L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 1

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1 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 1 Monocots vs. Dicots: Leaves LAB MANUAL OBJECTIVES 1. Differentiate between dicots and monocots by: a. Stem structure b. Leaf structure 2. Understand the development and arrangement of angiosperm tissues 3. Identify leaf arrangements LAB PREPARATION 1. Bring in completed assignments. a. In class, you will be sharing what you have learned about your canoe plant with your group members. In a few weeks, your group will be asked to rank your groups canoe plants in order of importance and present your reasoning to the class. 2. Read dichotomous key handout posted on Laulima. Google plant terms and characteristics that you do not know. 3. Read and study this laboratory handout. 4. Bring your Photo Atlas for Biology book. 5. Bring your personal protective gear (closed-toed shoes, lab coat) MONOCOTYLEDONS AND DICOTYLEDONS - A Review In addition to the number of cotyledons, monocots and dicots normally have other morphological differences that you can use to distinguish between them (Table 1 and Figure 1). Please note, that the characteristics of leaf venation and flower parts are not completely reliable. Occasionally you may encounter a monocot with net venation or a dicot with three flower petals. Be sure to use a combination of characteristics to confirm your determination. Roots Stems Stems Leaves Flowers Monocots 1 cotyledon Usually fibrous root system (may have started with a taproot) Vascular bundles in complex arrangement Does not produce vascular cambium Veins usually parallel Flower parts in 3 s, or multiples of 3 s Dicots 2 cotyledons Taproot usually present Vascular bundles arranged in a ring Can produce vascular cambium, secondary (woody) growth common Veins usually branched or net-like Flower parts generally arranged in multiples of 4 s and 5 s Table 1. Comparison of monocot and dicot characteristics

Palm trees), monocots do not increase in girth. When they do, it is not referred to as secondary growth since the growth process does not involve a vascular cambium.

2 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 2 Figure 1. Comparison of monocot and dicot traits Vascular Cambium and Secondary Growth In plants, increase in girth is called secondary (or 2 o ) growth. Except for a few notable exceptions (ex. Palm trees), monocots do not increase in girth. When they do, 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 the 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 that are 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 from the primary tissues in the original vascular bundles. Figure 2. Secondary growth in woody dicots 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

3 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 3 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. As the girth of a plant continues to increase, the growing epidermis that originally covered the plant normally can t keep up. 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 (Fig. 2). 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 (Fig. 2). 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 collectively referred to as bark. Figure 3. Common leaf arrangements In the first lab, we learned to prepare wet mounts of cork. Cork is the phellem layer of bark tissue harvested from an oak tree, Quercus suber, which has a thick layer of bark that helps it to recover quickly after forest fires. The cork cambium layer of bark becomes quiet thick, leading to its usefulness as a source of cork. Cork can be harvested every seven to ten years from the tree without harming the tree, making it a sustainable resource. Cork is widely used in the wine industry, but is also used in a multitude of ways, including insulation, floor materials, wall tiles, paper, clothing, and many other uses. Workers shave off the cork without cutting into the phellogen layer. LEAVES Leaves, perhaps more than any other plant organ, vary greatly in external form and internal structure. As the main photosynthetic organs of the plant, leaf morphology is influenced by the amount of sunlight they receive. Additional environmental factors, such as water availability, wind, temperature, and herbivores, also affect the morphology and arrangement of leaves on a plant. Leaves are the primary location for photosynthesis. A large surface area is desired to maximize light absorption and special openings, called stomata (singular: stoma), in the leaf surface function in gas exchange with the atmosphere. Plants utilize carbon from carbon dioxide in the air to produce carbohydrates. Water will also evaporate through the stoma in a process called transpiration. As water vapor is lost by transpiration in the leaves, more water is pulled into the roots and up the plant body via the xylem. Read about transpirational pull (Resource Acquisition and Transport in Vascular Plants) in the Campbell textbook. While transpiration is an integral component of the mechanism responsible for water movement throughout the plant, excessive water loss can result in dehydration (wilting) and plant death. For this reason, plants have adaptations that minimize or control water loss in most plants.

L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 4 Morphology of leaves The blade, or lamina, of a leaf is the large flat part. The margin is the end of the leaf.

Vascular bundles go from the stem through the petiole into the veins of the leaf. The mid-rib in the center of the leaf branches to produce veins formed with vascular tissue.

In simple leaves, there is a single blade, or lamina.

4 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 4 Morphology of leaves The blade, or lamina, of a leaf is the large flat part. The margin is the end of the leaf. The leaf can either be directly connected to the stem or attached by the stalk-like petiole. In some leaves, the petiole has two stipules, small leaf-like flaps, at its base. Vascular bundles go from the stem through the petiole into the veins of the leaf. The mid-rib in the center of the leaf branches to produce veins formed with vascular tissue. The veins are arranged in patterns, also called leaf venation. Simple Leaves Leaves are divided into simple leaves (Fig. 3) and compound leaves (Fig. 4 and 6). In simple leaves, there is a single blade, or lamina. Simple leaves will have very different shapes (see the leaf reference sheet which has just a few examples on your lab bench for examples) with varying margins. Leaves are attached to the plant at nodes on the stem in patterns. To maximize exposure to sun, plants may arrange the leaves to the stems in different patterns. Common Leaf Arrangements: Figure 4: Anatomy of a simple leaf Opposite: Two leaves attached at the node on opposite sides Alternate: one leaf per node with the second leaf being above the first but attached on the opposite side of the stem or where the successive leaves attach above the first in a spiral a variation of alternate leaf arrangement. Whorled: three or more leaves are attached at one node Compound Leaves Figure 5. Common leaf arrangements In compound leaves, the leaf is divided into multiple leaflets. There are two main types of compound leaves: pinnate and palmate. In pinnate laves, each leaflet has its own stalk, called a petiolule, attaching it to a rachis. In palmate leaves, the leaflets radiate out from the tip of the petiole (like the fingers of your hand) and lack a rachis. Figure 6. Compound leaf structure Sometimes it may be unclear whether a leaf is a compound leaf or a cluster of several simple leaves. When in doubt, remember that simple or compound leaves have axillary buds (lateral buds) where they attach to the stem and leaflets do not.

L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 5 Like simple leaves, each compound leaf is then arranged around the plant stem.

Common types of compound leaves The typical components of a leaf (Fig. 5) are discussed sequentially (as they occur from top to bottom) in the following sections.

The cuticle is a hydrophobic layer secreted atop the epidermal cells. This waxy layer functions to reduce water loss from the leaves.

5 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 5 Like simple leaves, each compound leaf is then arranged around the plant stem. The common leaf arrangements, opposite, alternate, and whorled are described above. Internal Anatomy of the Typical Leaf Figure 7. Common types of compound leaves The typical components of a leaf (Fig. 5) are discussed sequentially (as they occur from top to bottom) in the following sections. As you read, refer to the typical dicot leaf (Fig. 5 top). Keep in mind, however, that there are many variations of this basic plan. The cuticle is a hydrophobic layer secreted atop the epidermal cells. This waxy layer functions to reduce water loss from the leaves. The epidermis is the outermost cell layer of the leaf and originates from the protoderm. It provides function and regulates gas exchange through openings called stomata. The Greek word stoma means mouth. The stoma, the opening, has specialized guard cells surrounding it that open or close the stoma. Typically, stomata is located on the lower epidermis to reduce water loss and the number of stomata may depend on the environment around the plant. The bulk of leaf cells are chlorenchyma tissue called the mesophyll. In dicots, the mesophyll is divided into two layers, the palisade layer of elongated cells containing chloroplasts, and the spongy layer that is more loosely packed to allow oxygen and carbon dioxide to move through. Vascular bundles run through the veins of the leaf bringing in water and minerals from the roots and transporting sap out of the leaf. Figure 8: Cross-sections of a dicot leaf (top) and monocot leaf (bottom)

6 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 6 Figure 9: Examples of leaf characteristics

7 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 7 LABORATORY EXPERIMENT [STEM SECONDARY GROWTH] Tilia, older stem, Vascular cambium and secondary growth 1. Examine the prepared slide labeled Tilia, older stem with your compound microscope at low power. a. At this stage in the growth of a dicot, the vascular bundles are no longer present. Instead, we have one ring of meristematic cells, the vascular cambium, which now produces secondary xylem and secondary phloem. 2. Switch to a higher magnification and identify the pith in the center of the section. a. Directly outward from the pith is a thin layer of xylem tissue. This is what is left of the primary xylem. b. Proceeding further out you will encounter the first season s 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 a lower power to see the annual rings of wood. a. How old was the plant before it was sectioned? b. At the outer extreme of the secondary (2 o ) xylem is the vascular cambium (1-2 cells thick). c. The secondary phloem begins immediately outside the vascular cambium. Notice that it is a complex tissue made up of several cell types. d. There are sieve tube members (empty looking cells), fibers (extremely thick walled cells stained pink-purple in this section), parenchyma cells, and companion cells. e. Notice that the sieve tube members, fibers, and associated cells only make up part of the 2 o phloem. f. In between are large wedge-shaped areas dominated by parenchyma cells, these are phloem rays. g. 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. h. Moving further away from the stem s 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. i. Examine the periderm from the outside in. There are several layers of flat reddish cells exposed to the outside. These are cork cells that make up the phellem. j. If you continue inward, the phellogen (cork cambium) is the first layer of living, lessflattened cells. The phelloderm can be found in the layers (1-3) that follow immediately

8 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 8 after the phellogen. The 2 o phloem along with all the tissues outside it, make up the bark of the woody dicot. [LEAVES] Arrangements and VENATION 4. Examine the monocot plants on display. a. Record the scientific and common name of the plant in your notes. b. Identify the stem, leaves, and flower (if applicable). c. Sketch and label the plant and a close-up of a leaf and its venation. Describe its shape, venation pattern and margin. 5. Examine the dicot plant on display. a. Record the scientific and common name of the plant in your notes. b. Identify the stem, leaves, axillary buds (lateral buds) and flower (if applicable). c. Sketch and label the plant and a close-up of a leaf and its venation. Describe its shape, venation pattern and margin. [LEAVES] Cross-section DICOTS: Pre-prepared microscope slide of leaf cells 6. Examine the prepared slide of a typical dicot leaf under a compound microscope. Identify: cuticle upper epidermis stomata palisade mesophyll lower epidermis spongy mesophyll air spaces 7. Identify their xylem, phloem, and the bundle sheath cells surrounding them. Notice that not all bundles lie in the same plane. Some are in cross-section while others are longitudinal. How can you explain this? What is the function of the xylem, phloem, and bundle sheath cells? MONOCOTS: Pre-prepared microscope slide of leaf cells 8. Examine the prepared slide of a typical monocot leaf under a compound microscope. Identify: cuticle stomata vascular bundles upper epidermis mesophyll cells air spaces

9 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 9 Leaf Epidermis Slide MUST SHOW SLIDE TO TA OR TI 9. Obtain a piece of leaf and peel off a small strip of the LOWER, purple epidermis. (If you have difficulty obtaining only the epidermis, ask for help). Make sure to use forceps and exercise caution when using a razor blade. 10. Quickly mount the epidermis in a drop of water on a microscope slide before it dries out, making sure that the outside of the peel is facing up. There s no need to dye the plant tissue, the pigments in this type of leaf provide sufficient contrast. 11. Cover the peel with a coverslip and examine it with your compound microscope. 12. Locate and identify a stoma. 13. Examine the guard cells surrounding a stoma and the normal epidermal cells that make up most of the peel. How do these two cell types differ? [DISPLAY OF PLANT ADAPTATIONS] Angiosperms have developed modifications in their stems, roots, and leaves during the course of evolution to gain certain advantages in particular environments. Look through the examples on display and record what you see in notes. You should make note of the different modifications on display, but you can just use the common names. You do not need to memorize the scientific names. Dichotomous Key and Campus Botanical Tour Next week Your TA will be taking you for a short walk around campus. You will learn how to utilize a dichotomous key to identify organisms and review how to distinguish monocots and dicots.

10 L a b 5 : S t r u c t u r e s o f M o n o c o t s v s. D i c o t s P a g e 10 Assignment #6: Canoe Plant Scavenger Hunt (10 pts) You will need: -Your lab notebook -A pen/pencil -A camera or camera/phone -Someone to take your picture or creativity to capture it yourself -A ruler or measuring tape would be helpful Most of us suffer from plant blindness. It s a phrase coined to describe our tendency to ignore and be oblivious to plants in our environment. We are naturally zoocentric and more apt to notice animals but overlook the many plants surrounding us. That effect is even more apparent in urban settings. By now, you have chosen a canoe plant for your paper. Part of understanding its significance in the development of the Hawaiian Islands, is seeing how prevalent it is today. For this assignment, you re on the hunt to find your plant and get an idea of how and where it is still used. Find and photograph your chosen plant in as many settings as you can find it. Choose one location to record observations and notes for submission. Submit written portion at the beginning of next lab: 1. Description of location(s) where the plant can be found. 2. Choose one location to make and record observations Include a sketch birds-eye view showing the location of other plants and landscape elements around it a. Record data about the plant s habitat i. Location 1. ex. street address, GPS coordinates, if it is a large area, use landmarks and descriptors that would help someone unfamiliar with the location find it (ex. Northwest corner of St. John courtyard) ii. Date and time of day iii. Approximate temperature iv. Weather conditions 3. Record observations about plant a. Approximate height of plant b. Draw shape of leaf and pattern of leaf veins (record approximate size) c. Draw flower and describe appearance (make note if no flowers are present) d. Draw fruit and describe appearance (make note if no flowers are present) Answer questions at bottom of notes: Question 1: Describe the soil and light conditions of the area the plant is growing in. (Is it in full sun? Shade? Dry rocky soil? Question 2: Does the plant appear to have been planted purposely or introduced naturally? Photo prior to class Submit a wide photo that shows your plant and it s habitat and with your face also visible in the frame. Rename the photo with Your name Canoe Plant Name and submit electronically to your TA. If you see your canoe plant in other locations, snap a photo!

Plant Structure. Lab Exercise 24. Objectives. Introduction

Plant Structure. Lab Exercise 24. Objectives. Introduction Lab Exercise Plant Structure Objectives - Be able to identify plant organs and give their functions. - Learn distinguishing characteristics between monocot and dicot plants. - Understand the anatomy of