Modeling an Angiosperm Leaf (Part 2/2)

Today students will use the data they collected yesterday to construct two models of angiosperm leaf tissue.  Constructing the model will help students better understand the role of tissues in organs.  It will also help them improved their spatial understanding. Finally, at the end of the lesson students will compare and contrast angiosperm leaves with coniferous needles to understand how leaf modifications help plants adapt to extreme environments. Here is an overview of what they will learn today. 

Using the Frayer method, define leaf.  Next, ask student to reconsider their responses for yesterday's plant structure assessment probe.  

In their lab notebooks, students should answer:  

• What is an organ?

(Typical student answer:  An organ is a group of tissues doing specific functions.)

• How is a plant leaf an organ? 

(Typical student answer:  A leaf is an organ because it made of different types of plant tissues.)

(Note: While students have a better understanding about why a leaf is an organ, they still have some naive thinking. Once they have finished the activity today, they will revised their responses a third time to ensure they have a complete understanding why a plant leaf is an organ.)

Using the attached powerpoint, students will compare the sketches of the leaf that they made yesterday with professionally prepared slides. While showing the powerpoint, ask students to refer back to their drawings and consider the following questions:  

• Does the leaf used in yesterday's lab have a cuticle?  

Typical student answer:  Students are unsure of this.  

(Note:  A cuticle is difficult to seen because it is a clear waxy layer. The clear nail polish tends to hide the cuticle. Provide students with leaves and allow them to notice the shininess on the leaf.  This luster is due to the presence of the cuticle.)

• When viewed in the horizontal section, what shape are the cells seen in the upper epidermis (first layer)? 

Typical student answer:  The cells look like wavy square.  Every so often, round cells are seen and thin ovals seen in pairs of two. Some times the ovals open into a circle.  

• When viewed in the cross section, what shape are the cells seen in the upper epidermis (first layer)? 

Typical student answer:  From the side, the cell layer looks like flat rectangles.  

• Can you see any particular structures?  

Typical student answer:  There are round cells (stomata) interspersed throughout the slide.  There is no pattern to the order of round cells.  

• When viewed in the horizontal section, how do the cell shapes in the palisade layer (second layer) differ from the first layer?

 Typical student answer: The cells are square in shape when viewed from the top.  

• When viewed in the cross section, how do the cell shapes in the palisade layer (second layer) differ from the first layer from the side?

 Typical student answer: The cells are rectangular in shape when viewed from the side. 

• Are there any particular structures that you could see?

 Typical student answer: There are tiny green dots.  Those might be chloroplasts. 

• When viewed in the horizontal section, how do the cell shapes in the spongy layer (third layer) differ from the other previous layers?

Typical student answer: Some cells are square and some are circular in shape when viewed from the top. There is a lot of empty space between cells and cells are arranged irregularly.

• When viewed in the cross section, how do the cell shapes in the spongy layer (third  layer) differ from the other previous layers? 

Typical student answer: The cells are square and circular in shape when viewed from the side. There is a lot of empty space between the cells.  

• Are there any particular structures that you could see?

 Typical student answer: There are dense circles of cells (veins). The cells in within these dense circles are square in shape when viewed from the side. 

• When viewed in the horizontal section, how do the cell shapes in the lower epidermis layer (fourth layer) differ from the other previous layers?

Typical student answer: The cells are square in shape when viewed from the top. 

• When viewed in the cross section, how do the cell shapes in the lower epidermis (fourth layer) differ from the other previous layers? 

Typical student answer: The cells are thin rectangles in shape when viewed from the side. 

• Are there any particular structures that you could see?

Typical student answer: The same structures that are seen in the upper epidermis are seen in the lower epidermis. 

Using the projector, project the lilac leaf cross section 400X onto the board.  

Ask students the following questions while they construct the 3-D model of the upper epidermis:  

• What shape are the cells of the upper epidermis in cross section? (square)
• What shape are the cells of the upper epidermis in horizontal section? (square)
• When you combine those two shapes, what 3-D structure is made? (cube)
• How would you need to cut a paper lunch bag to make that 3-D structure? (Cut the bag at the line closest to the bottom of the bag.)

Students should cut 10 paper bags at the line closest to the bottom of the bag. Tape the bags together into a 2 X 5 array. Draw a nucleus on each bag at random spots on the bag. Tape the two layers together.

Next, ask students the following questions while they construct the 3-D model of the palisade layer:

• What shape are the cells of the palisade layer in cross section? (rectangle)
• What shape are the cells of the palisade layer in horizontal section? (square)
• When you combine those two shapes, what 3-D structure is made? (rectangular prism)
• How would you need to cut a paper lunch bag to make that 3-D structure? (The bags should not be cut.)

Students should open 10 paper bags and tape the bags together into a 2 X 5 array. Draw a nucleus on each bag at random spots on the bag. The upper epidermis array should be placed on top of the palisade layer array. Tape the two layers together.

Then, ask students the following questions while they construct the 3-D model of the spongy layer and air space (leaf model):

• What shape are the cells of the spongy layer in cross section? (square)
• What shape are the cells of the spongy layer in horizontal section? (square)
• When you combine those two shapes, what 3-D structure is made? (cube)
• How would you need to cut a paper lunch bag to make that 3-D structure? (The bags should be a little big than the epidermis. Cut the bags in half.)

Students should cut 6-7 paper bags in half and tape the bags together into a 2 X 5 array. Draw a nucleus on each bag at random spots on the bag. Arrange the bags so the will provide structural support to the model, but still show the air space. The spongy array should be placed below the palisade layer array.  Tape the two layers together.

Finally, ask students the following questions while they construct the 3-D model of the lower epidermis:

• What shape are the cells of the lower epidermis in cross section? (square)
• What shape are the cells of the lower epidermis in horizontal section? (square)
• When you combine those two shapes, what 3-D structure is made? (cube)
• How would you need to cut a paper lunch bag to make that 3-D structure? (Cut the bags exactly like the bags in the upper epidermis.)

Students should cut 10 paper bags at the line closest to the bottom of the bag. Tape the bags together into a 2 X 5 array. Draw a nucleus on each bag at random spots on the bag. The lower epidermis array should be placed on the bottom of the model. Wrap the entire model in clear contact paper to provide stability.

Students will build a model of a leaf by differentiating its separate tissues so they can better understand why the leaf is an organ.  While making the model, answer the questions on this handout, Modeling an Angiosperm Leaf. 

Equipment needed for leaf model (per lab group):  

• 1 green twin-size blanket (or approximately two yards of heavy fabric or green felt)
• 1 green twin-size sheet
• 2 sheets (approximately 2 X 2 meters) of clear heavy-weight vinyl (or shower curtain liners)
• 3 yards of 1/4 inch plastic tubing
• 1 permanent green marker
• craft glue or similar adhesive. 

Constructing the Model

1) Layer the 4 materials on top of one another in the following order

• Bottom layer: vinyl
• Second layer: blanket
• Third layer: sheet 
• Top layer: vinyl

2) Using a marker trace a leaf out of one of the layers of vinyl.  The leaf should be 2m wide from tip to tip.  

3) Use heavy duty shears to cut out the leaf shape from the vinyl.  Then use this piece as a cutting template for the rest of the layers.  

4) Add guard cells, open stomata, and closed stomata that are about 24 cm long and 14 cm wide.  

5) On the other layer of vinyl, have students draw capsule-shaped chloroplasts that are about 26 X13 cm. This portion of vinyl should be covered with chloroplasts. (Tip: Students can draw stomata and chloroplasts on a separate sheet of paper. Then they can place that paper template under the clear vinyl and trace the stomata and chloroplasts.)

6) Relayer the fabrics and whip stitch them together. Another option would be to staple them together.  

7) Have students cut a pillowcase sized window with pinking shear in the center of the sheet.  Students can fold back the flap to view the chloroplasts.  

8) On the stomata layer of the vinyl, have students made pores by cutting out the vinyl between the two guard cells on each open stomata.  (Tip: A scalpel will give the cleanest cut.)

9) Thread plastic tubing between the stitching down the center of the leaf between the blanket layer and the vinyl layers with stomata on it.  A small hole will need to be cut in the sheet of vinyl to allow the tubing to pass through it.  Extend the tubing 60 cm out the other end o the leaf.  The tubing represent the vein in the leaf.  

Based on Littlejohn, Patty. 2007. "Building Leaves and an Understanding of Photosynthesis." ScienceScope.