Bald Cypress knees, Taxodium distichum (Cupressaceae): An anatomical study, with functional implications

Abstract

Bald Cypress knees are polymorphous narrowly conical or multitipped vertical root outgrowths formed predominantly in swampy habitats by Taxodium distichum. Almost 180 years of sporadic botanical attention to the knees has failed to generate consensus on their adaptive significance, a situation exacerbated by inadequate data on their structure. The purpose of the present project is to provide a broad structural study of the knees and associated roots, including bark. The effort is based primarily on microtome sections, on macerations, and on biopsy punch specimens from living knees. The knee wood is soft and light yet solid, with tracheids highly varied in length and diameter, often notably broad. Rays and axial parenchyma contain abundant starch. Rays are mostly uniseriate in the wood, and often dilate to multiseriate in the bark. Axial parenchyma is limited in the wood, prolific in the inner bark. The porous knee outer bark, comprised mostly of fibers and collapsed thin-walled cells, flakes off leaving few layers adherent to the inner bark. The inner bark is less porous and initiates cavities upon cell deterioration between fibers. While not ruling out the common perception of knees serving as “pneumatophores” ventilating the roots, the results suggest a critical role aerating the phloem as well as the axial parenchyma, ray parenchyma, and cambium within the knee itself.