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Monocots and Dicots: A Crash Course in Cutting and Comparing Corn and Coleus

Overview After a lecture on monocot and dicot anatomy, fresh mounts of corn and coleus stem material are used to identify tissues and cells and make comparisons between the two groups.

Introduction Angiosperms are divided into groups called monocots and dicots based on a variety of anatomical characteristics. A few of the most important distinctions are evident when studying the internal anatomy of stems. Monocots have their vascular bundles (the “veins” of xylem and phloem) arranged in a more-or-less random pattern throughout the stem’s parenchyma tissue. Dicots, on the other hand, have a distinct “ring” of vascular bundles, surrounded by parenchymous tissue called pith and cortex. The dicot bundles also have a vascular cambium, which connects the ring of vascular bundles during secondary growth, producing secondary xylem (or what we know of as “wood”) and phloem. Monocots do not, as a rule, undergo secondary growth.

Motivation Angiosperms are the largest group of plants on the planet. They provide the fruits and vegetables and some of the pharmaceuticals that we use everyday. These flowering plants are divided into monocots and dicots. What makes trees that give us shade and shelter vastly different than the grass we lie on and use to make our yards beautiful? In this lab, we’ll look at the stems of dicots and monocots and discover the structural differences between the two.

Objectives Upon completion of this lab, students should be able to 1. List and describe the differences in tissue arrangements between monocots and dicots. 2. Identify, in a stem cross section, pith, cortex, xylem, phloem, vascular cambium, trichomes.

Materials • Corn stem (other monocots should work • Toluidine blue as well) • Microscope slides and cover slips • Lab gloves • Kimwipes or paper towel • Coleus stem (petiole works best) • Microcopes • Razor blades

Associated California State Biology Standards 7c. Students know how prokaryotic cells, eukaryotic cells (including those from plants and animals), and viruses differ in complexity and general structure. 7j. Students know how eukaryotic cells are given shape and internal organization by a cytoskeleton or cell wall or both.

Procedure 1. Use this lab in conjunction with lecture material about the structure and function of plant cells and tissues. Plant cells have unique features, and as they differentiate their features may even differ amongst each other. For instance, a typical cell from xylem tissue is dead at maturity, lacks most of the traditional cell components, and is mostly secondary cell wall material. On the other hand, a typical cell from the photosynthetic mesophyll tissue is very much alive, has only a primary cell wall, and is packed full of chloroplasts. 2. To highlight some of the structure and function details of cells and tissues, dissect stems from a common dicot and a common monocot. Monocots and dicots differ in a variety of ways, and one is the arrangement of their stem tissues. 3. Have students make a corn wet mount by cutting a section of stem from near the base of a corn plant. Ensure that students obtain stem material, and not just leaf sheaths. Razor blades are necessary to cut a very thin cross section of this stem, so use caution and cut the sections for students when necessary. Each student or group should make several. 4. Next, have students place the cross sections on a microscope slide. In this case, the cells and tissues will be most visible with a Toluidine Blue stain. To stain, students should float the cross sections in water, then add a drop or two of Toluidine Blue, let sit for about a minute, then wash clean with water applied to one side and absorbed by a paper towel or Kimwipe on the other. If possible, students should place a cover slip over the material. Caution: Toluidine Blue will stain skin and clothes, you may wish to have your students wear gloves. 5. Each student should view the cross section under a dissecting or compound scope. They should look primarily for the bundles or vascular tissue, which are in a scattered arrangement in monocots. They should look like monkey faces. The biggest cells are xylem, but students should also identify phloem and bundle caps. Instruct students to sketch and label the material in the lab notebooks. You may also wish to have students identify and sketch cortex and epidermal tissue. 6. Next, repeat the above procedure in order to prepare a section of coleus stem. Petiole material seems to work best for this. Toluidine Blue should not be needed. 7. Under the scope, have students look for the vascular bundles, which should be arranged in a ring around the pith. Help students identify pith, cortex, xylem, phloem, and even vascular cambium. Students should make comparisons to the monocot. Have students also look for trichomes. They should sketch and label all parts. Often, the razor blade preparation of the cross section will mutilate much of the soft tissue of the coleus, but there should be some parts of the cross section in which the bundles are quite visible. However, looking at the destroyed parts of the cross section, students will likely see the spiral strands of cellulose from the cell walls of the xylem that were ruptured and released in the preparation. You can use this opportunity to talk about the features of cell walls and contrast plant cells to animal cells. 8. Students may wish to repeat the procedure with a section from the main stem. In this case, it may be possible to view the connections of the vascular cambium between bundles, a trait that occurs in mature sections of dicots, but will not be visible in monocots because they lack a vascular cambium (their bundles are scattered).

Evaluation The following questions are listed under the Analysis section of the student handout and may be used as part of a report, class discussion, or assessment. 1. How do the monocot and dicot stems differ? Be specific. 2. How does the dicot’s stem anatomy facilitate secondary growth? 3. Where is xylem found in comparison to the phloem? 4. Why does there seem to be considerably more xylem than phloem? Why does xylem “hand around” but phloem is all relatively “new”? 5. What evidence from this lab is there to show the differences between plant and animal cells?

Extension activities 1. It is slightly more difficult to identify the structures in a root cross section, as they tend to vary (from models) in their spacing and exact placement. However, you could use the prop roots from corn and the roots from a carefully-dug dicot, prepare mounts (using stain when necessary) and challenge your students to try to identify the structures. You may not be able to form a consensus, but the activity would be a challenge in applying knowledge from the lecture and the lab above.

Test Preparation

The diagram to the right shows the stem of a coleus plant.

1. Refer to the illustration. The tissue labeled A is called (A) meristem. (B) xylem. (C) phloem. (D) ground tissue.

2. Refer to the illustration. The tissue labeled B, which conducts water and is made of elongated cells that connect end to end, is called (A) meristem. (B) xylem. (C) phloem. (D) ground tissue.

3. The vascular bundles of a monocot stem are arranged (A) in rings surrounded by ground tissue. (B) scattered throughout the ground tissue. (C) in pith scattered throughout the ground tissue. (D) in cortex scattered throughout the ground tissue.

Student Sheet: Monocot and Dicot Stem Anatomy Name:______

Procedure 1. To highlight some of the structure and function details of cells and tissues, dissect stems from a common dicot and a common monocot. Monocots and dicots differ in a variety of ways, and one is the arrangement of their stem tissues. 2. Make a corn wet mount by cutting a section of stem from near the base of a corn plant. Be sure to obtain stem material, and not just leaf sheaths. Razor blades are necessary to cut a very thin cross section of this stem, so use caution when cutting and ask for assistance if needed. You should make several cuttings. 3. Next, place the cross sections on a microscope slide. In this case, the cells and tissues will be most visible with a Toluidine Blue stain. To stain, float the cross sections in water, then add a drop or two of Toluidine Blue, let sit for about a minute, then wash clean with water applied to one side and absorbed by a paper towel or Kimwipe on the other. If available, place a cover slip over the material. Caution: Toluidine Blue will stain skin and clothes, you may wish to wear gloves if provided. 4. View the cross sections under a dissecting or compound scope. Look primarily for the bundles or vascular tissue, which are in a scattered arrangement in monocots. They should look like monkey faces. The biggest cells are xylem, but also identify phloem and bundle caps. Sketch and label the material in the lab notebooks. Identify and sketch cortex and epidermal tissue. 5. Next, repeat the above procedure in order to prepare a section of coleus stem. Petiole material seems to work best for this. Toluidine Blue should not be needed. 6. Under the scope, look for the vascular bundles, which should be arranged in a ring around the pith. Your teacher will help you identify pith, cortex, xylem, phloem, and even vascular cambium. Make comparisons to the monocot. Look for trichomes. Sketch and label all parts. Often, the razor blade preparation of the cross section will mutilate much of the soft tissue of the coleus, but there should be some parts of the cross section in which the bundles are quite visible. However, looking at the destroyed parts of the cross section, you will likely see the spiral strands of cellulose from the cell walls of the xylem that were ruptured and released in the preparation. 7. For more study, repeat the procedure with a section from the main stem. In this case, it may be possible to view the connections of the vascular cambium between bundles, a trait that occurs in mature sections of dicots, but will not be visible in monocots because they lack a vascular cambium (their bundles are scattered).

Analysis On a separate sheet of paper please complete the following:

1. What is different in arrangement of tissue between monocots and dicots? 2. How does the dicot’s stem anatomy facilitate secondary growth? 3. Where is xylem found in comparison to the phloem? 4. Why does there seem to be considerably more xylem than phloem? Why does xylem “hand around” but phloem is all relatively “new”? 5. What evidence from this lab is there to show the differences between plant and animal cells?