Name ___________________________ Per _________Date _______ Leaf Anatomy Lab The leaf is the primary photosynthetic organ of the plant. It consists of a flattened portion, called the blade, which 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. Find three different leaves (no larger than the spaces below). In each space below, place a different leaf under the paper so that is directly under the circle. Rub the side of a crayon lightly over each space, so that the impression of the leaves’ outer structure can be seen. On each leaf, label the following: blade; leaflet; petiole; vein. Under the circle, write whether the leaf is a simple or compound leaf. _________________________ ______________________________ ___________________________ 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 (C), and they create a circle around the xylem and the phloem. On the picture, xylem is the upper layer of cells (D) and is shaded a little lighter than the lower layer of cells – phloem (E). Recall that xylem transports water and phloem transports sugar (food). 1. What two tissues are found within a vein? ____________________________________________________ 2. The outermost layer of cells: ______________________________________________________________ 3. The waxy covering of the leaf: _____________________________________________________________ 4. Outer layer of the vein: __________________________________________________________________ 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 (F) and the spongy layer (G). 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. 5. What does the word “mesophyll” mean? _____________________________________________________ 6. Column like cells that lie just under the epidermis. _____________________________________________ 7. What two layers of the plant contain chloroplasts? _____________________________________________ Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata (H). Specialized cells called guard cells (I) surround the stomata and are shaped like two cupped hands. Changes in moisture inside the leaf cause the 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 open the stoma. This allows water to evaporate out of the leaf. During dry times, the guard cells close. 8. Openings that allow for gas exchange. ______________________________________________________ 9. These cells function to open and close stomata. ______________________________________________ Leaf Anatomy Diagram Color the structures on the diagram as noted. Make sure that the entire picture is colored. For simplicity only part of the picture is labeled. o o o o o Cuticle (light blue) Epidermis (yellow) Bundle Sheath (dark blue) Xylem (orange) Phloem (purple) o o o o Palisade Mesophyll (dark green) Spongy Mesophyll (light green) Stomata (red) Guard cells (pink) Leaf Anatomy Observations Materials: Green Leaf Microscope Microscope Slide Coverslip Microviewer slide strip Procedures: A. Viewing Stomata 1. Obtain a green leaf from the front of the class and prepare a wet mount slide with a small section of lower epidermis. Do this by snapping the leaf backwards but not completely in half and peel back the lower, translucent layer of tissue from the bottom of the leaf. 2. Lay the tissue on a glass slide. Add a drop of water and a cover slip. 3. Using the microscope, focus on low power and locate the stomata. They look like tiny clams or cheerios or mouths. Center the stomata in your field of view, switch to medium power and focus. 4. Draw your observations in circle #1 below. Label: stomata; guard cells and epidermal cells. 5. Clean your slide and coverslip and return to the front of the class. ________X B. Green Leaf Cells 1. Using the microviewer scope, insert the slide strip and focus on image #3, the “green leaf”. 2. Draw your observations in circle below. Label: 3. Remove the slide strip from the microviewer scope and return both to the front table. _______X Analysis Questions: Answer the following questions in complete sentences. 1. What is the advantage of the epidermis being transparent? 2. Does the upper or lower level of the leaf have more chlorophyll? How do you know? Why is this important to the function of the leaf? 3. What happens to the appearance of the guard cells when the stomata are open? When they are closed? Critical Thinking: Answer the following questions in complete sentences. 4. In order to keep your houseplants healthy, why should you periodically remove the dust from their leaves? 5. A cross-section of a pine needle (a modified leaf) has a thick cuticle. Why might this be an adaptation to colder climates? 6. Why do you think that stomata are open during the day and closed at night?
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