January 1, 2021
There is only one tree with knees and no one knows why.
At the turn of the year, in the depth of winter, I’m kept by the cold from paddling on the rivers and sounds so I’ve occupied myself with learning more about the place where I live, especially the swamps of North Carolina. And there is nothing more distinctive in those swamps than the bald cypress.
The cypress family—including the coast redwood and the giant sequoia—are inexpressibly old. They were growing on Pangea around 150 million years ago before the supercontinent broke up and distributed the trees globally. That is old.
But only one of them—the bald cypress—has knees. (Also, the pond cypress if you’re going to quibble but they’re so closely related you can’t casually tell them apart.)
The bald cypress grows best in the soft muck of a swamp. In fact, places where they dominate the canopy are called cypress swamps. They thrive in environments where little else can compete. Their roots spread horizontally, like a spider’s web, before plunging vertically, providing leverage against the trunk being downed by a strong wind, but the knees are often thrust several feet above average high water.
Biologists were once convinced that the knees helped aerate the roots in swamp water deprived of oxygen. It seemed a reasonable assumption until field experiments found there was no exchange of gases between the knees and the atmosphere. Cypress knees are also lacking the tissues—lenticels and aerenchyma—that plants use to move gases into their roots.
The alternate theory—that knees stiffen the roots and provide the tree with greater support—is belied by the fact that bald cypress growing in deep water don’t develop knees. Since the tree would seem to need as much support in deep water as shallow, this theory has no legs.
So, this unique feature of a unique tree remains an enigma to botanists and adds mystery to my admiration of swamps.