PREMISE Medullary bundles, i.e., vascular units in the pith, have evolved multiple times in vascular plants. However, no study has ever explored their anatomical diversity and evolution within a phylogenetic framework. Here, we investigated the development of the primary vascular system within Nyctaginaceae showing how medullary bundles diversified within the family. METHODS Development of 62 species from 25 of the 31 genera of Nyctaginaceae in stem samples was thoroughly studied with light microscopy and micro-computed tomography. Ancestral states were reconstructed using a maximum likelihood approach. RESULTS Two subtypes of eusteles were found, the regular eustele, lacking medullary bundles, observed exclusively in representatives of Leucastereae, and the polycyclic eustele, containing medullary bundles, found in all the remaining taxa. Medullary bundles had the same origin and development, but the organization was variable and independent of phyllotaxy. Within the polycyclic eustele, medullary bundles developed first, followed by the formation of a continuous concentric procambium, which forms a ring of vascular bundles enclosing the initially formed medullary bundles. The regular eustele emerged as a synapomorphy of Leucastereae, while the medullary bundles were shown to be a symplesiomorphy for Nyctaginaceae. CONCLUSIONS Medullary bundles in Nyctaginaceae developed by a single shared pathway, that involved the departure of vascular traces from lateral organs toward the pith. These medullary bundles were encircled by a continuous concentric procambium that also constituted the polycyclic eustele, which was likely a symplesiomorphy for Nyctaginaceae with one single reversion to the regular eustele.

中文翻译：

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Nyctaginaceae 髓束的多样性、分布、发育和进化

前提 髓质束，即髓中的维管单位，在维管植物中已经进化多次。然而，没有任何研究在系统发育框架内探索过它们的解剖多样性和进化。在这里，我们研究了 Nyctaginaceae 内主要血管系统的发育，显示了髓束在家庭中是如何多样化的。方法 用光学显微镜和显微计算机断层扫描对茎样本中的 31 个 Nyctaginaceae 属中的 25 个 62 种的发育进行了深入研究。使用最大似然方法重建祖先状态。结果发现了两种亚型的eustele，没有髓束的规则eustele，仅在Leucastereae 的代表中观察到，而多环eustele，含有髓束，在所有剩余的分类群中发现。髓束具有相同的起源和发展，但组织是可变的并且独立于叶序。在多环外胚层内，首先形成髓束，然后形成连续的同心原形成层，形成一圈维管束，包围着最初形成的髓束。规则的 eustele 作为 Leucastereae 的突触形态出现，而髓质束被证明是 Nyctaginaceae 的突触形态。结论 Nyctaginaceae 中的髓束通过单一共享途径发展，这涉及血管痕迹从侧部器官向髓部的离开。这些髓质束被一个连续的同心原形成层包围，该原形成层也构成了多环 Eustele，