Laboratory. Plant Structure

Transcription

1 Laboratory 4 Plant Structure

2 2 Laboratory 4: Plant Structure OBJECTIVES After completing this lab you will be able to: 1. Differentiate between dicots and monocots within the following categories: a. root structure b. stem structure 2. Note differences in the vasculature structure of monocots and dicots 3. Know the differences between monocot and dicot angiosperm flowers LAB PREPARATION In preparation for this laboratory you should do the following: 1. Read and study this laboratory. 2. Read Chapter 35 in Campbell (7 th Edition). 3. Bring your downloaded image pdf file to lab. 4. Bring personal protective gear (lab coat, goggles, gloves) to lab. 5. Bring to class four plants (two monocot species and two dicot species) not seen in the laboratory. (They may be whole plants or cuttings.) INTRODUCTION This lab and the next lab (Lab 5), examine the general morphology, anatomy, and physiology of some typical angiosperms. You can help yourself learn this material by not limiting your observations to the laboratory. The next time you sit down to take a break between classes look around at the plants surrounding you. Ask yourself, Is this an angiosperm Im looking at? If so, What kind is it, a monocot or a dicot? Is it a herbaceous plant or does it exhibit secondary growth? What kind of leaves does it have? Do these leaves show any obvious modifications? What kind of flowers does it possess? How does this plant reproduce? These are just a few of the questions you might ask. questions this way will likely tell you two things: (1) plants CAN be interesting, and (2) there s no such thing as a typical plant. Asking A. Apical and Primary Meristems Within the germinating seeds you watched grow last week, there were two regions of rapid cell division. One region was located near the base of the plumule (shoot of germinating seedling) and is called the shoot apical meristem (Gk. merizein, to divide). The other region

3 was located near the tip of the radicle and is called the root apical meristem. These two perpetually young meristems produce three types of primary meristems: protoderm, ground meristem, and procambium. These primary meristems are the first three specific tissues formed in the embryo. The term primary refers not to first, but to primary growth - the growth in length, as opposed to secondary growth, which is growth in girth. As the young sporophyte continues to grow, these primary meristems divide, giving rise to even more specific tissues. However, before discussing specific plant tissue types, we will review the types of cells that make up these tissues. B. Common Plant Cell Types 1. Parenchyma A parenchyma cell is the kind of cell you probably envision when you think of a typical plant cell. It is a living, relatively undifferentiated cell with a nucleus, normal cytoplasm, and only primary (abbreviated as 1 o ) cell walls that are non-lignified. (Lignified refers to the presence of lignin, which is a rigid polymer that serves to increase the strength, water impermeability, and resistance to decay of the cell wall.) Nearly every plant tissue contains some parenchyma cells, and in many tissues, it is the dominant cell type. 2. Collenchyma Collenchyma cells are extremely similar to parenchyma cells except that they have unevenly thickened cell walls. Sometimes the 1 o wall can be quite thick, forming tough, supportive tissues. Collenchyma is usually found in the periphery of the plant (where it can support most efficiently) and it often forms a layer of tissue directly beneath the epidermis. 3. Sclerenchyma There are two types of sclerenchyma: fibers and sclereids. Both have thick secondary (2 o ) cell walls which are often lignified. Fibers, however, are long unbranched cells which are normally dead at maturity. They are often found in vascular tissue where they function in support. Sclereids, on the other hand, are shorter cells that can sometimes be branched and living (although most types are dead). Their primary function is protection against herbivores, e.g., the hard seed coat of seeds is normally composed of sclereids. 4. Tracheids and Vessel Members These two types of cells are responsible for transporting water and minerals from the roots to the rest of the plant. They are elongated cells with lignified cell walls and are dead at maturity. When they die, their nucleus, cytoplasm, etc. is absorbed or washed away, Laboratory 4: Plant Structures 3

4 4 Laboratory 4: Plant Structure leaving only the cell wall. The cell then functions as a tube for transporting the water and dissolved minerals. Vessel members differ from tracheids by having a perforation (a hole) at each end of the cell where it connects with the adjacent vessel members. 5. Sieve Tube Members These cells function to transport the sugars that are produced in the photosynthetic parts of the plant, to other plant areas. Like tracheids and vessel members, they are elongated cells. However, sieve tube members are not dead cells, nor do they have lignified cell walls. At maturity, sieve tube members lose their nucleus and other cellular components, such as ribosomes and vacuoles. They develop special end-walls (containing pores), called sieve plates, which connect them to adjacent sieve tube members. Sieve tube members are only found in angiosperms. In the other vascular plants, a similar cell type called sieve cells, lacking sieve plates, is present. 6. Companion Cells Companion cells are so named because they accompany sieve tube members. Because the sieve tube members have lost their nucleus, ribosomes, etc., it is believed that the companion cell controls both their metabolism and their loading and unloading of nutrients. Companion cells have a dense, granular cytoplasm and are tightly bound via many cytoplasmic connections to their sister sieve tube members. 7. Cork Cells Cork cells look like small boxes. They are dead at maturity, and have cell walls which contain suberin, a compound that is highly hydrophobic (water resistant). Cork cells originate from a secondary meristem tissue called phellogen (or cork cambium) and function to protect the outer surface of the plant. C. Plant Tissue Arrangement All the various cell types and tissue types you will encounter in this laboratory are part of a simple three-part tissue system. As was pointed out earlier, the young sporophyte is made up of three primary meristems: the protoderm, the ground meristem, and the procambium. These three meristems are the precursors to three tissue systems, i.e., every tissue can be traced back to one of these three meristems. The three tissue systems are: the dermal tissue system (originating from the protoderm), the ground tissue system (originating from the ground meristem), and the vascular tissue system (originating from the procambium).

5 In todays lab you will be examining the arrangement of tissues in: (1) the monocot root, (2) the dicot root, (3) the monocot stem, and (4) the dicot shoot and stem. Learning these four patterns of tissue arrangement is not difficult, once you realize that they are all just elaborations of the three tissue systems. D. Primary Growth in the Young Sporophyte (Refer to Figures 4-1 and 4-2) 1. Dermal Tissue System The Greek word derma means skin, or hide. The dermal tissues, then, are those that form an outer protective covering, or skin, on the plant body. There are only two types of dermal tissue, the epidermis and the periderm. (You will always find one OR the other on the outside of a terrestrial plant.) The periderm is a secondary growth tissue, therefore, we will wait and describe it in the next section, The Vascular Cambium and Secondary Growth. a. Epidermis The epidermis is the outermost layer of cells on stems and roots that have not yet experienced an increase in girth, i.e., secondary growth. It can be considered the plants original equipment. That is, it is the first outer protective coating a plant stem or root will have. Furthermore, it is the only type of dermis found in a plant stem or root that does not undergo secondary growth. The epidermis is usually only one cell layer thick (normally parenchyma cells). However, in some plants like orchids, the epidermis can be several cells thick. In stems, the outer surface is normally covered with a waxy cuticle that reduces water loss and protects the plant from harmful ultraviolet radiation. In roots, where such protection is not necessary the cuticle is often absent, and root hairs emerge from the mature epidermal cells. 2. Ground Tissue System Ground tissue (also known as fundamental tissue) normally makes up the bulk of the young (herbaceous) plant body. But in many older plants with secondary growth, e.g., trees, the vascular tissue dominates. The various types of cells that make up the ground tissue (parenchyma, collenchyma, and sclerenchyma) function in wound healing, storage (both photosynthetic products and metabolic byproducts), strengthening, and support. Generally, the ground tissue in roots and stems is separated into two categories, the cortex and the pith, according to its location. a. Cortex Cortex is the name given to the ground tissue found between the epidermis and the ring of vascular tissue inside a stem or root. In the Laboratory 4: Plant Structures 5

6 6 Laboratory 4: Plant Structure Figure 4-1. Cross-section of a typical herbaceous monocot root (left) and a dicot root (right). Note that most dicot roots lack a pith and have a solid core of vascular tissue. Figure 4-2. Cross-section of a typical herbaceous monocot stem (left) and a dicot stem (right). The fasicular and interfasicular cambium are meristematic tissues. Note that most monocot stems do not have a ring of vascular tissue and therefore lack pith.

7 monocot stem, where the vascular bundles are scattered, and there is no ring of vascular tissue, the term cortex is inapplicable and the tissue is simply called ground tissue. In roots, the innermost layer of the cortex is called the endodermis. Unlike the rest of the cortex, the endodermis is compactly arranged and lacks intercellular air spaces. It is characterized by the presence of a Casparian strip on the anticlinal walls (the walls perpendicular to the roots surface) of its cells. The Casparian strip is a band of 1 o cell wall impregnated with a fatty substance called suberin which is impermeable to water. The presence of the Casparian strips, along with the compacted arrangement of cells, forces any substance entering or leaving the vascular cylinder to pass through the endodermal cells (via their cell membrane or plasmodesmata). With this arrangement, the endodermis can selectively control the passage of many solutes, e.g., ions, sugars. In many plants (particularly monocots), as the root grows older, the primary walls of the endodermis become completely suberized. Later still, a 2 o wall is often deposited and lignified. (Like suberin, lignin is impermeable to water.) b. Pith Pith is the term used for the ground tissue located in the middle of the ring of vascular tissue. Sometimes a pith is absent. There is no pith in a monocot stem, for reasons described above, and there is no pith in most dicot roots. (Most dicot roots have a solid core of vascular tissue.) 3. The Vascular Tissue System The vascular tissues are the tissues associated with transport. Just as the vascular system in higher animals conducts blood throughout the animals body, the vascular tissues in plants move water (with dissolved food molecules and/or nutrients) throughout the plants body. The vascular system is often organized into bundles, either in a ring, or scattered throughout the ground tissue. The bundles are made up of two tissue types: xylem and phloem. a. Xylem Xylem is the tissue that functions to conduct water and minerals from the roots upward to the other parts of the plant. Common xylem cell types include: tracheids, vessel members, parenchyma, and fibers (for support). Xylem is almost always located medial to (on the inside of) the phloem. Laboratory 4: Plant Structures 7

8 8 Laboratory 4: Plant Structure b. Phloem The phloem is the tissue that functions to transport the sugars made by photosynthesis (principally in the leaves) to the other parts of the plant. Common cells in the phloem are sieve tube members, companion cells, parenchyma, and fibers (for support). It is almost always located outside the xylem. c. Pericycle The pericycle is a ring of tissue (usually 1-3 cell layers thick) in the root, directly inside the endodermis and surrounding the vascular bundles. It does not function in transport like the xylem and phloem (although it does conduct water and minerals from the endodermis to the xylem). Rather, the pericycle is a meristematic tissue that can give rise to lateral roots, or in the case of woody plants (see below), contribute to the vascular cambium and periderm. E. Vascular Cambium and Secondary Growth Many plants, in particular many dicots, get fatter with age. In plants, the increase in girth is called secondary growth. Primary growth involves increasing in length, i.e., getting taller. Except for a few notable exceptions, e.g., palm trees, monocots do not increase in girth. When they do, however, it is not referred to as secondary growth since the growth process does not involve a vascular cambium. In dicots, plants that undergo secondary growth are said to be woody, as opposed to those that do not, which are called herbaceous. The process of becoming woody begins when the procambium between the xylem and phloem (in the vascular bundles) continues to divide (producing more xylem and phloem), and merges with quiescent meristematic cortical tissue between the bundles. This forms a continuous cylinder of meristematic tissue called the vascular cambium. As the cells of the vascular cambium continue to divide, those cells pushed to the inside differentiate into xylem, while those pushed to the outside differentiate into phloem. This new xylem and phloem is referred to as secondary xylem and secondary phloem, distinguishing it from the primary tissues in the original vascular bundles. Eventually, the secondary xylem and phloem become the dominant tissues in the plant. If the plant experiences a variation in temperature or rainfall over the year (usually associated with seasonal change), the morphology of the secondary xylem can change slightly, producing the familiar annual rings of tissue observed when a tree is cut down. These rings of xylem are collectively called wood. In contrast, the layers of annual phloem tissue normally change very little in morphology.

9 Laboratory 4: Plant Structures 9 Figure 4-3. Cross-section of a typical woody dicot stem which displays secondary growth. As the girth of a plant continues to increase, the growing epidermis that originally covered the plant normally can not keep up (by cell division). Eventually, it is stretched and broken. As a result, the plant needs to produce a new skin. Cells in the underlying cortex then become active, dividing to produce a ring of tissue called the periderm. The actively dividing cells of the periderm are called the phellogen, or cork cambium. These cells produce 1-5 layers (usually 2) of cells to the inside which are called phelloderm, and many, many layers of cells to the outside called phellem, or cork. While the phelloderm remains alive, the phellem, which is composed of cork cells (containing suberin), dies and collapses, forming a new water-impermeable skin to protect the growing woody plant. All of the tissues outside the vascular cambium are collected referred to as bark.

10 10 Laboratory 4: Plant Structure F. Monocotyledons and Dicotyledons Botanists divide angiosperms into two groups: monocotyledons and dicotyledons. Although you probably do not recognize them as such, you are already familiar with many monocots and dicots. As the names imply, the two groups differ in that the monocots have seeds with one cotyledon and the dicots have seeds containing two cotyledons. The word cotyledon literally means seed leaf, so you may want to think of it as the type of leaf found in the seed of a plant. However, these are not the same kind of leaves that are normally seen on plants. Generally, the function of true leaves is to photosynthesize and produce food for the plant. In contrast, the cotyledon of a monocot functions as a food-digesting organ, while the cotyledons of a dicot serve to store food for the developing embryo. Table 4-1. Comparison of monocot and dicot characteristics. Monocots 1 cotyledon Vascular bundles in the stem are scattered Vascular cambium is not produced Flower parts in 3 s, or multiples of 3 Leaf venation is generally parallel Dicots 2 cotyledons Vascualr bundles in the stem are arranged in a ring Vascular cambium can be produced, so secondary (woody) growth is common in the dicots Flower parts generally arranged in 4 s and 5 s Leaf venation is generally net-like In addition to the number of cotyledons, monocots and dicots normally have other morphological differences that you can use to distinguish between them (Table 4-1). Please note, however, that the characteristics of leaf venation and flower parts are not absolutely reliable. Occasionally you may encounter a monocot with net venation, or a dicot with three flower petals. Be certain, therefore, you use a combination of characteristics before making a determination.

11 Laboratory 4: Plant Structures 11 LABORATORY EXPERIMENTS You should work individually for all experiments. Be sure to look at the posters of the histology of the stem and the root put in the lab. Experiment 1: Monocotyledons and Dicotyledons PROCEDURE 1. Examine the monocot plants (Trimeza martinicensis) on display and identify the stem, leaves and flower. 2. Sketch and label the leaf. Submit as part of your lab summary 3. Examine the dicot plant (Hibiscus spp.) on display and identify the stem, leaves, axillary buds (lateral branch buds), and flower. 4. Sketch and label the plant. Submit as part of your lab summary. Experiment 2: Macroscopic Anatomy of Young Roots PROCEDURE 1. Examine the mustard or lettuce seedlings on display under the dissecting microscope. 2. Identify the root hairs. Root hairs greatly increase the surface area and therefore the absorptive powers of the root. Virtually all the absorption of water and minerals occurs by way of the root hairs in herbaceous (non-woody) plants. Experiment 3: Microscopic Anatomy of Roots PROCEDURE 1. Examine a prepared slide of a longitudinal section of the corn (Zea) root-tip under the low power of your compound microscope. 2. Identify the root cap, and the region of cell division (apical meristem). 3. Scan up and identify the region of cell elongation, and the region of cell maturation. Root hairs are normally only present in the region of cell maturation.

12 12 Laboratory 4: Plant Structure 4. Re-examine the cells in the region of cell division under high power (100X or 400X). Notice that some of the cells are undergoing mitosis. 5. Also, take a closer look at the cells of the root cap. Notice that a number of the centrally located cells have large starch grains within them. These are called statoliths, and help the root determine which way is up. 6. Examine a prepared slide showing a typical monocot root in cross-section. Locate the inner stele, which is separated from the cortex by the endodermis. (Stele refers to all the tissues within the ring of endodermis.) 7. Notice that only the outer wall of the endodermis remains nonlignified in this monocot root. What other tissues appear to be lignified? 8. In this monocot, the xylem and phloem tissues are arranged in a ring. 9. Locate the single layer of cells with thickened outer walls just within the epidermis. This is the exodermis, a structure similar in appearance (mirror image) to the endodermis but not always present in roots. Its function is also similar, to regulate the passage of water and minerals in and out of the cortex. 10. Examine a prepared slide of the typical herbaceous dicot root in cross-section. In this slide, locate the X-shaped core of xylem within the stele. 11. Identify the cortex, pericycle, and endodermis. 12. Under high power, view the cells of the endodermis and identify their Casparian strips. The root epidermis is normally damaged in the preparation of these slides. Experiment 4: Microscopic Anatomy of Stems PROCEDURE 1. Examine the prepared slide labeled Coleus stem tip. Identify the apical meristem located between the last (smallest) leaf primordia. Also identify the axillary buds and young leaves. You should be able to see some primary vascular tissue developing in the young leaves.

13 Laboratory 4: Plant Structures Carefully using a razor blade, attemptto make a fresh crosssection of the Trimeza martinicensis provided in lab. Make the sections as thin as you can and make more than one (that way if you dont see everything in your first section you have others already on the slide to examine). 3. After looking at the unstained section try one, or more, of the stains available in the lab. Identify the epidermis, cortex, and vascular bundles (xylem and phloem). 4. Stain a section with Lugols and locate where most of the starch is stored in the stem. How does this correspond with the location of chloroplasts? Directly in from the layer of parenchyma cells with all the chloroplasts is another layer of cells with very thick cell walls. These are fibers. (Fiber caps are also present on some of the vascular bundles.) 5. What stain(s) would you use to confirm that these were indeed fibers? 6. Make fresh cross-sections of the Coleus stem provided. Identify the epidermis and cortex. 7. Take a close look at the Coleus epidermis to see the many types of epidermal hairs present. Locate the vascular bundles and identify the xylem and phloem. Between the xylem and phloem is the fascicular cambium, which is not really distinguishable in this type of section. However, you should be able to make out the interfascicular cambium between the bundles. 8. Identify the pith. Experiment 5: Vascular Cambium and Secondary Growth PROCEDURE 1. Examine the prepared slide labeled Tilia, older stem with your compound microscope at low power. At this stage in the growth of a dicot, the vascular bundles are no longer present, and the cambial layer is no longer differentiated into fascicular and interfascicular cambia. Instead, we have one ring of meristematic cells, the vascular cambium, which now produces secondary xylem and secondary phloem.

14 14 Laboratory 4: Plant Structure 2. Switch to high power and identify the pith in the center of the section. Directly outward from the pith is a thin layer of xylem tissue. This is what is left of the primary xylem. Proceeding further out you will encounter the first seasons growth of secondary xylem, which you can identify by the radiating xylem rays composed of parenchyma cells, and the larger vessel members present. 3. Switch back to low power for a moment to see the annual rings of wood. How old was the plant before it was sectioned? At the outer extreme of the 2 o xylem is the vascular cambium (1-2 cells thick). The secondary phloem begins immediately outside the vascular cambium. Notice that it is a complex tissue made up of several cell types. There are sieve tube members (the empty looking cells), fibers (the extremely thick walled cells stained pink-purple in this section), parenchyma cells (both in rays, and scattered) and companion cells. Notice that the sieve tube members, fibers, and associated cells only make up part of the 2 o phloem. In between are large wedge-shaped areas dominated by parenchyma cells; these are phloem rays. At the very outer extreme of the secondary phloem are some sieve tube members that appear crushed and torn, this is what remains of the primary phloem. Moving further away from the stems center is the much reduced (in size) cortex. It too has been ripped and torn, unable to keep up with the expansion due to secondary growth. Finally, we reach the periderm. Examine the periderm from the outside in. Notice that there are several layers of flat reddish cells exposed to the outside. These are cork cells that make up the phellem. If you continue inward, the first layer of living, less-flattened cells is the phellogen (cork cambium) followed immediately by 1-3 cell layers of phelloderm. The 2 o phloem, along with all the tissues outside it, make up the bark of the woody dicot. F. Display: Stem and Root Modifications Stems and roots have undergone various modifications in the course of evolution to perform different functions. Look at the displays that show examples of such modifications. For your convenience, the text of the displays is included here:

15 STEM MODIFICATIONS Stems have been modified to perform many different functions during the course of evolution. The swollen stems of water hyacinth (Eichornia crassipes) are filled with air sacs, which aid the plant in floating. Stems have taken over the function of photosynthesis in some plants. For example, the large, succulent stems of cacti are protected from herbivores (plant-eaters) by their spines, which are highly modified leaves that do not function in photosynthesis and contain no living tissue at maturity. Photosynthesis is instead carried out by the fleshy green stems. TUBERS are underground stems modified to store food. Potatoes and taro are familiar examples of tubers. The eyes of the potato are buds, each arising in the axil of a modified leaf and capable of giving rise to a new potato plant. The thorns of Euphorbia milli (crown-of-thorns plant) are an example of stem modification. The thorns arise at the axils of leaves, and serve to discourage herbivory (plant-eating). ROOT MODIFICATIONS Roots have also undergone various modifications in the course of evolution to perform different functions. Many dicots have a single taproot as the major underground structure; in some, such as carrots and radishes, the taproot is fleshy and modified for food storage. Water lettuce (Pistia sp.) is a monocot angiosperm that floats on top of the water and is adapted to still, freshwater habitats. Because diffusion of oxygen from the water into the roots is very slow, the root cortex contains air channels that conduct oxygen to the roots from the leaves. Many orchids have aerial roots with a modified epidermis called velamen. The roots provide additional support for the plant, and the modified epidermis allows absorption of water from the air. Some orchids also have chloroplasts in their roots for photosynthesis. Laboratory 4: Plant Structures 15

16 16 Laboratory 4: Plant Structure LAB SUMMARY 1. You were instructed to bring to class four plants (two monocot species and two dicot species) not seen in the laboratory. (They may be whole plants or cuttings.) Label each plant as monocot or dicot, and include your name. On a separate sheet explain (for each plant) how you determined whether it was a monocot or dicot. 2. Submit copies of the drawings you made in the lab. These figures should be properly labeled with complete legends. 3. Complete Table 4-2, which contrasts the main differences between monocot roots and stems, and dicot roots and stems. 4. Answer the following questions, briefly, in no more than 2-3 sentences: Experiment 1 Monocot plant What type of leaf venation does this plant have? How many tepals does the flower have? (Tepal is the term used when both the sepals and petals of the flower look the same.) Is this a complete or incomplete flower? Perfect or imperfect?) Does this plant fit all the criteria of a typical monocot plant? Experiment 1: Dicot plant What type of venation is present in these leaves? How are the flower parts arranged? How many cotyledons would a seed from this plant have? Experiment 2: Root hairs Which tissue gives rise to root hairs? Experiment 3: microscopic root anatomy What three primary meristems arise from this region? Monocot slide: Does the stele contain a pith? From which tissue do lateral roots originate? Xylem or Phloem--which of these tissues sends water up to the rest of the plant? Dicot slide: How is the phloem arranged? Experiment 4: Anatomy of stems The primary vascular tissue in the Coleus tip: what primary meristem gave rise to this tissue? In the cross section of the monocot, how did you differentiate the vessel members of the xylem from the sieve tube members of the phloem?

17 Interfasicular and fasicular cambium--when these two cambium layers link up what are they collectively called? In the Coleus plant, what type of cells make up the pith? Experiment 5: In the cork cells which comprise the phellum, what hydrophobic compound do they contain? Phellogen (cork cambium) followed immediately by 1-3 cell layers of phelloderm---which of these layers produces the other two?. Why would a tree die if you removed a strip of bark all the way around the trunk? 5. Answer the following questions in not more than 3 sentences. a.what is a meristem? b.how do parenchyma cells differ from collenchyma and sclerenchyma cells? c.how do tracheids and vessel members, used for water transport, differ from sieve tube members, used for sugar transport? d.what are the three tissue systems of a vascular plant? From which three meristems do they arise? e.what is a Casparian strip? What is its function? f.what four types of cells are found in phloem tissue? What is the function of each? g.how does secondary growth change the arrangement of the vascular tissue in a plant? h.what is bark? How is it formed? i.how would you distinguish a monocot plant from a dicot plant? j.what is the difference between primary growth and secondary growth? k.what are the functions of the root cap? Laboratory 4: Plant Structures 18