Introductory Biology II (BIOL 115) Laboratory Manual Spring Table of Contents

Transcription

1 Introductory Biology II (BIOL 115) Laboratory Manual Spring 2018 Table of Contents Topic Page Lab Schedule... 3 Plant Cells... 5 Mosses & ferns... 9 Seed plants Seeds & fruits Vegetative organs Plant anatomy Animal Development Porifera Cnidaria Platyhelminthes Molluscs Annelids Nematodes Arthropods Echinoderms Chordates I: Urochordates and Cephalochordates Chordates II: Vertebrates... 59

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3 Laboratory Schedule Spring 2018 WEEK TOPIC(S)/ACTIVITY 1 Plant cells 2 Plant Diversity: Mosses & ferns 3 Plant Diversity: Seed plants 4 Seeds & fruits 5 Vegetative organs: Roots and shoots 6 Plant anatomy 7 Lab Practical I 8 Animal Development Basal Animals: Porifera 9 Basal Animals: Cnidaria Lophotrochozoa: Platyhelminthes 10 Lophotrochozoa: Molluscs & Annelids 11 Ecdysozoa: Nematodes & Arthropods 12 Deuterostomes: Echinoderms & Chordates I 13 Deuterostomes: Chordates II- Vertebrates 14 Lab Practical II 3

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5 Plant Cells Plant cells have features characteristic of all eukaryotic cells, including a plasma membrane, a nucleus, and a cytoplasm containing many membrane-bound compartments. In this they are similar to cells of animals, fungi, and protistans. However, plant cells also have some unique features that differentiate them from the various other types of eukaryotic cells: Plant cells generally have a cellulose-based cell wall that forms external to the plasma membrane, providing increased cell strength and rigidity. The presence of the cell wall is significant. For instance, it changes the way these cells respond to their osmotic environment, and requires different mechanisms to allow for intercellular communication. Plant cells also contain plastids, a family of related organelles not found in cells of other organisms. Included in this group are chloroplasts, which provide an efficient site for carrying out the reactions of photosynthesis; chromoplasts, which serve as storage sites for some non-photosynthetic pigment molecules; and leucoplasts, which are involved in storage of a variety of colorless substances, such as starch, oils, and proteins. Which plastids are present in a given cell depends on the location and function of the cell e.g., chloroplasts will only be found in photosynthetic tissues (mostly leaf cells and some stem cells). The vacuoles of plant cells serve a different role than the vacuoles of other cell types. In plant cells, a very large central vacuole is often present, consuming up to 95% of the total cell volume. This large vacuole can serve as a storage site for many substances, is involved in the process of intracellular digestion (similar to that occurring in the lysosomes of other cell types), is involved in the osmotic regulation of the cell, and greatly increases the surface area of the plant cells and therefore of the whole plant. As the field of cell biology expands, and previously unrecognized organelles are discovered and studied, more structures unique to plant cells may become apparent. For example, in animal cells, distinct cell regions responsible for the assembly of microtubules have been identified, and have been associated with structures called centrioles. Plant cells do not contain centrioles yet do contain microtubules. Therefore the nature of the microtubule-organizing centers in plant cells must differ from that of animal cells. LEARNING OUTCOMES: Successful completion of this lab requires that you be able to: properly and safely set up and use a compound microscope prepare wet mount slides from living plant materials, and observe them using a compound microscope identify cell structures that are unique to plant cells including cell walls, plasmodesmata, various types of plastids, and various intracellular crystals recognize the variability of plant cells, and demonstrate an understanding of the connection between the structure of a given cell and its specific function 5

6 PROCEDURE: 1. Make a fresh mount slide using a piece of epidermal tissue stripped from a scale of red onion (Allium). Examine the cells of this tissue. Draw two adjacent cells. Label the cell wall, the nucleus, and the cytoplasm. Add a few drops of hypertonic salt solution to the edge of the cover slip and draw it under the cover slip with a piece of absorbent paper. Wait for about a minute; then observe the cells again. The presence of the hypertonic solution surrounding the cells should have caused the cells to plasmolyze (plasmolysis is the shrinkage of the protoplasm of plant cells, causing it to pull away from the cell wall). Plant cells generally are connected to one another by structures called plasmodesmata that, although structurally complex, superficially appear to be thin strands of protoplasm passing through the cell wall to join the adjacent cells. Thus the plasma membrane of each cell is continuous with the plasma membrane of surrounding cells and the cytoplasm of each cell is continuous with the cytoplasm of surrounding cells. Plasmodesmata are too small to be visible with our compound microscopes, but when cells are plasmolyzed protoplasmic strands called Hechtian strands are visible that remain attached to the cell wall. It is believed that where these Hechtian strands contact the cell wall they allow plasmodesmatal connections to adjacent cells to remain intact. Draw two adjacent plasmolyzed cells. Label the cell wall and the cytoplasm. In this drawing also label a Hechtian strand which may indicate the presence of plasmodesmata passing through the cell wall. 2. Obtain a leaf from an Elodea plant, and make a wet mount slide from it. Observe and draw a single cell of this leaf tissue. Refer to figure 1.5 in the Atlas for photomicrographs of a Elodea cells. Label the cell wall and the green, pigment-containing chloroplasts. 3. Use a razor blade to obtain a very thin slice of red pepper. Make a wet mount slide of this tissue and observe with a microscope. The red organelles present in the cells of this tissue are examples of chromoplasts. Notice the difference in size between the chloroplasts that were seen in Elodea and the chromoplasts seen here. Draw a single cell of this tissue. Label the cell wall, the cytoplasm, and a chromoplast. 4. An interesting feature of plant tissues is that they often include specialized cells (called idioblasts) containing large calcium-based crystals (most commonly, calcium oxalate) in a variety of shapes. The frequency with which these crystals are found, and the wide range of species that produce them, suggest that they have important plant functions, with evidence for roles in regulation of plant calcium levels, metal detoxification, and protection from herbivory. A. A druse is a group of crystals in a single idioblast, taking on the appearance of a small ice crystal. Obtain a prepared slide of a cross-section of a Tilia stem. Scan it, focusing on the cortex and pith, looking for idioblasts containing druses. Draw an idioblast of Tilia. Label the cell wall, and the druse. 6

7 B. A second type of crystal, called raphides, is found in the leaves of many plants, including Dieffenbachia. Raphides are long, thin, needle-like crystals that usually occur in bundles within an idioblast. Removing a section of leaf tissue and grinding it lightly in a small amount of water can release the raphide-containing idioblasts from the surrounding cells, making them easy to observe. Your laboratory instructor will prepare such a slurry for you. Using a dropper, take a small amount of water from the bottom of the Dieffenbachia slurry, place it on a slide, and cover it with a cover slip. Scan the slide to find an isolated idioblast. Draw an idioblast of Dieffenbachia. Label the cell wall and the raphides. 7

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9 Introduction to Plant Diversity - Mosses and Ferns INTRODUCTION: One major way in which plants differ from animals, fungi, and most other organisms is that plants all exhibit a phenomenon called alternation of generations during their life cycle. This life cycle includes two distinct multicellular forms - a diploid sporophyte generation and a haploid gametophyte generation. The Sporophyte Generation In a plant there is always, at some time during the life cycle, a multicellular adult organism whose cells contain diploid nuclei. These adults are called sporophytes. At maturity, specialized cells (usually called sporocytes or spore mother cells) within the sporophyte undergo meiosis. The resulting cells, with haploid nuclei, are called spores. The process of spore formation is generally confined to organs called sporangia that develop on the mature sporophyte. The Gametophyte Generation The haploid spores produced by the sporophyte generation eventually germinate, undergoing a series of mitotic divisions and developmental processes, eventually producing a second form of multicellular adults, called gametophytes. The cells of a gametophyte have haploid nuclei. At maturity, specialized cells within the gametophyte develop into gametes - eggs and sperm. The gametes also have haploid nuclei, since they arose from haploid cells of the gametophyte. In some, but not all, plants the formation of gametes occurs within distinct organs, called gametangia, which are a part of the mature gametophyte. A gametangium within which sperm are produced is known as an antheridium. A gametangium within which eggs are produced is known as an archegonium. Fertilization and the Formation of a New Sporophyte Generation Eventually a sperm and an egg fuse in a process called fertilization (or syngamy), resulting in a cell called a zygote. The zygote has a diploid nucleus resulting from the fusion of the two haploid nuclei. Mitotic divisions of the zygote, and developmental events associated with these, produce a new multicellular sporophyte and complete the plant's life cycle. The term alternation of generations comes from the fact that during a plant's life cycle the two generations, a diploid sporophyte generation and a haploid gametophyte generation, occur alternately. The sporophyte and gametophyte differ markedly in appearance, and thus are said to exhibit heteromorphic alternation of generations. Although all plants exhibit alternation of generations, different evolutionary groups vary with respect to : whether the gametophyte generation or the sporophyte generation is larger how long the gametophyte generation lives vs. how long the sporophyte generation lives whether both generations can produce their own food or (more commonly) one generation must depend on the other to obtain a food supply the specific details of the overall life cycle 9

10 Offspring Dispersal Since plants are generally non-motile organisms, and therefore cannot move to a more favorable location, the success of plant species is enhanced by mechanisms that allow for offspring dispersal - i.e., the dispersal of one stage of the life cycle away from the "parent" plant to potentially grow in a different location. If many offspring are produced, and they are dispersed widely, it improves the chances that some offspring will land in a favorable location. In mosses and ferns, fertilization, zygote formation, and embryo formation occur in archegonia located on the surface of the gametophyte generation, after sperm are released from an antheridium and swim to an archegonium. The embryo is provided minor protection by the archegonium and surrounding gametophyte tissue. The adult sporophyte develops from the embryo, and is at least initially surrounded by and attached to these tissues. For these plants, the only opportunity for dispersal of offspring away from the parent plant is when spores are released from a sporangium. The spore is therefore the "dispersal unit" for these types of plants. The spores will land at some distance from the parent plant, and though they consist of just a single haploid cell, will need to survive on their own until conditions are favorable for them to germinate. Sporopollenin is a unique chemical found in plant spore walls, which is desiccation-resistant (i.e., helps to prevent the spore from drying out) and extremely resistant to chemical degradation and other decay. LEARNING OUTCOMES: Successful completion of this lab requires that you be able to: properly make wet mount slides of small specimens in order to observe them with a compound microscope properly use compound and dissecting microscopes to examine features of living and preserved specimens identify the gametophyte generation and sporophyte generation of a typical moss, and compare them to the gametophyte generation and sporophyte generation of a typical fern determine which generation in mosses is the dominant generation, and how the subordinate generation obtains food; determine which generation in ferns is the dominant generation, and how the subordinate generation obtains food identify the location of spore production in mosses, and in ferns, and demonstrate an understanding of the way in which each influences spore dispersal recognize structural components of a moss sporangium, and of fern sporangia, and demonstrate an understanding of how each contributes to spore dispersal identify the location and structure of archegonia and antheridia on moss gametophytes, and fern gametophytes, and demonstrate an understanding of how this impacts the process of fertilization in each type of plant 10

11 PART I - MOSSES (an example of NONVASCULAR PLANTS) Division Bryophyta, Division Hepatophyta, Division Anthocerophyta - Mosses, liverworts, hornworts Plants in these divisions have features that are considered to be ancestral. These are plants with little specialization of tissue and comparatively simple reproductive processes. They are the only plants that have a dominant gametophyte generation. A study of the features of mosses will illustrate the major characteristics of these plant divisions. In mosses, the gametophyte is a small plant body with very little specialization of cells and tissues (sometimes called a thallus). The moss thallus has: a shoot portion that appears leafy rhizoids that emerge from it to attach it to the substratum upon which it grows. The thallus is generally green and photosynthetic, and obtains water and other nutrients from the soil by direct absorption into its cells. It contains no cells specializing in the long-distance transport of water and/or nutrients (vascular tissue) and therefore cannot grow so large as to prevent contact between the soil and the majority of its cells. Based on the extent of branching and how erect the growth is, mosses are said to be either acrocarpous or pleurocarpous. The acrocarpous mosses are those that branch little or not at all, and which grow entirely erect. The pleurocarpous mosses are those that are extensively branched. In these mosses the growth is generally horizontal, with erect branches. A mature moss gametophyte is capable of developing gametangia on its surface. Sperm-producing antheridia can arise amongst the leaf-like structures along the length of the thallus, or at the tip of an erect branch. Egg-producing archegonia most often develop at the tip of the erect gametophyte. When fully developed, flagellated sperm are released from an antheridium and swim through a film of water to reach an egg-containing archegonium (on the same, or a different, plant). Fertilization of an egg by a sperm produces a zygote within the archegonium. This zygote undergoes mitosis to produce an embryo, again retained within the archegonium. Finally, the embryo matures into a sporophyte, consisting of: a sporangium ("spore capsule") a seta (stalk) a foot (embedded in the gametophyte tissue) The mature moss sporophyte remains attached to the gametophyte on which it was produced. Why is the sporophyte not considered to be a part of the gametophyte to which it is attached? Why does the sporophyte remain attached to the gametophyte? Meiosis occurring within the sporangium produces spores. Following spore production, the capsule opens up to release the spores, which germinate to produce new moss gametophytes. 11

12 PROCEDURE: 1. Obtain and draw a specimen of a moss, including a gametophyte and attached sporophyte. Consult Atlas Figs. 6.32, 6.33, 6.38, 6.40, and 6.41 for features of moss morphology. Identify and label the gametophyte, and its specific portions (the rhizoids and shoot) Identify and label the attached sporophyte, and its structures (the seta and sporangium) 2. Place the moss capsule on a microscope slide in a drop of water. Using a dissecting microscope, examine and draw the capsule in greater detail. At the tip of the capsule a cap-like structure called an operculum MAY be present. If so, include it in your drawing and label this structure. Using a dissecting needle, gently pry the operculum off the capsule to reveal a row of structures called peristome teeth. The peristome teeth help regulate the release of spores from the capsule. Draw the capsule again showing the peristome teeth and label these structures. Crush the capsule to observe the large number of small, green spores contained within. 3. Examine a prepared slide of moss archegonia attached to the tip of a gametophyte branch (Atlas Fig. 6.35). Each archegonium consists of an elongate neck, with a canal in it leading down to the egg-containing venter. Draw an archegonium. (NOT the entire branch tip - just draw a single archegonium) Label the neck, venter, and egg, and the stalk that attaches it to the rest of the gametophyte. 4. Examine a prepared slide of moss antheridia, also attached to the tip of a gametophyte branch. Each antheridium has an external jacket of sterile cells, and a large mass of spermatogenous cells which will develop into sperm (Atlas Fig & 6.37). Draw an antheridium (again - just a single antheridium, NOT the whole branch tip). Label its sterile jacket and spermatogenous cells, and the stalk that attaches it to the rest of the gametophyte. 12

13 PART II - FERNS (an example of SEEDLESS VASCULAR PLANTS) Division Pterophyta (ferns), Division Lycophyta Plants in these divisions are typically more complex in form than mosses. These plants contain some specialized vascular tissue and, like all vascular plants, have a dominant sporophyte generation. In ferns, the sporophyte most often consists of a rhizome (a horizontal stem) roots fronds (prominent erect leaves) A mature sporophyte may produce sporangia, usually located on the underside of fronds. Each sporangium consists of a stalk and capsule. Sporangia are often located in clusters called sori (singular=sorus). Each sorus may be covered by a protective flap of tissue growing from the leaf surface; this protective flap is called an indusium. Meiosis of cells within the sporangia results in the production of spores. When the sporangia within a sorus are mature the indusium shrivels to expose them, the wall of each sporangium capsule opens, and the spores are released to the surroundings. Each fern spore germinates to produce a fern gametophyte (sometimes called a prothallus), which is small and photosynthetic. rhizoids extend from the surface and help to anchor the gametophyte each prothallus will produce antheridia and/or archegonia on its surface. Procedure: Sperm produced within the antheridia swim across a surface film of water to reach, and fertilize, an egg within an archegonium. The zygote produced from fertilization grows into an embryo within the archegonium, and then develops into an independently growing sporophyte. 1. Draw a fern specimen and label its fronds. 2. Obtain one sori-bearing pinna (leaf segment) from a fern frond (Atlas pp ). Observe and draw a single sorus on this pinna, using a dissecting microscope Label the sporangia and the indusium (if present). 3. Obtain a small sample of fresh fern sporangia in a small petri dish. Using a dissecting scope at the highest power, observe and draw a sporangium. Label the annulus (the distinctive row of thick-walled cells running across the top of the capsule) and the spores contained within the capsule. After you have drawn a sporangium, place a drop of 90% ethanol onto a group of sporangia in the petri dish, observing them through the scope as you do. After a short time, draw a sporangium again, illustrating the changes that occurred in response to the ethanol. 13

14 4. Obtain a live fern gametophyte from an agar plate where it has grown from a germinated spore. Place it on a slide with a drop of water, and place a cover slip over it. Observe using a compound microscope. Draw the gametophyte. Label the prothallus, and the rhizoids. It is possible that you will be able to find two forms of gametophyte on the agar plate - one form that is larger and vaguely heart-shaped, and a second form that is smaller and more strap-like in shape. If so, draw both types. The heart-shaped form usually produces archegonia, at least initially; the strap-like form carries antheridia. 14

15 Introduction to Plant Diversity - The Seed Plants In last week's lab you had the opportunity to examine two groups of seedless plants - the mosses and ferns. In these groups of plants, it is easy to distinguish the gametophyte generation and the sporophyte generation of the plant's life cycle (in ferns, the two generations are each independently growing structures; in mosses, the two generations are each clearly visible, although the sporophyte generation can only grow when attached to a gametophyte from which it gains its food. In those plants, the dispersal units (diaspores) are the spores of the plant, which are able to survive away from the parent plant because of the desiccation and degradation resistant sporopollenin-containing spore wall. SEED PLANTS: Plants in the divisions Cycadophyta, Ginkgophyta, Gnetophyta, Coniferophyta, and Anthophyta are seedproducing plants. In seed plants, the sporophyte generation is the dominant generation, and is composed of the roots, stems, and leaves that are most commonly recognized as plant vegetative organs. In these plants, there are two distinct forms of spores produced - microspores that form spermproducing microgametophytes, and megaspores that form egg-producing megagametophytes. Megaspores are NEVER released from the parent sporophyte, and spores do not serve as the dispersal units of seed plants. Reproductive processes involving the microgametophyte and megagametophyte eventually produce an embryo enclosed in special protective tissues of the "parent" sporophyte. This structure is a seed. The seed is the dispersal unit (diaspore) of seed plants. Ovules, megasporangia, and egg formation In seed-producing plants the mature sporophyte produces megaspores in a structure called an ovule. The ovule consists of a nucellus (a megasporangium) one or more layers of protective tissue (integuments) surrounding the nucellus a stalk (funiculus) that attaches it to the rest of the sporophyte. Meiosis of certain cells of the nucellus leads to the production of haploid megaspores enclosed within the ovule. The megaspores are never dispersed from the ovule; they remain enclosed in its protective integuments. One or more of the megaspores are involved in producing a single megagametophyte within the ovule. Each megagametophyte eventually produces one or more egg cells The eggs remain within the ovule until their fusion with sperm to produce diploid zygotes. The zygote remains in the ovule and develops into an embryo. 15

16 Microsporangia and pollen grain formation In seed-producing plants the sporophyte also produces microspores in microsporangia. Cells of the sporogenous tissue within each microsporangium undergo meiosis to produce haploid microspores. Each microspore produces a small microgametophyte. The microgametophytes are extremely small (consisting of only a few cells) and develop enclosed within the microspore wall. These structures are pollen grains. Pollination Pollen grains are released from microsporangia and must be transferred to the vicinity of a mature, eggcontaining, ovule. This process is called pollination and occurs by a variety of mechanisms - the pollen grains can be carried by wind, or insects, or birds, etc. Pollen germination Following pollination, the pollen grain germinates by producing an elongate protrusion called a pollen tube that grows into the ovule and extends toward the egg cell. At the same time, microgametophyte development continues, resulting in the formation of sperm that travel through the pollen tube to reach an egg. Seed production When a sperm reaches, and fuses with an egg cell within an ovule, seed production begins. Fusion of the sperm and egg produces a diploid zygote. The zygote undergoes mitosis leading to the formation of an embryo (young sporophyte). The integuments harden to form a protective coat around the embryo and a seed is formed. The hardened integuments are called the seed coat. A mature seed consists of the seed coat (a diploid tissue of the parent sporophyte) an embryo (a very young offspring sporophyte nutrition storage tissues - sometimes derived from the earlier nucellus and/or megagametophyte tissues, or sometimes formed as new tissue produced along with the embryo. Mature seeds detach from the parent sporophyte and are dispersed. The seed coat provides desiccation-resistance and resistance to chemical degradation, thus protecting the embryo within the seed in a manner similar to the protection provided to a spore by the spore wall. Under the appropriate environmental conditions the seed will germinate, and the embryo will grow to produce a new adult sporophyte. 16

17 Gymnosperms versus Angiosperms Seed plants are often informally referred to as gymnosperms (the non-flowering seed plants, in the divisions Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta); and angiosperms (the flowering plants, in the division Anthophyta). Gymnosperms and angiosperms, while they all produce seeds, differ with respect to the location of seed development some of the processes leading to, and including, seed formation some of the tissues present in the vegetative plant body. A major difference between gymnosperms and angiosperms is the placement of the microsporangia and ovules on the parent sporophyte. In gymnosperms they are borne on microsporophylls and megasporophylls. The sporophylls often occur in clusters called strobili, or cones. Clusters of microsporophylls form microstrobili (staminate cones). Clusters of megasporophylls form megastrobili (ovulate cones). In angiosperms, microsporangia and ovules are borne within flowers. Flower structure varies among species, but often consists of the following components: 1. a peduncle, or flower stalk 2. a receptacle, to which other floral parts are attached 3. one or more pistils; each consisting of a pollen-receptive stigma, a style, and an ovary within which ovules are found 4. stamens; consisting of a filament, and a lobed anther (four fused microsporangia) 5. colorful petals, which may serve as attractants for pollinators 6. small, leaf-like sepals LEARNING OUTCOMES: Successful completion of this lab requires that you be able to: demonstrate an understanding of the location of megaspore, megagametophyte, and egg production in seed plants. demonstrate an understanding of the location of microspore, microgametophyte, and sperm production in seed plants. demonstrate an understanding of the influence that non-dispersal of megaspores has on the location of embryo formation in seed plants. demonstrate an understanding of the role of pollen grains in seed formation. identify the reproductive structures (strobili) of a typical gymnosperm, and their components. identify the reproductive structures (flowers) of a typical angiosperm, and their components. compare the location of pollen production in gymnosperms to pollen production in angiosperms, and demonstrate an understanding of how these influence pollen dispersal. compare the location of seed production in gymnosperms to seed production in angiosperms, and demonstrate an understanding of how these influence seed dispersal. compare the diaspores of seedless plants to those of seed plants. 17

18 PROCEDURE: Gymnosperms 1. Examine examples of gymnosperm cones available in the laboratory. Note the scale-like sporophylls and, on megasporophylls, note the depressions where seeds were previously located. Draw one of these cones. Label a megasporophyll; indicate the area where a seed had been located. 2. Examine a slide of a Pinus (pine) megastrobilus (ovulate cone) using a compound microscope (Atlas p. 117). Draw one megasporophyll bearing an ovule. Label the nucellus, the integument, and the micropyle of that ovule. 3. Examine a slide of a Pinus microstrobilus (staminate cone) using a compound microscope (Atlas p. 118). Draw one microsporophyll bearing a microsporangium. Label the pollen grains contained within the microsporangium. Angiosperms 4. Study the model of a flower available in the laboratory to find the sepals, petals, stamens, and pistil. Compare this to the live flowers also available. Remember, few flowers will be as simple in appearance as the idealized flower model (see Atlas p. 133 & 135 for photographs of floral anatomy and p. 138 for examples of flower types). Draw a live flower. Label the peduncle,a sepal, a petal, a stamen (filament and anther), and a pistil (stigma and style). Determine where the ovary is located at the base of the pistil (in some types of flowers, it may be embedded in the receptacle portion of the peduncle). Use a razor blade to cut through the ovary. Use a dissecting scope to observe and draw the ovary and the ovules found within it. Label the ovary wall and the ovules. Put a drop of water on a slide. Scrape the surface of the anther portion of a stamen to remove some pollen grains, and transfer them to the drop of water. Put a cover slip over the drop, and observe with a compound microscope. Draw a pollen grain. 5. Obtain a slide containing a cross-section through an anther carrying mature pollen grains (Atlas p. 141). Draw a single lobe of the anther (one microsporangium). Label the pollen grains. The external layer of each pollen grain is an ornate layer, called an exine. 6. Obtain a slide of germinating pollen grains (Atlas Fig ). Draw a single germinating pollen grain. Label the pollen grain and the pollen tube. Examine a pollen tube carefully to find, draw, and label two dark-red staining sperm. 7. Examine a slide of a cross-section through a Lilium ovary (Atlas p. 142). Draw the ovary. Label the ovary wall and the ovules. 18

19 Seeds and Fruits Plants in the division Anthophyta (angiosperms) have evolved complex mechanisms to provide for protection and dispersal of their offspring. Two key features of this system are the production of embryos within seeds, surrounded by a protective seed coat the production of these seeds in ovaries that mature into fruits Both the hard seed coat and the hard or fleshy fruit walls create an effective buffer preventing harsh environmental conditions from causing damage to the enclosed embryo. In addition, both seed coats and fruit walls often include specific structures that facilitate seed dispersal. Seeds In angiosperms, seeds consist primarily of the external seed coat, the embryo, and (at least initially) a nutritive tissue called the endosperm. The embryo consists of a primary root (radicle) one or two cotyledons the hypocotyl, a stem-like region below the cotyledon(s) connecting to the radicle a plumule above the cotyledons consisting of an embryonic stem (the epicotyl) and attached immature leaves. Plants in the class Monocotyledones (monocots) contain just one cotyledon; plants in the class Dicotyledones (dicots) contain two cotyledons. The endosperm is a nutrient-storing tissue that develops from the triploid primary endosperm nucleus, formed when the two polar nuclei of a megagametophyte fuse with a sperm nucleus. This process does not occur in gymnosperms, and gymnosperm seeds do not include an endosperm. In the seeds of some plants, such as corn, the endosperm forms a large portion of the mature seed and serves as the major source of nutrition for the embryo during seed germination. In the seeds of other plants, such as pea and bean, the endosperm has been almost entirely consumed by the time the seed is mature. In these seeds the cotyledons are enlarged and fleshy and serve as the major nutrient source during germination. 19

20 Fruits In flowering plants, ovules develop into seeds that remain enclosed in an ovary. Fruits are formed when the ovary undergoes changes, becoming either dry and hardened or enlarged and fleshy. This maturation process is generally triggered by signals accompanying seed formation. Characteristics of the fruit in a particular plant will depend, to a large extent, on the characteristics of the ovary from which it develops. For example: the arrangement of the ovules carried in a particular ovary, and their placentation (points of attachment to the ovary wall), will determine the eventual placement of seeds within the fruit. the separate ovule-containing chambers (locules) in the ovary may remain visible in the fruit. an ovary that is embedded within receptacle tissue may result in a fruit that also includes modified receptacle tissue (producing what is known as an accessory fruit). LEARNING OUTCOMES: Successful completion of this lab will require that you be able to: Distinguish the various structures present in a bean seed. Distinguish the various structures present in a corn kernel. Demonstrate knowledge of the defining difference between dicot seeds and monocot seeds. Identify the structures present in a young bean seedling and in a young corn seedling. Demonstrate an understanding of the role of each seed structure in the development of a seedling. Demonstrate an understanding of the development of a fruit from the ovary of a flower. Utilize information about ovary and fruit structure to identify and describe the defining characteristics of various fruit. Recognize and describe some of the ways in which some fruit walls may facilitate seed dispersal (and therefore contribute to species success). 20

21 PROCEDURE: Seeds and seedlings 1. Obtain a soaked bean seed. Remove the seed coat to reveal the two cotyledons that are the prominent internal structures. Carefully separate the two cotyledons and observe the embryo held between them. Draw the opened seed (see Atlas Fig ) Label the cotyledons, the plumule, and the radicle. (In this type of seed very little endosperm tissue is present and it is not readily apparent.) 2. Observe a young bean seedling, and draw it. Label the root system, the hypocotyl, the cotyledons, the stem, and the true leaves. 3. Obtain a soaked corn grain. This grain is actually an entire fruit, because the outer layer is ovary tissue that has become fused to the seed coat of the seed it encloses. Use a razor blade to longitudinally split the soaked grain. Draw the open surface of the corn grain. Label the endosperm and the embryo. (In this type of seed the cotyledon is small, and the endosperm is the primary nutrient source.) 4. Observe a young corn seedling, and draw it. Label the root system and the shoot. (In these seedlings, the hypocotyl and cotyledon do not emerge from the seed, and the leaves wrap around the stem making it impossible to view.) Fruits Botanically, the term fruit refers very specifically to the modified ovary (or sometimes ovary and surrounding tissues) that surrounds/contains the seeds of flowering plants. However there is great diversity in the size, shape, and other characteristics of fruit that can make the fruit of some plants difficult to recognize for what it is. This confusion is added to by the fact that the term fruit is used in the grocery store almost entirely to indicate those fruit that are fleshy, often juicy, and usually sweet. Several items that are actually fruits, but don t fit those criteria, are incorrectly referred to as vegetables. In truth, vegetables should be vegetative organs i.e., parts of the plant that are not directly involved in reproduction, such as carrots (a type of root) or lettuce (leaves). Peppers, on the other hand, are fruits, as can be seen by the fact that they contain seeds. Many fruits are not edible at all, or are not typically eaten by people. Below, there is a key to many of the different kinds of fruits. Your lab instructor will provide you with a variety of different fruits so that you can see some of the characteristics that are used to distinguish them. Some of the terms you will need to know in order to use the key include: simple, aggregate, and multiple fruit simple fruit develop from a single ovary from a single flower aggregate fruit result from many ovaries from a single multipistillate flower multiple fruit develop from ovaries from many flowers in a single inflorescence true fruit vs. accessory fruit true fruit form from ovary tissue only accessory fruit develop from ovary tissue and surrounding tissue (e.g., receptacle tissue) 21

22 dehiscent fruit vs. indehiscent fruit dehiscent fruit open at maturity to release the seeds that have developed within indehiscent fruit do not specifically open to release their seeds; instead the seeds may be released when the fruit tissue itself degrades, or the fruit wall may be thin enough that the young seedling can push through it during seed germination pericarp, endocarp, mesocarp, exocarp the fruit wall that forms from the modification of an ovary wall during fruit formation is called the pericarp, and consists of three layers: endocarp the innermost layer of the pericarp, closest to the seed mesocarp the middle layer of the pericarp exocarp the outermost layer of the pericarp 5. A few types of fruit will be made available for you to examine. For each of these, examine the fruit wall, and briefly describe a way in which it might be involved in seed dispersal. (Common dispersal mechanisms are wind dispersal and animal-related dispersal, either by the fruit being eaten by an animal or by the fruit attaching to an animal s hair.) 6. Use the key below to identify the type of each fruit provided to you by your lab instructor. Key to the Classification of Fruits 1. Simple fruits, formed from a single ovary or carpel Compound fruits, formed from several carpels or ovaries Dry fruits Fleshy fruits Fruit dehiscent Fruit indehiscent Fruit derived from one carpel only Fruit derived from one to several carpels Fruit splitting at maturity along one suture...follicle (e.g., milkweed) 5. Fruit splitting at maturity along two sutures...legume (e.g., peanut) 6. Fruit formed of 2 carpels, separating at maturity, leaving a persistent partition upright between them...silique (e.g., money plant) 6. Fruit formed of several carpels...capsule (e.g., cotton, poppy) 7. Fruit several to many-seeded, breaking at maturity into several one-seeded segments...loment (e.g., tick trefoil) 7. Fruit not as above, generally one-seeded

23 8. Seed coat firmly united with pericarp at all points...caryopsis (e.g., corn kernel) 8. Seed coat not attached to pericarp at all points Pericarp thin, with one or two wings...samara (e.g., elm) 9. Pericarp lacking wings Fruit coat very hard...nut (e.g., hazelnut) 10. Fruit coat not particularly hard Fruit composed of 2 carpels, separating at maturity into two 1-seeded halves or mericarps which are indehiscent...schizocarp (e.g., mallow) 11. Fruit coat thin, not separating; fruit small, 1-seeded... Achene (e.g., meadow rue, Devil s beggar tick) 12. Fleshy part of fruit derived from ovary only Fleshy part of fruit derived, at least in part, from the receptacle Entire ovary becoming fleshy at maturity Outer part of ovary fleshy, inner part stony...drupe (e.g., plum, walnut) 14. Fruit fleshy or juicy, composed of several (usually 10) carpels, each with 2 seeds; rind leathery...hesperidium (e.g., orange) 14. Fruit fleshy, of several carpels, each with one to several seeds; leathery rind lacking...berry (e.g., cranberry, kiwi) 15. Pistils several, separate, non-fleshy, enclosed by the fleshy or semi-fleshy receptacle....hip (e.g., rose) 15. Ovary compound, carpels united Ovary wall fleshy, berry-like, with hard rind...pepo (e.g., pumpkin) 16. Inner part of ovary wall papery or cartilaginous, outer part fleshy, surrounded by and united with a fleshy receptacle...pome (e.g., pear) 17. Many simple fruits, usually achenes or drupes, derived from separate carpels of one flower, located on a single receptacle...aggregate Fruit (e.g., strawberry) 17. Many simple fruits derived from the carpels of separate flowers Flowers borne within an enlarged hollow, fleshy receptacle...synconium (e.g., fig) 18. Flowers borne upon the surface of a more or less fleshy receptacle...multiple Fruit (e.g., pineapple) 23

24 24

25 Vegetative Organs Roots and Shoots Each plant in the division Anthophyta shares the same basic vegetative body plan consisting of a root system, and a shoot system including stems and leaves. However, the division Anthophyta is incredibly diverse, including upwards of 250,000 extant species, and the variability in the structure of these vegetative organs is immense. ROOT SYSTEM Roots are plant organs with a number of functions to perform. One of the major functions of roots is to anchor the plant into the soil on which it grows; another is to efficiently absorb water and minerals from the soil and pass these on to other plant organs. Roots may also be specialized to serve as sites of food storage and perform many other functions as well. In seed plants, the development of a root system begins during seed germination as growth of the radicle portion of the embryo. The root produced at this point is known as the plant s primary root. Lateral roots (root branches) may form from the primary root. These roots originate in a meristematic region located in the center of the original root. In most dicots, the primary root continues to grow over time. This results from activity in the root apical meristem. The apical meristem also produces a protective root cap that covers the apical meristem at the tip of each root. Lateral roots form and also grow by activity of their own root apical meristems. Additional lateral roots may form as branches off lateral roots. In this case the root system has a single main root produced from the primary root, referred to as a taproot. The overall root system is called a taproot system. In most monocots, the primary root ceases growth early in the plant's development and is replaced by adventitious roots. Adventitious roots are roots that originate from stem or leaf tissue instead of from the radicle. Lateral roots then form from these adventitious roots. In this case there is no single main root, but instead several adventitious roots of equal prominence, along with numerous lateral roots. The overall root system is called a fibrous root system. In both types of root system, the ability of the plant to absorb water and minerals from the soil is greatly enhanced by the presence of many root branches, and thus many actively growing root tips. These root tips are vital to absorption because: in these regions the root surface is covered by many single-celled filamentous root hairs that add to the root surface area and can penetrate between soil particles. in older portions of the root, the root hairs have been lost and changes to tissues within the root limit water uptake. 25

26 The overall form of a plant's root system (its root architecture) will influence the extent to which the system provides anchorage for the plant, and the regions of soil from which it will best absorb water and minerals. This form is created by factors such as the extent of branching, where branches form, and the direction of growth of various branches. SHOOT SYSTEM A plant's shoot consists of stems and leaves, and originates from growth of the plumule portion of an embryo during seed germination. The primary function of a plant's stem (including stem branches) is to position leaves and reproductive organs. Leaf positioning is generally designed to optimize photosynthesis by providing exposure to sunlight and access to carbon dioxide. Positioning of reproductive organs is designed to optimize pollination and fruit dispersal. The primary function of leaves is to carry out photosynthesis. Growth of the shoot results from activity in the shoot apical meristem. New cells produced at this meristem may contribute to increases in stem length formation of leaf primordia that expand to form mature leaves formation of axillary buds a bud primordium is produced in the axil of every leaf primordium each bud primordium develops into an axillary bud that includes its own shoot apical meristem each axillary bud may develop into a stem branch, bearing its own leaves In woody perennial plants (shrubs and trees) growing in temperate climates, the leaves may drop from the stems during the cold winters, and growth at the shoot tip may be temporarily halted. In such plants, the winter twig will consist of a terminal bud, and a series of nodes (where leaf scars and axillary buds will be visible) and internodes. There is much more variability in the form of plant shoots, especially in leaf form and the placement of leaves on the stems, than what is seen in plant roots, presumably a reflection of the much more variable environment within which shoots are located. Leaf Phyllotaxy - the arrangement of leaves on a stem. The points where leaves are attached to stems are nodes; stem regions between nodes are internodes. How many leaves are located at each node, and their orientation with respect to one another, will have a strong effect on the extent to which lower leaves are shaded by upper leaves (and thus ineffective at photosynthesis) Major types of phyllotaxy are spiral and alternate phyllotaxy - one leaf per node opposite phyllotaxy - two leaves per node whorled phyllotaxy - more than two leaves per node 26

27 Leaf Morphology - the overall form and appearance of a leaf A typical leaf includes a narrow petiole that attaches the leaf to a stem and a broader, flattened leaf blade. Some leaves may be sessile, i.e., have a blade but no petiole. The leaf blade may be many different shapes, and may be simple (in which the entire blade exists as a single unit), or compound (in which the blade is divided into smaller segments, or "leaflets") Compound leaves may be: palmately compound (in which all leaflets are attached to the end of the petiole), or pinnately compound (in which the leaflets are attached at different points along the length of an extension of the petiole) The pattern of major veins running through a leaf (leaf venation) may be netted venation (in which the major vein or veins entering the leaf form clearly visible branches that extend toward the leaf margins), or parallel venation (in which the major veins entering the leaf do NOT form clearly visible branches, and remain roughly parallel to one another from the leaf base to the leaf tip) Branching Axillary buds are produced in the axil of every leaf. The axillary buds may remain inactive (especially true of buds close to the stem tip), or may become active and produce stem branches. Since the location of axillary buds is determined by the location of leaves, differences in phyllotaxy are reflected in differences in branching patterns. LEARNING OUTCOMES: To successfully complete this lab, you will need to be able to recognize the primary root and lateral roots of a typical plant, and compare the origin of the two demonstrate an understanding of the role of the root apical meristem, and of the root cap that covers it identify the root hairs present near root tips, and demonstrate an understanding of their importance to plant success compare taproot systems with fibrous root systems, with respect to structure and its impact on root function identify the main functions of stems and leaves identify the structures found at a shoot tip, and the role that each plays in shoot growth demonstrate an understanding of the role of phyllotaxy in plant success demonstrate an understanding of the variability of leaf structure, and its connection to plant strategies for success 27

28 Procedure: 1. Obtain a young, recently germinated, radish seedling. Use a razor blade to separate the young root from the rest of the seedling. Put a drop of water on a slide, place the root into this drop, and place a cover slip over it. Use a compound microscope to observe and draw the root. Label the root hairs, and indicate the location of the root cap. 2. Obtain a young corn seedling, and use a dissecting microscope to observe its young root. Once again, you should be able to see the root hairs emerging in the region just past the root tip. If you look in the upper region of the root, you should also be able to see the lateral roots beginning to emerge from the primary root surface. Draw the root. Label the root hairs and the newly developing lateral roots. 3. Obtain a slide of a lateral root. Using a compound microscope, you should be able to see a cross-section of the original root, and a lateral root that originates from tissues near the center of the original root and has grown through the surface layers. Draw this specimen. Label the main root and the lateral root. 4. Observe, and briefly sketch, the examples of a taproot system and a fibrous root system in the plants provided by your instructor (see Atlas Fig ). 5. Obtain a prepared slide of a Coleus stem tip (see Atlas Fig ). Draw the stem tip. Label the apical meristem, the leaf primordia, and the bud primordia. 6. Observe and draw a young bean plant. Label the terminal bud at the shoot tip, an internode, and a node. What type of phyllotaxy is present in this plant? In bean plants, the first leaves produced differ in structure from later leaves. Make separate drawings of the first (oldest) leaves, and the second leaves. On each drawing, label the petiole and the blade. Are the first leaves simple, pinnately compound, or palmately compound? Are the second leaves simple, pinnately compound, or palmately compound? What type of venation is present in these leaves? 7. Observe and draw a winter twig from a woody perennial (see Atlas Fig ). Draw this twig. Label the terminal bud, an internode, a node, a leaf scar and an axillary bud. 8. Two additional plants will be provided to you by your instructor. For each plant, answer the following questions: What phyllotaxy is present? Do the leaves have a petiole, or are they sessile? What type of venation is present? 9. Additional specimens will be available to provide an opportunity to see some of the variation that occurs in roots, stems, and leaves, among different species and different stages of development. Observe these plants and note how their vegetative organs have been modified to carry out additional functions. 28

29 Plant Anatomy Cells and Tissues Plant organs have major differences in their basic form, and the functions they carry out. However, the cells and tissues these organs are composed of have strong similarities. All organs consist of tissues from three tissue systems dermal tissues, ground tissues, and vascular tissues. Every plant organ has a protective dermal tissue at its surface. The cell types and arrangements in dermal tissue are generally designed to prevent water loss and/or pathogen entry, while simultaneously providing an opportunity for gas exchange. In organs that have undergone only primary growth, that dermal tissue is one to a few layers of an epidermis. In above ground organs the surface of the epidermal cells is covered with a waxy cuticle to resist water loss There are openings in the epidermal layer (stomates) to allow for gas exchange. In organs that have undergone secondary growth, the epidermis has been replaced by a periderm. The periderm consists of many layers of cells that are continually replaced by a cork cambium, and the walls of all of these cells include hydrophobic substances to resist water loss. Much of the metabolic activity of a plant organ is carried out in ground tissue and this type of tissue generally makes up most of the organ s interior. Ground tissue can consist of a mix of a variety of cell types, with the specific types present dependent on the plant organ and its function. E.g., most of the ground tissue in leaves is specialized to carry out photosynthesis. Embedded within the ground tissue, each plant organ contains some vascular tissue. Xylem is specialized for the distribution of water and mineral nutrients throughout the plant. The cell walls of xylem cells contain lignin and are extremely rigid. These cells provide the structural support needed for upright growth of cells. Phloem is responsible for the transport of plant metabolites through the plant. Although roots, stems, and leaves are all composed of these three tissue groups, the specific cell types and arrangements of cells vary in the different organs, reflecting the unique functions of each. 29

30 Root Anatomy The surface of roots is formed by a layer of epidermis, usually with little or no cuticle at its exterior. Present under the epidermis is ground tissue called the cortex. The cortex consists of mostly unspecialized parenchyma cells, which often provide a site for storage of starch and similar metabolic products. The innermost layer of the cortex, the endodermis, has cells with lignified and/or suberized cell walls which help to control water movement through roots. Directly next to the endodermis is a layer of pericycle. The cells of the pericycle undergo rapid division to produce lateral roots. In the center of roots is a vascular cylinder. This consists of a central core of xylem surrounded by patches of phloem. In the roots of monocots, an additional area of ground tissue (called the pith) is present in the center of the vascular cylinder. Leaf Anatomy In a typical leaf, the epidermis is a single layer of cells with a significant cuticle. The thickness of the cuticle varies, being very thick in plants adapted to arid environments and very thin or nonexistent in plants adapted to a watery environment. Throughout the epidermis, stomata are present, formed by specialized guard cells. The majority of the leaf interior is composed of photosynthetic parenchyma cells called mesophyll. In many plants the layers of mesophyll near the upper leaf surface are composed of elongate parenchyma cells packed tightly together, called the palisade mesophyll. This is an efficient arrangement for collection of light as it hits the leaf surface. The layers of mesophyll near the lower leaf surface are composed of more nearly spherical parenchyma cells that are loosely packed, and are called the spongy mesophyll. This cell arrangement facilitates the movement of gases through the mesophyll. Directly beneath each stomate at the leaf surface there is usually a small air space, called a stomatal chamber, where no mesophyll cells are present. This area aids in gas exchange. The vascular tissue of the leaf is found in veins that are surrounded by the mesophyll tissue. Each vein consists of xylem and phloem The xylem is generally closest to the upper leaf surface. The phloem is generally closest to the lower surface. Most veins are supported within the mesophyll by a bundle sheath of smaller, tightly packed parenchyma cells, or collenchyma or sclerenchyma cells. In some monocots, usually grasses, the arrangement of leaf tissues differs from the typical structure described above. In these plants, there is no distinction between palisade and spongy mesophyll and the bundle sheath surrounding the veins is formed by a group of large parenchyma cells with a special role in photosynthesis. 30

31 Stem Anatomy The surface of stems of consists of an epidermis, similar in structure to that of the leaves, though with fewer stomates present. In a typical dicot: Vascular tissue is present in vascular bundles arranged in a ring passing through ground tissue. Each vascular bundle includes phloem (located nearest the stem surface) and xylem (located nearest the center of the stem). Each vascular bundle may be surrounded by a ring of fibers forming a supportive bundle sheath. The region of ground tissue between the epidermis and the vascular bundles is the stem cortex. This tissue may be photosynthetic. The region of ground tissue in the center of the stem is the pith. This tissue is not photosynthetic, but may be involved in storage of starch or other metabolites. There is also ground tissue located between adjacent vascular bundles. This is the interfascicular parenchyma, or pith rays. In a typical monocot: The vascular bundles are arranged in two or more concentric rings within the ground tissue, or are scattered randomly through the ground tissue. In these stems there is no distinct cortex or pith regions and the ground tissue is collectively called the ground parenchyma. In the majority of these plants, the vascular bundles have bundle sheaths enclosing them. Secondary Growth in Stems Some dicots undergo secondary growth, and the addition of secondary tissues means that older stems differ significantly from younger ones. In these plants, as the stem matures a vascular cambium forms (passing between the xylem and phloem in each vascular bundle) and begins to produce new cells. New cells formed to the outside of the ring of vascular cambium become a part of the secondary phloem. New cells formed to the inside of the vascular cambium become a part of the secondary xylem. In most species, the amount of secondary xylem produced greatly exceeds the amount of secondary phloem produced. It is secondary xylem that is commonly known of as wood, and it is the strength and rigidity of the cell walls of tracheids and vessel members that gives wood its strength. During seconday growth a second lateral meristem, the cork cambium, develops from cells within the cortex. The cells of the cork cambium divide, producing several layers of cork cells to its exterior. These cell layers form the periderm. In a stem that has undergone secondary growth, the bark is all of the tissues outside of the vascular cambium. This includes the periderm, the cortex (if any of it remains), and the secondary phloem. 31

32 LEARNING OUTCOMES Successful completion of this lab requires that you be able to: Demonstrate an understanding of the function of the dermal tissues, ground tissues, and vascular tissues in plant organs. Identify the tissues found in a typical root, and describe their arrangement. Demonstrate an understanding of the differences between palisade mesophyll and spongy mesophyll, and their roles in photosynthesis. Demonstrate an understanding of the role of stomates in leaves, and how this relates to their placement with respect to other leaf structures. Demonstrate an understanding of the role of leaf veins, and how this relates to their placement in leaves. Identify the primary tissues found in a typical dicot stem, and describe their arrangement. Demonstrate an understanding of secondary growth in stems, and describe the arrangement of secondary tissues in a woody dicot stem. Procedure: 1. Obtain a slide of a mature Ranunculus root x.s (Figs & 6.183). Draw a wedge shaped section of this root. Label the epidermis, cortex, endodermis, pericycle, phloem, and xylem. 2. Examine a slide of a Syringa leaf x.s. and draw a portion of the leaf (similar to Atlas Fig ). Label the upper epidermis, lower epidermis, stomate, guard cell, palisade mesophyll, spongy mesophyll, xylem, phloem, and bundle sheath. 3. Examine a slide of a x.s. of a Ranunculus stem. Ranunculus is a dicot with a typical arrangement of mature tissues. It is somewhat unusual however, in that, at maturity, the pith partially disintegrates, leaving a lysigenous space in the center of the stem. The cortex of this stem is composed of photosynthetic parenchyma, which is more loosely arranged than the cortex of some other stems. Draw a wedge-shaped portion of this stem. Label the epidermis; the cortex; a vascular bundle including xylem, phloem, and a bundle sheath; and the pith. 4. A slide of a Zea stem in x.s. will provide a good example of the arrangement of tissues in a monocot. Draw a wedge-shaped portion of this stem (Atlas Fig ). Labeling the epidermis; the ground parenchyma; and a vascular bundle including xylem, phloem, and a bundle sheath, which includes a number of fibers located next to the phloem. 5. Tilia is a dicot that undergoes a considerable amount of secondary growth (similar to Atlas Fig ).. A slide of an older Tilia stem should exhibit some of the secondary tissues. Draw a wedge-shaped portion of this stem. Label the periderm, cortex, secondary phloem, vascular cambium, secondary xylem, and pith. 6. Examine a slice taken from a woody dicot stem. Sketch this stem. Label the bark, the sapwood, and the heartwood. Note the annual rings present within the wood. These result from differences in the size of xylem cells produced at different times in the growing season. 32

33 Animal Development Animals are multicellular organisms that develop from a fertilized egg (zygote) through a complex, geneticallycontrolled process called embryogenesis. Patterns of embryological development may be shared by animals derived from a common ancestor, and embryological evidence has been extremely useful in constructing hypotheses about animal evolution. In this lab, we will examine early development of a sea star. Refer to your textbook (Ch. 32, Figs and 32.9) and lab atlas (p.23) to help you understand and interpret the embryos you will observe. Sea stars are members of the phylum echinodermata which also includes sea urchins and sea cucumbers. Echinoderms have been used extensively in the study of embryology, in part due to the transparency of their embryos. Echinoderms and chordates comprise the deuterostomes, a major lineage of coelomate animals. Deuterostomes are bilaterally symmetrical coelomates that develop from embryos with three germ layers. Deuterostomes have radial, determinate cleavage. The first opening in the embryo, called the blastopore, usually becomes the anus of the animal. The mouth forms from a second opening. The protostomes are a second major lineage of coelomate animals that differ fundamentally from deuterostomes in details of their embryology. In protostomes, the blastopore becomes the mouth and cleavage is spiral. Molluscs, annelids, and arthropods are the major protostome phyla. Sea stars are dioecious and they release gametes into the surrounding seawater where fertilization occurs. Unfertilized eggs contain a large nucleus called a germinal vesicle (GV). The nucleolus is prominent within the GV. Upon fertilization, the germinal vesicle breaks down and the fertilization membrane elevates. The fertilization membrane acts to prevent a second sperm from entering the egg. After fertilization, the zygote begins to divide via mitosis. These early cell divisions are called cleavages and the cells that result are called blastomeres. The first cleavage results in a two cell stage embryo. A four cell stage embryo results from the second cleavage. The planes of the first two cleavages are perpendicular to one another and produce equally sized blastomeres. The third cleavage occurs at right angles to the first two and results in an eight cell embryo. Sea stars undergo radial cleavage, so the daughter cells produced the third cleavage lie directly on top of one another. Cleavages continue until the embryo consists of a solid ball of cells called a morula. Sometime around the 32 cell stage, a fluid-filled cavity called the blastocoel begins to form in the interior of the embryo. A blastula stage embryo is a hollow structure consisting of a single cell layer surrounding the blastocoel. Gastrulation is a process by which an embryo becomes layered. Gastrulation can occur in a number of different ways. In the sea star, the first sign of gastrulation is a flattening of one side of the blastula. This flattened area will invaginate into the embryo, creating a new internal space, the archenteron, which will form the gut of the animal. As the wall of the embryo pushes into the interior, a two layered embryo is formed. The outer layer, called ectoderm, will become the nervous system and outer covering of the animal. The inner layer, called endoderm, will develop into the gut. As invagination progresses, the archenteron elongates. Eventually it will make contact with the interior wall of the embryo and a second opening will form the mouth of the animal. The blastopore will become the anus of the animal. A third germ layer called mesoderm will form from coelomic pouches that bud from the walls of the archenteron. This sea star embryo will develop through two free-swimming, feeding larval stages. The bipinnaria larva is bilaterally symmetrical with a complete gut. Ciliary bands on the larva function in locomotion and to sweep particles into the mouth for ingestion. This larva develops into a brachiolaria larva, which will eventually settle to the bottom and metamorphose into a juvenile sea star. 33

34 LEARNING OUTCOMES: Successful completion of this lab requires that you be able to: examine prepared slides, locate developmental stages of the sea star, and make accurate, correctly labeled drawings of the stages. understand the sequence of events that occurs during development. recognize and name the developmental stages. describe the difference between radial and spiral cleavage. Procedure. Obtain a prepared slide containing developing sea star embryos at all stages. Most of the embryos will be in early cleavages, but by scanning the slide you should be able to find all stages discussed. In addition to the labels below, each drawing must have a title (e.g. blastula) and the total magnification indicated. 1. Find an unfertilized egg on your slide. Sketch the egg labeling the plasma membrane, the ooplasm (= egg cytoplasm), the germinal vesicle (nucleus), and the nucleolus. 2. Locate an egg that has been fertilized and label the fertilization membrane. Where did the fertilization membrane come from and what is its function? 3. Find and sketch examples of two cell, four cell, and eight cell embryos. Label the fertilization membrane and a blastomere on each sketch. Scan the slide and observe embryos at about 16 cells and 32 cell stage but do not draw them. 4. Locate and draw a blastula stage embryo. Label the blastocoel. 5. Find an early gastrula, sketch it, and label the blastocoel, archenteron, blastopore, ectoderm, and endoderm. 6. Sketch a late gastrula stage embryo. Label the blastocoel, archenteron, blastopore, ectoderm, and endoderm. The embryo should have coelomic sacs budding from the developing gut. Label the area from which mesoderm will develop. 7. Find several bipinnaria larvae and sketch one from a lateral view. Label the oral lobe, mouth, esophagus, stomach, intestine, and anus. What type of symmetry is present in the larva? In an adult sea star? 34

35 Basal Animals: Phylum Porifera (sponges) INTRODUCTION Members of the kingdom Animalia, which are all multicellular eukaryotes, are thought to have evolved from single-celled eukaryotes (protistans). Among the simplest of all animals are the members of the phylum Porifera (sponges). Sponges are little more than loose aggregations of cells with no true tissue organization. There is some division of labor amongst the cells, but there are no organs. The basic body form of all sponges consists of a sac-like structure with an outer layer of flat cells called pinacocytes, an inner layer of flagellated choanocytes, and a middle mesohyl layer containing amoeboid cells that produce skeletal structures of various sorts. These layers are perforated by a large number of small pores, which give the group the name Porifera (= pore bearer). The cavity of the sac is called the spongocoel and it has at least one opening to the outside, the osculum. Figure 1. Cutaway diagram of an asconoid sponge. After Margulis & Schwartz, In all sponges, the body is designed to facilitate filter feeding. Water is drawn into the pores and canals by the beating of the choanocytes' flagella. The water moves into the spongocoel and is eventually forced out through the osculum. As the water passes across the choanocytes, food particles (microscopic algae, bacteria, and organic debris) are trapped by the collar and taken into food vacuoles via phagocytosis for intracellular digestion. There are three morphological types of sponges (refer to Atlas Fig. 7.3). Asconoid sponges are the simplest. The body is vase-shaped with a central space (spongocoel) and a single large opening called the osculum through which water exits. Water is drawn in through numerous pores called ostia; the water passes into the spongocoel and exits the sponge via the osculum. The body wall of syconoid sponges is folded into canals to increase the surface area of choanoderm. Leuconoid sponges have the largest ratio of choanoderm surface area to water volume. In these sponges, the large central spongocoel has been replaced with numerous small flagellated chambers. The canals formed by the folded body wall are extensively branched in leuconoid sponges. The more complex internal structure of leuconoid sponges increases surface area relative to volume, slowing the flow of water through the canals and making it easier for the choanocytes to trap food. The vast majority of living sponges have the leuconoid body plan. Asconoid and syconoid sponges are far less common and tend to be small. 35