Autumn is the season of ambivalence and reconciliation, soft-carpeted training ground for the dissolution that awaits us all, low-lit chamber for hearing more intimately the syncopation of grief and gladness that scores our improbable and finite lives — each yellow burst in the canopy a reminder that everything beautiful is perishable, each falling leaf at once a requiem for our own mortality and a rhapsody for the unbidden gift of having lived at all. That dual awareness, after all, betokens the luckiness of death.

But autumn is also the season of revelation, for the seeming loss unveils a larger reality: Chlorophyll is a life-force but it is also a cloak, and when trees shed it from their leaves, nature’s true colors are revealed.

Photosynthesis is nature’s way of making life from light. Chlorophyll allows a tree to capture photons, extracting a portion of their energy to make the sugars that make it a tree — the raw material for leaves and bark and roots and branches — then releasing the photons at lower wavelengths back into the atmosphere. A tree is a light-catcher that grows life from air.

Although the human mind has puzzled over why leaves fall and change color at least as far back as Aristotle, chlorophyll — which shares chemical kinship with the hemoglobin in our blood — was only discovered and named in 1817, by the French pharmacist-chemist duo Joseph Bienaimé Caventou and Pierre Joseph Pelletier. In a lovely touch of humility that distinguishes, always, the scientist from the explorer — the explorer, so eager to name the lands and landmarks he has “discovered” after himself — they wrote in their landmark paper:

We have no right to name a substance long-known, and to the story of which we have added only a few facts; however, we will propose, without granting it any importance, the name chlorophyll, from chloros, color, and φυλλον, leaf: a name that would indicate the role it plays in nature.

But chlorophyll, which is yet to be fully understood, is not the only pigment in trees. Throughout a leaf’s life, four primary pigments course through its cells: the green of chlorophyll, but also the yellow of xanthophyll, the orange of carotenoids, and the reds and purples of anthocyanins.

In spring and summer, when the days grow long and bright, chlorophyll saturates leaves as the tree busies itself converting photons into the sweetness of new growth.

As daylight begins fading in autumn and the air cools, deciduous trees prepare for wintering and stop making food — an energy expenditure too metabolically expensive in the dearth of sunlight. Enzymes begin breaking down the decommissioned chlorophyll, allowing the other pigments that had been there invisibly all along to come aflame. And because we humans so readily see in trees metaphors for our emotional lives, how can this not be a living reminder that every loss reveals what we are made of — an affirmation of the value of a breakdown?

A similar process take place as fruit ripen from green to varying shades of red, purple, orange, or yellow.

Two centuries after the discovery of chlorophyll, a new generation of scientists armed with a new arsenal of tools unimaginable in 1817, in that abiding way science has of only revealing new layers of reality when it lets go of its assumptions, placed bananas in various stages of ripeness under UV light and discovered that as the world’s favorite yellow fruit ripens and its chlorophyll breaks down, it not only reveals the xanthophyll of yellow, but produces the chlorophyll catabolite hmFCC — a previously unknown blue fluorescent compound.

Subsequent research has found signs of this blue pigment in devil’s ivy — the evergreen golden pothos thriving in the corner of my library in Brooklyn at this very moment — rendering the mystery of chlorophyll ongoing and filling the human heart with exhilaration. How thrilling to think that something we discovered two centuries ago, something nature created more than a billion years ago when the first green plants evolved from prokaryotes, can still shimmer with mystery — a molecular microcosm of the ultimate thrill: the knowledge that however much we might uncover, nature will never cease to be filled with surprise ripe for the reaping. And how humbling to think that we too are animals doing their best to make sense of the world with their creaturely limitations — animals whose vision evolved to peak in so narrow a band of the spectrum, in the tiny wavelength range between red and violet, blind to everything between radio and cosmic rays, blind to ultraviolet light. But if we were butterflies or reindeer, bees or sockeye salmon, bananas might be blue.

The poetic astronomer Maria Mitchell captured this best in her rueful and rapturous observation that “we have a hunger of the mind which asks for knowledge of all around us, and the more we gain, the more is our desire,” and yet “we reach forth and strain every nerve, but we seize only a bit of the curtain that hides the infinite from us.”

Complement with the late, great nature writer Ellen Meloy on the conscience of color from chemistry to culture, then revisit this fascinating read on Turing, trees, and the science of how alive you really are.

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My name is Maria Popova — a reader, a wonderer, and a lover of reality who makes sense of the world and herself through the essential inner dialogue that is the act of writing. The Marginalian (which bore the unbearable name Brain Pickings for its first 15 years) is my one-woman labor of love, exploring what it means to live a decent, inspired, substantive life of purpose and gladness. Founded in 2006 as a weekly email to seven friends, eventually brought online and now included in the Library of Congress permanent web archive, it is a record of my own becoming as a person — intellectually, creatively, spiritually, poetically — drawn from my extended marginalia on the search for meaning across literature, science, art, philosophy, and the various other tendrils of human thought and feeling. A private inquiry irradiated by the ultimate question, the great quickening of wonderment that binds us all: What is all this? (More…)

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