Plant Structure and Function Extension NGSSS: SC.912.L.14.7 Relate the structure of each of the major plant organs and tissues to physiological processes. (AA) Part 1A: Leaves The leaf of a plant serves two basic functions: photosynthesis and cellular respiration. Photosynthesis is a chemical reaction in which plants convert radiant energy (light energy) into chemical energy (food energy or more specifically, glucose). Cellular respiration is the chemical reaction in which chemical energy (glucose) is converted into usable energy for the plant. Can you list the reactants and products for the processes of photosynthesis and cell respiration? Photosynthesis Equation: Cell Respiration Equation: Leaf Cross-Section The leaf is the primary photosynthetic organ of the plant. It consists of a flattened portion, called the blade that is attached to the plant by a structure called the petiole. Sometimes leaves are divided into two or more sections called leaflets. Leaves with a single undivided blade are called simple, those with two or more leaflets are called compound. The outer surface of the leaf has a thin waxy covering called the cuticle (A), this layer's primary function is to prevent water loss within the leaf. (Plants that leave entirely within water do not have a cuticle). Directly underneath the cuticle is a layer of cells called the epidermis (B). The vascular tissue, xylem and phloem are found within the veins of the leaf. Veins are actually extensions that run from to tips of the roots all the way up to the edges of the leaves. The outer layer of the vein is made of cells called bundle sheath cells (E), and they create a circle around the xylem and the phloem. On the picture, xylem is the upper layer of cells (G) and is shaded a little lighter than the lower layer of cells - phloem (H). Recall that xylem transports water and phloem transports sugar (food). Within the leaf, there is a layer of cells called the mesophyll. The word mesophyll is greek and means "middle" (meso) "leaf" (phyllon). Mesophyll can then be divided into two layers, the palisade layer (D) and the spongy layer (F). Palisade cells are more column-like, and lie just under the epidermis, the spongy cells are more loosely packed and lie between the palisade layer and the lower epidermis. The air spaces between the spongy cells allow for gas exchange. Mesophyll cells (both palisade and spongy) are packed with chloroplasts, and this is where photosynthesis actually occurs. Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata. Specialized cells, called guard cells (C) surround the stomata and are shaped like two cupped hands. Changes within water pressure cause the Biology HSL Page 107
stoma (singular of stomata) to open or close. If the guard cells are full of water, they swell up and bend away from each other which opens the stoma. During dry times, the guard cells close. Identify the structures: Color the structures underlined above. Make sure that the entire picture is colored and that the color matches the words. For simplicity only part of the picture is labeled. Cuticle (light blue) Epidermis (yellow) Guard cells (pink) Palisade Mesophyll (dark green) Phloem (purple) Xylem (orange) Spongy Mesophyll (light green) Bundle Sheath(dark blue) Biology HSL Page 108
Answer the following questions: 1. What two tissues are found within a vein? 2. What does the word "mesophyll" mean? 3. What two layers of the plant contain chloroplasts? 4. The outermost layer of cells: 5. The waxy covering of the leaf.: 6. These cells function to open and close stomata. 7. Outer layer of the vein: 8. Column like cells that lie just under the epidermis. 9. Openings that allow for gas exchange. 10. The stalk that connects the leaf to the stem. Extension: 1. Write a paragraph discussing how leaf structure contributes to the efficiency of the process of photosynthesis. 2. Predict adaptations that would be found in leaves located in the following environments. a. Desert b. Tropical rain forest c. Aquatic environment Part 2A: Stomata and Guard Cells: Materials: Plant leaves Clear fingernail polish Clear cellophane tape (clear package sealing tape) Microscope Microscope slides Oyster plant leaf Procedure: 1. Obtain a leaf from a plant; generally any plant will work for this procedure. 2. Paint a thick patch of clear nail polish on the leaf surface being studied. Make a patch at least one square centimeter. 3. Allow the nail polish to dry completely. 4. Tape a piece of clear cellophane tape to the dried nail polish patch. (The tape must be clear. Do not use Scotch tape or any other opaque tape. Clear carton-sealing tape works well.) 5. Gently peel the nail polish patch from the leaf by pulling on a corner of the tape and peeling the fingernail polish off the leaf. This is the leaf impression you will examine. (Only make one leaf impression on each side of the leaf, especially if the leaf is going to be left on a live plant.) 6. Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any excess tape. Biology HSL Page 109
7. Scan the slide until you find a good area where you can see the stomata. Each stoma is bordered by two sausage-shaped cells that are usually smaller than surrounding epidermal cells. These small cells are called guard cells and, unlike other cells in the epidermis, contain chloroplasts. 8. Cut a small piece of the Oyster plant leaf. Observe directly under the microscope. A cross section or slide is not needed. Observe the stomata and guard cells and compare them to the ones previously observed. Analysis: 1. Sketch and label the Stoma, Guard Cells, Epidermal Cells, and Chloroplasts 2. Estimate the number of stomata in both of your samples. 3. Where were most of the stomata found? Why do you think this is? Explain your answer in evolutionary terms. Extension: 1. Question: Will plants have more stoma open during the day than during the night? 2. Develop a hypothesis about the number of open stomata found in a plant kept in the dark compared to a plant in the light. 3. Repeat the procedure above for preparing your slide. You will make two impressions, one from a "Dark Plant" and one from a "Light Plant" You will compare the two impressions. 4. Write a short paragraph that answers the question; use your data to support your conclusions. 5. Discuss the factors that cause the stomata to be open during the day and closed at night. Under what circumstances the stomata might be closed during the day? Biology HSL Page 110
Part B: Roots Materials: Plant roots (monocot and dicot) Microscope Microscope slides Cells in plants cells are organized into four main tissues protective, vascular, meristematic, and fundamental. Protective tissue surrounds the outside of a root and the rest of the plant. This tissue is composed of epidermis and/or cork cells. Vascular tissue contains cells which conduct materials such as water, minerals, and food to the organs of the plant. It also provides support. Two of the major vascular tissues are xylem and phloem. Fundamental tissue includes storage areas and cells where food is manufactured. This tissue also provides support. Meristematic tissue is growth tissue. It produces new cells which develop into the other three types of tissues. In this investigation, you are to observe and identify the various tissues in herbaceous monocot and dicot roots, the area of lateral root origin, and onion root. You are to compare the relative sizes of each cell type observed. You are also to observe and identify primary and secondary roots from different types of root systems. Herbaceous Monocot and Dicot Roots: 1. Observe the monocot and dicot root slides on low power, then on high power of your microscope. 2. For each root, locate and identify the epidermis, cortex, endodermis, pith, xylem, pericycle, and phloem (Figure 53-1). Determine whether each cell type is protective, vascular, fundamental, or meristematic tissue. 3. Compare the relative size of each type of cell. 4. Make a sketch of each type of cell. Biology HSL Page 111
Lateral Root Origin 1. Observe the slide of the area of lateral root origin on low power, then on high power. 2. Determine the tissue from which the lateral root is growing. (See Figure 53-2). 3. Make a sketch of your observations. Onion Root Tip (Longitudinal Section) 1. Observe the onion root tip slide on low then medium power. 2. Label the apical meristem (the area where the cells are dividing). Note that the cells are smaller and less developed closer to the meristem and that they elongate and mature as they get older (farther up the root). 3. What is the function of the root cap at the very tip of the root? Sketch what you see. Biology HSL Page 112
Root Systems Using Figure 53-3, observe the different types of root systems. 1. The blue-stained material in the cortex of the dicot root is starch. Why is it found in the root and how did it get there? 2. From what specific tissue does a lateral root originate? Is it meristematic? Why? 3. How do xylem patterns differ in monocot and dicot roots? 4. How do tap root systems differ from fibrous root systems? 5. Which root system, tap or fibrous, is more similar to an adventitious root system? Explain? 6. What are four basic functions of roots? Biology HSL Page 113
Part C: Stems Materials: Plant stems (monocot and dicot) Microscope Microscope slides As in roots, protective, vascular, fundamental, and meristematic tissues are found in stems. The difference between roots and stems may be slight or significant. In this investigation, you are to observe and identify the tissues in an herbaceous monocot stem, and herbaceous dicot stem, and a woody stem. You are to compare the relative size of each cell to the other cells and make sketches of each. You are to observe and identify the external parts of a woody twig and determine the pattern of leaf arrangement. Herbaceous Monocot and Dicot Stems: 1. Observe the monocot and dicot slides on low power, then on high power of your microscope. 2. Make a sketch of each stem and label the epidermis, cortex, vascular bundles, xylem, phloem, cambium, and pith (Figure 54-1). 3. List the functions of each cell type. 4. Determine whether each cell type is protective, vascular, fundamental, or meristematic tissue. 5. Compare the relative size of each cell to the other cells. Biology HSL Page 114
Tilia Stem (Woody growth): 1. Observe the Tilia stem slide on low power, then on high power of your microscope. 2. Sketch and label the epidermis, cork, cortex, phloem, cambium, spring wood, summer wood, xylem, pith rays, and pith (Figure 54-2). 3. List the functions of each cell type. 4. Determine whether each cell type is that of protective, vascular, fundamental, or meristematic tissue. 5. Compare the relative cell sizes. Woody Twig: 1. Observe the woody twig. 2. Locate and identify the terminal bud, lateral bud, lenticels (small pores in the bark), bud scale, bud scale scar, node, internode, and leaf scar (Figure 54-3). 3. List the functions of each part of a woody twig. 4. Determine whether the stem has an alternate or opposite leaf arrangement. Biology HSL Page 115
5. The area between two bud scale scars is one year s growth. Count the years of growth on the twig. Analysis Questions: 1. State the various functions of a stem. 2. How is woody xylem in Tilia different from xylem in herbaceous monocots and dicots? 3. What other major differences exist between woody stems and herbaceous stems? 4. How do monocot and dicot stems differ in location of cambium, xylem, and phloem? 5. Explain how you would tell if one growing season was more favorable for growth than others. 6. What function do lenticels serve? What would happen to a woody twig with no lenticels? 7. How can you determine the past arrangement of leaves on a bare twig? Biology HSL Page 116