seen on their side Note their length Fibers are like cables and lend strength to plant tissues Stone Cells These cells are not elongated but have massive secondary walls Observe the preparation on the third microscope of the macerated pear esh These stone cells have simple pits which are also forked Stone cells give pear fruit its gritty texture a i a E I 1 W Stone Cells of Pear III Studying Vascular Tissues in Cross and Longitudinal Sections of the Stem of Cucurbita Take a prepared slide of Cucurbita and examine it by holding it up to the light Note that there is both a cross section and longitudinal section on the slide Now observe the slide under your microscope IIIa The Xylem Cross Section Survey the cross section of the stem at 40x Note the two rings of vascular bundles arranged around a hollow pith Observe a vascular bundle at 100x Note the huge empty cells with the secondary walls These are vessel members The xylem tissue is made up of these vessel cells together with the parenchyma around them Longitudinal Section Move your stage to view the stem in longitudinal section Note because of the hollow pith it forms two sections seemingly unconnected In this view the vessels can be viewed for what they are continuous tubes running longitudinally through the stem The most prominent vessels are those that differentiated after the stem stopped elongating Xylem tissue that differentiates after elongation ends in a plant organ is termed metaxylem The metaxylem vessels have a complete cylinder of pitted secondary wall If you look carefully you should also see vessels with incomplete secondary walls The secondary walls here consist of either hoops or spirals These are protoxylem vessels Protoxylem differentiates while elongation is occurring The incomplete secondary walls inside the pIimary walls prevents the vessel from collapsing and yet allows it to be stretched without ripping Protoxylem vessels with hoops are said to have annular thickenings Those with spirals are said to have helical thickenings It is unlikely that you will see both on your slide so obseive the demonstration scopes of these vessels on the side bench IIIb The Phloem Cross Section Look again at a vascular bundle in cross section Note that in Cucurbita phloem lies both to the inside and outside of the xylem Carefully search the area of the phloem for views of sievetube members with a sieve plate in clear view Locating a sieve plate is the surest way of identifying this cell type Once you locate a sieve tube member note its size relative to the other cells Also note that in Cucurbita these cells have primary walls that seem thicker than that of their neighbors If you have difficulty locating a sieve tube after three minutes ask your TA for help The smaller cells with the dense cytoplasm associated with the sieve tube members are companion cells Also note that in Cucurbita phloem includes not only sieve tube members and companion cells but also parenchyma cells Longitudinal Section Switch back to the longitudinal section of this stem Observe an area inside or outside of the xylem to locate a region of phloem Note the arrangement of the sieve tube members These are stacked directly one on the other to form a continuous structure the sieve tube Look for sieve plates at the junctures of these cells to confirm their identity Also look for companion cells in this section Observe and label the figures on the next two pages Cucurbita Longitudinal Sections of Vascular Tissue Vessels 1n the Xylem I Hec Segondary Wall Annular Secondary Wall Thickenings Fitted Secondary Wall Thickenings Thickenings Phloem Tissue companion cells CC sieve tube member stm Cucurbita Cross Sections of Vascular Tissue Pith Hollow IV Function of Vascular Tissues IVa Movement of Water Through the Xylem In the xylem water moves through tracheary elements which are dead at maturity Living protoplasts are unnecessary for this movement to occur In this activity you will determine the validity of the idea At the start of lab your TA placed a shoot of Coleus into a dye solution containing a poisonous saltCuSO4 The copper ions will kill any cells contacted and the dye will stain the areas to which the water is drawn Procedure Work with the people who sit at your table three groups per section Take the shoot at your table and determine if the dye was drawn up the stem and how far Make a cross section of the stem stained with the dye and determine in what tissue the dye is located Record the distance the dye moves up the stem Also record the results from the other two tables below One bench will have the Coleus shoot placed in moving air from a fan Another will have the shoot exposed to ambient room conditions and the third will have the shoot covered with a plastic bag Record you observations and those of your classmates in the table below Moving Air Ambient Air Covered Did the dye move up the stem If so how far In what tissue did you View the dye in your cross section Explain the differences in the rate of movement if any between the three conditions IVb Movement through the phloem The most widely accepted theory to explain the movement of photosynthate from source to sink often from leaf to root is the pressure flow hypothesis By this idea substances move through the sieve tube members along a gradient of turgor pressure This is created by the active transport of sucrose into the phloem at the source and by its active export out at the sink The loading and unloading of sucrose results in the osmotic movement of water into the phloem at the source and out of the phloem at the sink For this mechanism to work an area bounded by living membranes is necessary hence sieve elements must be alive to function If the pressure flow hypothesi is true would you expect the pressure in the phloem at the hypocotyl of a squash seeding to be check one At ambient air pressure Less than ambient pressure Greater than ambient pressure In this activity we will see which is correct Bmdum Work in pairs On the side bench is a flat of squash seedlings with a cylinder of alcohol in front of the flat While both you and your partner are observing quickly cut off the shoot at the ground level and place the cut stump into the alcohol and observe the emersed stump What do you observe Make a second cut above the first and again place the stump into the alcohol What do you obseive Based on your obseivations is the phloem under pressure relative to air in the room Discuss your observations after cutting the shoot a second time With your TA This relates to the p protein found in the sieve tube elements Notes Topic 6 The Plant Cell Introduction Cells are the fundamental units of all life Structurally nothing simpler than a cell is actually alive Plant cells like animal cells and unlike bacteIia are eukaryotic Eukaryotic cells have nuclei Eukaryotic cells also have structures called organelles Where different biological processes occur The entire eukaryotic cell is pervaded by a membrane system called the endoplasmic reticulum Plant cells unlike animal cells have a cell wall and plastids In this exercise you will obse1ve examples of relatively undifferentiated plant cells DuIing the exercise you will be asked either to label figures of the cells you observe or to draw those cells Lots of time has been allocated to these tasks so take your time and make careful observations I The Elodea leaf cell a photosynthetic factory Procedure Make a wet mount of a leaf that has been vitally stained with Janus Green B This is a vital stain It colors the living mitochondria and membranes of the nucleus and vacuole making them more visible The leaves are in bowls at each bench Make a wet mount of a leaf Observe the leaf with your scanning objective 40x total magnification Even at low power it will be obvious that most of these cells are full of chloroplasts Chloroplasts These are green membrane bound structures that are the site of photosynthesis Chloroplasts belong to a class of organelles called plastids Plastids are thought to have been derived from free living photosynthetic bacteria that were engulfed by a unicellular organism This joined organism gave Iise to algae which evolved into plants Evidence for the idea include the presence of DNA in plastids This DNA is arranged in rings like that of bacteria Plastids also have ribosomes the structures that are the site of protein synthesis These ribosomes are different from those found in the rest of the plant cell and are like those of bacteIia Plastids are surrounded by a double membrane system The inner membrane is thought to have been derived from the bacterium and the outer one from the host cell Chloroplasts make vegetation green The gross structure of the Elodea leaf and cells Switch to high power 400x and carefully through focus 1 How many cell layers thick is this tissue Continue through focusing but carefully study one cell 2 Based on your observations are the chloroplasts evenly distIibuted throughout the cell 3 How are they distributed 4 Carefully observe the profiles of the different chloroplasts in your cell Given that the chloroplasts are all the same shape how would you describe that shape Switch back to a lower magnification and look for cells manifesting any internal movement What you are looking for isn t subtle keep scanning the cells in your leaf until you find one demonstrating an obvious flowing movement of the chloroplasts around the periphery of the cell This movement is called cyclosis or cytoplasmic streaming It is facilitated by the same two proteins responsible for muscle contraction in animals actin and myosin While observing a streaming strand of cytosol look carefully for spherical structures about 10 of the diameter of the disc of the chloroplasts These are mitochondria Locate a nucleus if you haven t already Often they will be found against one side of the cell in which case they will be hemispherical in outline If you have difficulty finding one ask your TA for help but search for a good five minutes first Can you see a nucleolus in your nucleus Drawings a Make two drawings of the same cell one with the focus just below the cell wall and the second with the focus at the midpoint of the cell Label chloroplasts in each view note their different orientations In the second view the area in the center without chloroplasts is occupied by a large central vacuole This structure contains a solution called cell sap and can occupy up to 90 of the volume of the cell Label the vacuole in your drawing If you look closely while through focusing you will observe strands of cytosol transversing the vacuole Include these in your second drawing If you were successful in observing mitochondria include these in your drawing also Diameter f ov Drawing 1 Drawing 2 b If a nucleus was not included in the drawings for b make a third drawing with a nucleus label the nucleolus if you see it clearly 44444 mm Diameter fov Based on your observations why are vegetables such as lettuce and cabbage low in calories Assume that their cells are similar to these of Elodea II The Tradescantia Stamen Hair a source of plant cells without chloroplasts In Elodea cells chloroplasts interfere with the view of the overall structure of the plant cell In this activity you will view plant cells without chloroplasts Procedure The cells you wish to observe are associated with the male parts of the Tradescantia ower the stamen The starnen has two parts the anther containing the pollen and a supporting structure called the lament The filaments in Tradescantia are hairy and these hairs are what we wish to view Remove an entire starnen with your forceps and place in a drop of water on a slide Tradescantia Flower With your teasing needles slice off the anther and carefully lower a cover slip over the filament Many of the hairs will trap air Ignore those parts of the slide with entrapped air Carefully view a living cell totally emersed in water at high power Your cell may contain a water soluble pigment in the vacuole If so discerning the boundary of the vacuole will be obvious If your cell does not have pigment dissolved in the cell sap this boundary can be identified as the point of transition betweent e granular cytosol and the clear solution in the vacuole Identify cytostolic strands transversing the vacuole Note any cytoplasmic streaming and the prominent nucleus In these cells the nucleus is often suspended in an island of cytosol surrounded by the vacuole This island is connected to the sheets of cytosol just below the cell wall by the cytoplasmic strands that cut across the vacuole Label figure El below gt5 143 111 Other Plastids IIIa1 Chromoplasts in Flower Petals Tear a piece of tissue from an Allamanda ower and make a wet mount Carefully view the intact margin at 400x Throughfocus and observe the yellow translucent chromoplasts Draw a petal cell label the chromoplasts Diameter f ov IIIaZ Watersoluble pigments in Impatiens ower petals Many flowers are not colored by chromoplasts Prepare a wet mount of Impatiens flower petal tissue using the same procedure used for Allamanda Again observe an intact margin and through focus 6 What part of the cell is colored Draw a cell from the intact margin and label the boundary of the vacuole and label the vacuole itself Diameter f ov IIIb Chromoplasts in Fruit Plastids can be transformed into other forms of plastids As some fruits ripen chloroplasts are transformed into chromoplasts The resulting color change serves as a visual signal to animals that the fruit is ripe This ensures that the seeds inside the fruit will not be dispersed until they are mature Make wet mounts of thin sections of green unripe and red ripe pepper fruit Make drawings of each tissue label chloroplasts and chromoplasts Diameter foV Green Fruit Tissue Ripe Fruit Tissue IIIc Pigment Bodies in Carrot The pigments in chromoplasts are fat soluble and can often be observed as discrete droplets or crystals These structures are pigment bodies In some tissues the chromoplasts become disrupted and pigment bodies are found directly in the cytosol This is the case in carrot tissue Carotene pigment the source of Vitamin A in the tissue is produced in chromoplasts and becomes incorporated into a long narrow crystal twisted and under tension inside the plastid Eventually these chromoplasts burst releasing the crystals which become straight Make a wet mount of carrot tissue and observe these pigment bodies If you see an intact chromoplast call it to the attention of your TA Draw pigment bodies in this tissue include a chromoplast if possible Diameter foV IIId Leucoplasts in Zebrina Plastids without color are called leucoplasts Leucoplasts can be readily observed in the intact margin of Zebrina leaves Cut a sliver of tissue from the edge of a Zebrina leaf and prepare a wet mount Focus with high power on the intact leaf margin These cells have a water soluble pigment anthocyanin dissolved in their vacuole Note that the nucleus appears to be suspended in the vacuole To understand the structure here think back to your observations of the Tradescantia stamen hair and its nucleusThe structures surrounding the nucleus making it appear bumpy are leucoplasts Draw a cell along an intact margin label nucleus vacuole and leucoplasts Diameter fov IIIe Starch Grains in Potato Tuber All plants store starch in plastids In some cases plastids become so full of starch that there appears to be nothing else present except a huge starch grain This is the case with the storage leucoplasts in the potato tissue g rise to the green tissue often found in potato Prepare a wet mount of potato Observe the starch grains inside the cells of this tissue Set your microscope up to observe these grains through crossed polaroids ask your TA to demonstrate how Note that through crossed polaroids the grains glow in front of a dark background This is a characteristic of starch grains and also of crystals Make drawings on the next page Recall the iodine test for starch used in the last topic Please repeat this exercise here by adding 12KI to your wet mount Make a third drawing as outlined on the next page Drawings Diameter f ov Starch Grains Stained with IZKI Through Crosses Polaroids These leucoplasts can be converted into chloroplasts if potatoes are subject to light This is associated with the generation of toxins Potatoes should always be stored in the dark and any green potato tissue should be discarded The stages of this transition between leucoplast and chloroplast can easily be observed in green potato tissue Make a wet mount of green potato tissue and make four drawings illustrating the transition between leucoplast and chloroplast Diameter fov Starch Grain Stage 2 Stage 3 Chloroplast leucoplast IV The Cell Wall In plant tissues adjacent cells have a boundary between their walls called a middle lamella and adjacent protoplasts are connected together by membrane bound cytoplasmic channels taht transverse their walls These channels are called plasmodesmata Plasmodesmata facilitate the transport of certain materials between cells These features are usually difficult to see through the light microscope However in the food storage tissue the endosperm of persimmon Diospyros seeds these features are readily visible because the walls are huge Take a prepared slide of Diospyros endosperm and observe the tissue at 400x Identify the middle lamella the lines running so as not to intersect with either protoplast and plasmodesmata the fine black suiau39ons running from one protoplast to another Label Figure E2 V Crystals in Plant Cells Crystals of calcium oxalate a salt of oxalic acid are frequently found in plant tissues How they function to enhance the survival of plants is not always clear though in some cases they may protect the plant from herbivores In all cases these crystals form in the vacuole They take on different shapes in different tissues even though they are composed of the same chemical In this exercise we will observe two types of crystals raphides needleishaped and druses jagged spheres 10 Druses These are common in the tissues of pot herbs such as mustard greens spinach Swiss chard and collard greens While all of these are nutritious all of these foods increase our risk of forming kidney stones Druses can be easily observed in the petioles of Begonia Make a thin section through the petiole of a Begonia These crystals may be found in cells positioned just below the epidermis Drawing Draw a druse in this tissue Diameter f 0V Rhaphides These are common in many different plant tissues and often function in defense Make a longitudinal section of the leaf tissue of Sansieveria Look for enlarged cells with darkened structures Switch to high power and note that these are bundles of needle shaped crystals raphides Obserye these through crossed polaroids Drawing Draw raphides in this tissue Diameter foV VI The Plant Cell Through the Transmission Electron Microscope The invention of the scanning electron microscope revolutionized the study of plant cells and tissues With the electron microscope details of cellular structure barely visible or invisible with the light microscope can be viewed with startling detail Other structures such as ribosomes may be viewed only with the electron microscope Below is an EM of a young plant cell For Figure E 3 below note the nucleus N surrounded by a double unit membrane the cell wall CW with its laminated often amorphous structure mitochondria M with their internal cristae the vacuoles surrounded by a single membrane tonoplast and the endoplasmic reticulum ER The dots throughout are ribosomes Identify and label these structures on the figures that follow Nucleus identified by its size double unit membrane and granular texture due to chromatin Cell Wall identified by its laminated or amorphous texture Mitochondria identified by their size by their double unit membrane and by the enfoldings of the inner membrane called cristae Plastids Identi ed by their double unit membrane Leucoplasts can be identified by their absence of cristae or chromatin Leucoplasts may have amorphous starch grains or crystalline protein Chloroplasts can be identified by their stacks of thallakoid membranes called grana Vacuole Vacuole membrane Vacuoles are surrounded by a single unit mem rane The texture inside is clear evidence of the absence of other cellular components Microbodies Have a single unit membrane and are usually dense in appearance Golgi Bodies In cross section appear as a stack of membranebound compartments resembling a cross section of a stack of pancakes Endoplasmic Reticulum Membranes that pervade the cell seemingly not associated With any of the structures listed above If ribosomes are clustered along these membranes is called rough ER Ribosomes dotlike structures often associated With endoplasmic reticulum Examine the other electron micrographs on the side bench Be sure to identify the structures listed earlier in bold During the respiration and photosynthesis laboratories we Will study details of electron micro graphs of the chloroplast and the mitoc ondiion to detail how these physiological processes are sequestered in the plant cell Web Review httpbotitbotany wise educoursesbotanyl30Euka1yotic7CellIndexitoiEChtml
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