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.

柏柏树膝盖是多形的狭窄圆锥形或多根垂直的根部生长物，主要由紫杉（Taxodium distichum）在沼泽地中形成。几乎180年以来，植物学对膝盖的零星关注一直未能就其适应性意义达成共识，但有关其结构的数据不足，使这种情况更加恶化。本项目的目的是对膝盖和相关的根部（包括树皮）进行广泛的结构研究。这项工作主要基于切片机切片，浸软和活膝活检穿孔样本。膝盖木材柔软，轻巧但坚固，气管的长度和直径变化很大，通常很宽。射线和轴向薄壁组织含有丰富的淀粉。射线在树林中大多是单生的，并且通常会膨胀以在树皮中复裂。轴向薄壁组织局限于木材中，多产于内部树皮中。多孔的膝盖外皮，主要由纤维和塌陷的薄壁细胞组成，剥落下来，几乎没有一层附着在内部树皮上。内部树皮的孔隙较少，并且在纤维之间的细胞变质时会引发空腔。虽然没有排除人们普遍认为膝盖是使根部通风的“气穴”，但结果提示了对韧皮部以及膝盖本身的轴向薄壁组织，射线薄壁组织和形成层充气的关键作用。