Interlocked grain occurs when the orientation of xylem fibers oscillates, alternating between left- and right-handed spirals in successive wood layers. The cellular mechanisms giving rise to interlocked grain, thought to involve the slow rotation of fusiform initials within the vascular cambium, remain unclear. We suggest that observations of wood structure at the cellular level, but over large areas, might reveal these mechanisms. We assayed timber from several commercially-important tropical angiosperms from the genus Khaya (African mahogany) that exhibit interlocked grain using X-ray computed microtomography followed by orthogonal slicing and image processing in ImageJ. Reconstructed tangential longitudinal sections were processed with the ImageJ DIRECTIONALITY plug-in to directly measure fiber orientation, and showed grain deviations of more than 10^o from vertical in both left- and right-handed directions. Grain changed at locally constant rates, separated by locations where the direction of grain change sharply reversed. Image thresholding and segmentation conducted on reconstructed cross sections allowed the identification of vessels and measurement of their location, with vessel orientations then calculated in Matlab and, independently, in recalculated tangential longitudinal sections with the DIRECTIONALITY plug-in. Vessel orientations varied more than fiber orientations, and on average deviated further from vertical than fibers at the locations where the direction of grain change reversed. Moreover, the reversal location for vessels was shifted ~ 400 μm towards the pith compared to the fibers, despite both cell types arising from the same fusiform initials within the vascular cambium. We propose a simple model to explain these distinct grain patterns. Were an auxin signal to control both the reorientation of cambial initials, as well as coordinating the end-on-end differentiation and linkage of xylem vessel elements, then it would be possible for fibers and vessels to run at subtly different angles, and to show different grain reversal locations.

中文翻译：

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通过 X 射线计算机显微断层扫描分析非洲桃花心木 (Khaya spp.) 中互锁晶粒的形成

当木质部纤维的方向发生振荡，在连续木层中的左旋和右旋螺旋之间交替时，就会出现互锁纹理。产生互锁颗粒的细胞机制，被认为涉及维管形成层内梭形首字母的缓慢旋转，仍不清楚。我们建议在细胞水平上观察木材结构，但在大面积上，可能会揭示这些机制。我们使用 X 射线计算机显微断层扫描，然后在 ImageJ 中进行正交切片和图像处理，分析了来自Khaya（非洲桃花心木）属的几种具有商业重要性的热带被子植物的木材，这些被子植物显示出互锁的纹理。使用 ImageJ DIRECTIONALITY处理重建的切向纵向截面直接测量纤维取向的插件，并显示左右手方向与垂直方向的纹理偏差超过 10 ^o。颗粒以局部恒定速率变化，由颗粒变化方向急剧反转的位置分隔。对重建的横截面进行图像阈值处理和分割，可以识别血管并测量其位置，然后在 Matlab 中计算血管方向，并独立地在重新计算的切向纵向截面中使用DIRECTIONALITY插入。血管取向的变化大于纤维取向，平均而言，在晶粒变化方向相反的位置，与纤维相比，垂直偏离更远。此外，与纤维相比，血管的反转位置向髓部移动了约 400 μm，尽管两种细胞类型都来自血管形成层内的相同梭形初始。我们提出了一个简单的模型来解释这些不同的颗粒图案。如果生长素信号控制形成层首字母的重新定向，以及协调木质部血管元件的端到端分化和连接，那么纤维和血管就有可能以微妙的不同角度运行，并显示不同的晶粒反转位置。