Fall Leaf Colors!

This week, I went to Fairmount Park in Philadelphia armed with my Foldscope. Wandering around with the microscope pressed to my face, I definitely amassed some weird looks, which I initially chose to ignore. However, after a couple of minutes, a man came up to me and asked what I was doing. I happily explained and he thought the Foldscope concept was really cool!

Soon, I came to a big maple tree that I walk by often and have always thought was pretty. This time though, evaluating the tree with the mind of a scientist, I noticed the vast array of colors the one tree was displaying. Some leaves were yellow, some were green, some were brown, and some were mottled. I became curious about how these colors would look under the microscope, and the extent to which the actual structure of the leaves changes along with the color.

Such an array of colors, all from the same tree!

First, I decided to examine the green leaf. I figured this was kind of the “default” leaf state, to which I’d compare all the other colors. I managed to get a really good picture under the Foldscope:

It’s so cool how you can see the transparent veins and then the green blocks, which must include cells in which the chlorophyll is located.

Next, I looked at the yellow leaf. Here is the image that I was able to capture:

The structure of the leaf looks the same as in the green leaf, with the colored blocks partitioned by more transparent veins.

Next, I studied the brown leaf. Unfortunately, it was much harder to take a clear picture of this one under the microscope, which I think is because its lower moisture level makes it less transparent and less easily illuminated. However, I was still able to get a general sense of what it looked like and, surprisingly, it still has the same veiny structure as the other colored leaves:

Based on these observations, I decided to do more research about the mechanisms behind leaf color changes. Apparently, during the spring and summer, leaves have so much chlorophyll that its green color obscures other pigments that are present. However, in the fall, trees are able to detect changing levels of sunlight and temperature, which triggers the breakdown of chlorophyll. This exposes other pigments (normally orange and yellow) such as carotenes and xanthophyll, which changes the color of the leaves and produces the beautiful fall foliage that humans love! It makes sense that in my microscope observations, the leaves all look the same structurally, but with different colors; the pigment composition of the cells changes, rather than the structure of the leaf itself.

My leaf exploration still left me with one big question: why are leaves from the same tree experiencing different stages of seasonal transition? While some are still green, many are yellow, and others are brown and dried up. Perhaps this is due to differences in temperature and light levels experienced by different branches of the same tree? If anyone knows the answer to this question or has another theory, feel free to let me know in the comments!

I conducted this project as part of Professor Pringle’s EEB321 class at Princeton University.

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