They fall silently, a great, hushed rustling of blessings.
Let us then, give thanks for leaves — far more than for pilgrims or turkeys. Let us count leaves on our list of blessings, for these ancient flutterers have made all the rest possible.
I study the arrangement of the leaves on the tip of a branch, trying to glimpse the mysterious mathematical proportions of the Fibonacci numbers. Turns out, this number sequence articulated hundreds of years ago by a Renaissance genius of the same name captures some mysterious symmetry of the universe — from the Golden Mean by which we determine whether a woman’s face is beautiful to the arrangement of leaves on a branch.
Lying here amidst the rustle and the splendor, with winter already closing in and summer but a dappled memory, the world seems so simultaneously simple and intricate that it seems miraculous at every scale.
Consider the humble leaf, in all its misleading simplicity.
In a few hundred million years, leaves have made the planet’s great diversity of living things possible, for leaves have dramatically increased the ability of plants to make energy from sunlight — which fuels the whole three-ringed circus of life.
You can search the planet and not find a more diverse and absorbing illustration of the miracle of leaves than Payson — perched on an ecological boundary. Here, the cottonwoods and sycamores along the East Verde go lurid every fall, dropping the leaves of summer in an act of shrewd extravagance. But growing right alongside them the patient junipers and pines hold onto their evergreen leaves all through winter’s hardships.
So I let the leaves half bury me, gulping the gift of their oxygen and marveling at the ingenuity of their tradeoffs. For leaves have taken us all on a long, strange trip.
The first land-based plants emerged from the great nursery of ocean some 1.2 billion years ago, but they amounted to variations on pond scum.
The first true land-dwelling plants didn’t creep ashore until about 500 million years ago, having already mastered the art of photosynthesis. Green chlorophyll allows plants to use the energy of sunlight to turn water and carbon dioxide into sugars and a little gasp of oxygen, which the plant sheds as waste to the great delight of air-breathers everywhere.
But here’s a mystery: Early plants hit upon roots, stems, trunks, veins, photosynthesis and many other hallmarks of plantiness a full 50 million years before they came up with leaves — followed promptly by flowers.
That long wait has perplexed botanists, given the huge advantages plants gain from leaves. If leaves dramatically increase growth, why did plants wait so long to invent them?
One set of researchers who published their speculations in the journal Nature suggested vascular plants had to wait until they had significantly reduced the carbon dioxide levels in the atmosphere and thereby cooled the planet before they could evolve leaves. That’s because leaves may produce energy, but they also lose a lot of water from evaporation and soak up a lot of extra heat.
The researchers suggested that in the hot, carbon dioxide-rich atmosphere 500 million years ago, leaves would all shrivel.
Others dispute that theory, pointing out that those early plants lived in swamps where they had plenty of water, which means they could use extravagant levels of evaporation to keep cool.
Instead, some botanists look to the root of the problem — arguing that plants didn’t need leaves until the roots became efficient enough to make use of the energy the leaves could produce. And that, in turn, may date to the start of an ancient partnership between plant roots and fungi growing in the soil. Most plants now depend on a whole little world of bacteria and fungi growing on their roots to dramatically increase their uptake of water and nutrients from the soil. In return, the fungi and bacteria essentially feed on the roots.
For instance, at least 78 different species of bacteria grow on the roots of a cottonwood, according to a study by researchers from Brookhaven National Laboratory. Seeking ways to make plants like cottonwoods and aspen grow faster, the researchers isolated these different bacteria then grew seedlings with different combinations. They discovered that certain bacteria increase the plant’s growth rate by 50 percent.
So one theory on the origins of leaves suggests that once plant roots found their key partners in the soil, they could finally take advantage of the energy boost offered by the evolution of leaves.
Now, consider a remarkable feedback effect once plants hit on leaves.
Leaves provide a second great benefit to plants — based on what looks at first glance to pose a disadvantage: evaporation.
A cottonwood every day loses huge quantities of water through its leaves. But those sun-baked leaves are connected directly to the roots through the vascular system of the plant. As a result, the pressure created by the evaporation from the leaf’s surface effectively pumps water and nutrients up from the roots. This again dramatically increases the growth rate of the tree.
So plants could never have escaped the swamp nor grown more than a few feet tall until they invented leaves, which became the foundation of the explosion of diversity in life on this planet.
A leaf lands on my forehead. I watched it all the way down, fascinated by the spiraling flutter. At the last possible moment, it veered to crash land on my forehead.
I am startled — but grateful.
I think all the world rustles now in the branches of my cottonwood — and the maples and sycamores and aspen that flare for their own tremble of glory in the stately progression of fall.
They have nourished us, made up our beds, pulled the comforter snug against our chins.
I can’t say that we’ve expressed our gratitude very well.
We’ve laid waste to the riparian areas once dominated by cottonwoods and sycamores and the other giants of fall. By some estimates, these cottonwood-willow ecosystems sustain a greater mass and diversity of life than any place outside the rainforest. But we’ve destroyed or degraded 90 percent of those areas in the past century.
Mercifully, the cottonwood that shelters me now does not appear to hold it against me.
The great poplar gently rains its blessings down upon me.
It soothes my soul.
It sings with the wind.
It pays its debts.
For it exchanges my carbon dioxide for its oxygen — and then returns the gifts of a star to the waiting earth — and all through the miracle of a leaf.