Soon, ghosts and goblins will take to the streets to celebrate this season for the macabre. Today, Halloween is an occasion for dressing up in costumes and trick-or-treating. Its origins include pagan and Christian observances commemorating seasonal harvests and the memories of loved ones who have passed away.
Many people are uncomfortable with the topic of death. Admittedly, the unknowns surrounding the event can be unsettling at best. But one of the core truths about life is none of us are getting out of here alive. The Stoic philosophers encouraged “memento mori” or “remember that you must die.” The idea is that through memento mori, we develop a greater appreciation for life and beauty in the present. After all, the present is all we have ever had and all we will ever have.
Observing plants provides us with opportunities to appreciate the present. Examples include the miraculous germination of seeds, the fleeting blossoms of spring ephemerals, sunflowers dutifully following the summer sun across the sky, or leaves achingly beautiful death fade from green to shades of yellow, orange, and red to brown in the crisp autumn air.
As deciduous plants transition from green sunlight-absorbing leaves filled with chlorophyll to bare stems and branches during winter dormancy, their leaves blaze spectacularly in a process called leaf senescence. Senescence is initiated by shorter days and cooler temperatures in the fall. It is the process of leaf deterioration with age.
Chlorophyll is an essential pigment in plant leaves, helping plants absorb sunlight to make food through photosynthesis. As leaf senescence occurs, the plant breaks down chlorophyll into sugars and amino acids for the plant to reabsorb for use the following spring.
Carotenoids are yellow and orange pigments in the leaves that become visible as chlorophyll breaks down. Anthocyanin pigments are red, purple, or crimson and are not present in leaves before the initiation of leaf senescence. Anthocyanins develop in response to sunlight exposure and the sugars that accumulate in the leaves from the breakdown of chlorophyll. The different processes in which these pigments are expressed explain the broad spectrum of autumn leaf colors we experience. For more information about leaf color changing, visit HGIC 1029, Color Changes in Autumn Leaves.
At the same time these color changes occur, a leaf abscission layer starts forming at the base of the leaf’s petiole. The abscission layer is a barrier that eventually stops the flow of water and nutrients between the leaf and the plant. When the abscission layer completes its development, leaves fall from the plant due to rain, wind, or gravity. The recycling of nutrients begins as the leaves decompose on the ground. Visit Leave the Leaves Revisited for more information about nutrient recycling in fallen leaves.
As we witness the spectacular autumnal death of this season’s leaves, let us practice memento mori, and rejoice in the present by doing so.