Cypress Knees
Mysterious structures that have baffled botanists through the centuries, until recently
I first discovered cypress knees while on a pontoon boat on the Chickahominy River in southeastern Virginia. As we weaved through a creek, I noticed dull spears of wood rising out of the water, concentrated around the trunks of cypress trees. My partner’s mother said they were cypress knees. They were curious structures, and I wanted to know why cypress trees produced them.
Cypress knees have long mystified botanists. In 1819, French botanist François André Michaux wrote: “The roots are charged with protuberances eighteen to twenty-four inches high. […] These protuberances are always hollow, and smooth on the surface, and are covered with a reddish bark, like the roots, which they resemble in softness of wood. They exhibit no sign of vegetation, and I have never succeeded in obtaining shoots from wounding the surface and covering it with earth. They are peculiar to the cypress, and begin to appear when it is twenty to twenty-five feet high. […] No cause can be assigned for their existence.”
Over the centuries, botanists have suggested various hypotheses for cypress knees’ existence, including root system aeration, structural support, nutrient accumulation, and carbohydrate storage. A few clues around when the trees produce the knees (or not) helped narrow the likely explanations. For instance, cypress knees form on trees whose roots are sporadically flooded, but they are lacking or absent in trees growing in dry soils and, intriguingly, trees growing where their roots are continuously flooded.
In 1890, a naturalist named Robert H. Lamborn wrote an article in Science about cypress knees. He was a champion of the hypothesis that cypress knees were for mechanical support, which he believed they needed to weather severe storms. The end of his article, which tends towards the literary, depicts a lone cypress tree at war with cyclones:
“The cyclone, the loose sand, the morass,—these are the enemies they contend with, as it were, in unbroken phalanx, shoulder to shoulder, their shields locked, their spears bristling against the foe; but the graceful plumed cypress, the knight-errant of the sylvan host, bearing with him his trusty anchor,—the emblem of hope,—goes forth alone and defiant, afar from his fellows, scorning the methods of his vassals, and planting himself bolding amid a waste of waters, where no other tree dare venture, stands, age after age, erect, isolated, but ever ready to do battle with the elements. Twenty centuries of driving rain and snow and fierce hurricane beat upon his towering form, and yet he stands here, stern, gray, and solitary sentinel of the morass, clinging to the quaking earth with the grasp of Hercules, to whom men were building temples when his wardenship began.
—Robert H. Lamborn, 1890
While the cypress tree’s root system certainly bolsters its place in uneven waters and mud, the leading hypothesis is that cypress knees function as pneumatophores, structures that allow gas exchange via diffusion. (Pneumato- meaning “air” or “breath” and -phore meaning “bearer of” or “carrier”.) A quick biology refresher may be useful here. Animals eat other animals or plants, and they require cellular respiration to survive. Plants make their own food with the Sun’s energy, so they must do both photosynthesis (creating food) and cellular respiration (breaking it down for energy). Just for fun, and since my high school biology teacher would be proud that I still remember these (thanks Ms. Veenstra!), here are the equations for photosynthesis and cellular respiration. Notice how they are essentially the reverse of each other.
Photosynthesis: 6CO2 + 6H2O + solar energy —> C6H12O6 + 6O2
(carbon dioxide, water, and solar energy yield glucose and oxygen)
Cellular respiration: C6H12O6 + O2 —> 6CO2 + 6H2O + energy
(glucose and oxygen yield carbon dioxide, water, and energy in the form of adenosine triphosphate or ATP)
Plants require oxygen for cellular respiration in all living tissues, including in underground roots. In environments where oxygen is lacking, plants have adapted. For example, many mangrove species have air roots that function as pneumatophores which allows them to thrive in anaerobic environments.
Cypress knees may be similar. A paper published in the International Journal of Plant Science in 2015 investigated the root aeration/gas exchange hypothesis. The authors, Craig E. Martin and Sarah K. Francke from the University of Kansas, measured oxygen concentrations in air extracted from submerged cypress roots when attached knees were above water versus submerged. They found that when roots were submerged in water, the root’s internal air oxygen concentration was much higher when the knee was above water than when both the knee and root were submerged. They concluded: “This result unambiguously supports past assumptions that cypress knees do indeed function as pneumatophores in supplying submerged roots with oxygen.”
The gas exchange hypothesis makes sense, except for the fact that, as noted earlier, cypress knees are lacking in trees where the roots are continuously flooded. How do these trees obtain sufficient oxygen? It’s unclear, but perhaps 100 years from now a scientist will find an answer.
Note: This week I’ll be sending out two posts since I missed last week’s. The second one is coming Sunday. Turns out I’m still adjusting to having a full-time job, a 10-month-old, taking two classes towards my Masters degree, and writing on Substack. Your grace and forgiveness is appreciated.
Thoroughly enjoyed this blog. I will never look at our dear Cypress trees the same way again. I was even mentioned!