From Seed to Seed:
Plant Science for K-8 Educators

Activity 7: Leaves: A Slice of Life

Grades: 5-8

Associated Lesson Topics:

• Characteristics of plant cells
• Internal leaf structure
• Chloroplasts

National Standards:

• SCIENCE: Standard C

Planting the Seed...

How can we investigate what is going on inside a leaf? What do you think the best procedure is for doing this? What do we expect to see inside the leaf? Encourage students to experiment with their procedural suggestions. Although we include instructions below for doing so, it is very difficult to make clear hand slides. Have professional leaf cross-section slides available for them to use if their procedures fail. Are there any differences between the cells located near the upper epidermis and those near the lower epidermis? This question should guide them toward the discovery of chloroplasts. Why are the tiny green circles in that location and what function might they serve?

Teacher Information:

In this activity, students develop microscope skills while looking at the cross-section of a leaf under the microscope. The tiny green spheres, as you may infer, are chloroplasts. Chloroplasts are the site of photosynthesis and are green because of the presence of chlorophyll pigments. These pigments are responsible for the capturing of light necessary for photosynthesis to take place. They are also easy to spot! After students have made their observations, you can launch into a discussion on photosynthesis and the role that chloroplasts play in this important plant process.

Necessary Materials:

• Lilac (Syringa) plant.
• Sharp knife or razor blade.
• Microscopes.
• Microscope slides and coverslips.
• Small, plastic water-dispenser bottle with spout.
• Professional leaf cross-section slides.

Procedure:

• Remove a leaf from the plant and place it flat on the table.
• With the sharp knife or razor blade (and adult supervision), cut the leaf straight down the center. Next to this cut, make a thin, vertical slice-as thin as possible-of the leaf. Students may need to repeat this step several times to get a thin enough slice.
• Add a few drops of water to the microscope slide and place the thinly sliced leaf section in the water. The leaf section should be placed on its side-your students want to be able to look inside the leaf, not at just the upper or lower epidermis. Add the coverslip.
• Place the slide under the microscope. If the student can see only a thick, dark mass when looking under the microscope, the slice is too thick. Have them try again or use the professional slide.
• Encourage students to look at the slide at the highest magnification possible. They can draw a sketch of what they see in their journals, label the different parts they are able to identify, and note any observations or new questions.

Harvesting the Crop...

By looking at the cross-section of the leaf under the microscope, students will also be able to see the stomata and guard cells. These are the openings in the epidermis where oxygen and carbon dioxide are exchanged between the plant and the atmosphere. Most plants have the highest concentration of stomata on the lower surfaces of the epidermis. This allows for the maximum uptake of carbon dioxide for photosynthesis while minimizing the loss of water by protecting the stomata from direct heat from the sun. Grasses, however, have stomata evenly divided between the upper and lower surfaces of the epidermis. Why? Because as grasses get blown around in the wind, they do not know which way is up! Structure is intimately related to function.

Sources

• Microscopes, slides (including leaf cross-section), coverslips, water-dispenser bottle
  Carolina Biological Supply www.carolina.com
  
  

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