To begin the lesson, read through the book, Fall Foliage by Charles Smith as a class. Have students bring in a variety of leaves, or pictures of leaves and trees. Have some of your own examples as well, but be sure to include coniferous branches and pine cones. Pass the evergreen samples around the class so the students can actually feel and see firsthand the difference between the two groups.
If you don't have a microscope handy to show the class plant cells, a printout will suffice. Visual ads are often more effective and encourage engagement, especially in science, as opposed to just writing down notes and reading text. It's usually a good idea to keep some props handy for future reference. Just drilling facts and vocabulary won't get you very far. Stimulate the class with whatever tools are available to you, and you'll see more promising results. Ask the class if they've noticed any particular trees in their neighborhood that have begun to change.
Build on prior student knowledge on the phases of a tree's life cycle, and how it relates to the seasons. Some students may already know about photosynthesis. Expand on that, and discuss the different elements involved, and how the rate of photosynthesis and amount of chlorophyll affect the leaves.
Explain how chlorophyll is not only important to food production, but its green color hides all the other pigments in the leaf. If you already have some leaf samples where the leaves have begun to turn different colors, show the class what happens as the chlorophyll disappears from the leaf. You can even play a matching game with different colored leaves and identify the pigment hidden behind the chlorophyll.
You may also want to engage them in a little art project. Drawing or coloring is another fun way to help them identify various cell parts of leaves. Draw two sets of plant cells, one for a summer leaf, and one for an autumn leaf. Have them label the parts of the cells, and the differences between them. You might also want to include a drawing exercise in an assessment quiz or worksheet.