It is assumed that special structures in aerial roots of leafless orchids, aeration units, are involved in gas exchange regulation. In this study, the structure of aeration units in three leafless taxa (Microcoelia exilis, M. aphylla, Dendrophylax funalis) and a leaf-bearing species (Campylocentrum micranthum) are investigated to obtain more evidence of a possible mechanism for gas exchange control in aerial orchid roots. Extensive analyses of the anatomy of the velamen radicum, exodermis and cortex were performed using light and electron microscopy and µCT. In all considered leafless taxa, the aeration unit consists of pneumathodes (non-wettable regions in the velamen radicum), an exodermal aeration cell (EAC) with a thin cell wall (often equipped with a special internal layer showing a fluffy structure) and specialized cortical cells (SCC). The SCC provide gaps for gas exchange, show differentially thickened cell walls and are arranged in a staggered manner. Based on the 3D structure of the aeration unit, we suggest the possibility of dehydration-triggered gap closure (driven by root shrinkage), in the process of which first the SCC are pushed against each other and second the EAC are squeezed shut by the exodermis.

中文翻译：

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无叶兰花气体交换装置的结构和功能解剖：控制机制的证据？

假设无叶兰花气生根中的特殊结构，即曝气单元，参与了气体交换调节。在这项研究中，研究了三种无叶类群（Microcoelia exilis、M. aphylla、Dendrophylax funalis）和带叶物种（Campylocentrum micranthum）中的曝气单元结构，以获得更多证据证明空气中气体交换控制的可能机制。兰花根。使用光学和电子显微镜以及 µCT 对维拉门、外皮和皮质的解剖结构进行了广泛的分析。在所有考虑的无叶分类群中，通气单元由 pneumathodes（维拉门 radicum 中的非润湿区域）、具有薄细胞壁（通常配备显示蓬松结构的特殊内层）的外胚层通气单元 (EAC) 和专门的皮质细胞（SCC）。SCC 为气体交换提供间隙，显示不同增厚的细胞壁，并以交错方式排列。基于曝气单元的 3D 结构，我们建议脱水触发间隙关闭（由根部收缩驱动）的可能性，在此过程中，首先 SCC 相互推挤，然后 EAC 被外皮挤压关闭.