Pit membranes between xylem vessels play a major role in angiosperm water transport. Yet, their three-dimensional (3D) structure as fibrous porous media remains unknown, largely due to technical challenges and sample preparation artefacts. Here, we applied a modelling approach based on thickness measurements of fresh and fully shrunken pit membranes of seven species. Pore constrictions were also investigated visually by perfusing fresh material with colloidal gold particles of known sizes. Based on a shrinkage model, fresh pit membranes showed tiny pore constrictions of ca. 20 nm, but a very high porosity (i.e. pore volume fraction) of on average 0.81. Perfusion experiments showed similar pore constrictions in fresh samples, well below 50 nm based on transmission electron microscopy. Drying caused a 50% shrinkage of pit membranes, resulting in much smaller pore constrictions. These findings suggest that pit membranes represent a mesoporous medium, with the pore space characterized by multiple constrictions. Constrictions are much smaller than previously assumed, but the pore volume is large and highly interconnected. Pores do not form highly tortuous, bent, or zigzagging pathways. These insights provide a novel view on pit membranes, which is essential to develop a mechanistic, 3D understanding of air-seeding through this porous medium.

中文翻译：

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高孔隙率，微小的孔口收缩和不弯曲的通道是被子植物木质部间窝孔膜的3D结构的特征。

木质部血管之间的坑膜在被子植物的水运输中起主要作用。然而，它们的三维（3D）结构作为纤维状多孔介质仍然是未知的，这主要是由于技术挑战和样品制备伪像。在这里，我们基于七个物种的新鲜和完全收缩的凹坑膜的厚度测量结果，应用了一种建模方法。还通过在新鲜物质中注入已知大小的胶体金颗粒进行目测缩孔。基于收缩模型，新鲜的凹坑膜显示出约微小的孔收缩。20 nm，但具有很高的孔隙率（即孔体积分数），平均为0.81。根据透射电子显微镜，灌注实验显示新鲜样品中相似的孔收缩，远低于50 nm。干燥导致凹坑膜收缩50％，导致更小的孔收缩。这些发现表明，凹坑膜代表介孔介质，其孔隙空间具有多个收缩特征。收缩比以前假定的要小得多，但是孔体积很大并且高度互连。毛孔不会形成高度曲折，弯曲或曲折的路径。这些见解提供了关于凹坑膜的新颖观点，这对于发展对通过这种多孔介质的空气播散的机械3D理解至关重要。