Magnolia Leaves: Why are they so weird?

After taking a year of AP Biology, we have been acclimated to apply what we see around us to everything that we have learned in class. In a biology scavenger hunt that Ms. Eckert made for us, she asked us to find a non-native plant whose leaves are used to make wreaths. Our group immediately went for the Blue Spruce Tree, thinking that the leaves would be like pine leaves, but as it turns out the Blue Spruce was not what she was looking for. Although the Blue Spruce is both non-native and used to make wreaths, the answer Ms. Eckert was looking for was the magnolia tree. Surprised, we decided to go take a closer look at the tree. When examining the leaves, we found that the underside was fuzzy and soft. Immediately our first question was: what kind of evolutionary advantage would these strange adaptations have for the plant? (of course)

Our own amphitheater is home to dozens of plant species, one of which is the fascinating magnolia tree. You might know the magnolia tree as a tree that sprouts beautiful flowers in the springtime, (which it does), but to us, they are a lot more than just pretty plants, they are a product of millions of years of intricate evolution. The top of magnolia leaves have a waxy coating that makes them resistant to damage from salt and air pollution. On the underside, leaves have yellow-brown pubescence, which is soft, short, erect trichomes (hairs). See the image below to compare the top and underside of the magnolia leaf.

While many plants have trichomes, the concentration of trichomes on the magnolia leaf that make its underside fuzzy is unique. We wondered what evolutionary advantage this adaptation might provide for the magnolia plant. Creating and maintaining such a large number of trichomes takes a lot of energy and resources from the plant that could be used for other necessary life functions, so why would natural selection favor plants that invested so much of their energy into this? The trichomes must serve an important function that makes the amount of energy used up by their maintenance worthwhile for the plant. The function of trichomes on plants in general can be to reflect excess light and provide shade for the plant, to reduce water loss, and to defend against insects by forming a barrier or secreting sticky fluids and toxic compounds.

Magnolia leaves have an abundance of trichomes on the underside, but none on the tops. This leads us to hypothesize that trichomes on magnolia leaves may be used more for the purpose of reducing water loss from the plant through transpiration, the process by which a plant loses water by evaporation. This evaporation occurs through the stomata, pores in the epidermis of the leaf located on the underside, just as the trichomes are. Stomata on a magnolia leaf are surrounded by trichomes which can trap moisture from some of the water evaporating. The unique structure of magnolia leaves then causes the plant to require less water input to maintain homeostasis and carry out life functions.

As our fellow classmates walk across the bridge at graduation, they will be able to see the magnolia trees from their beautiful flowers to their intriguing textured leaves. If you happen to stumble upon a magnolia tree, take a closer look! Maybe you’ll uncover something yourself.

This is a picture of a Magnolia wreath, which are becoming very popular for the holidays.

This is a magnolia tree from the amphitheater. Here, you are able to see the glossy surface on top of the leaf and the fuzzy trichomes on the underside of the leaf (and Sammy admiring it :) )

Samantha Rego and Leela Rangachar