Betulaceae have simple flowers but complex inflorescences. Recent phylogenetic analyses using molecular data have produced robust phylogenetic trees of Betulaceae. In this study, we evaluated the phylogenetic value of comparative organogenetic data of reproductive organs in the context of molecular phylogenies. Flower and inflorescence developmental processes of 21 species from all six genera in Betulaceae were documented with scanning electron microscopy. In each pistillate cyme, there are one primary bract, two secondary bracts, and two or three flowers in the six genera; only in Alnus are there two tertiary bracts on the abaxial side. The pistillate flower of all genera but Alnus has tepal primordia. Two tepals stop developing early on, resulting in the lack of tepals in mature flowers of Betula; while the tepals are initiated from a common circular primordium at the base of pistil in Corylus, Ostryopsis, Carpinus, and Ostrya, and the developed tepals with irregular shape and unstable number of lobes are adnate to the top of the pistil. In staminate organogenesis, each cyme has one primary bract and three flowers in all genera; two secondary bracts are only present in Alnus, Betula, and Corylus. Staminate flowers have no tepals except in Alnus and Betula, and exhibit high variation in number of stamens among genera. The number of secondary and tertiary bracts in each pistillate and staminate cyme, as well as the presence of tepals in pistillate and staminate flowers was clarified in all genera. Micro-morphological characters were used to infer the phylogenetic relationships of genera and sections of Betulaceae. Our analyses support the division of two subfamilies: Betuloideae (Alnus and Betula) and Coryloideae (Corylus, Carpinus, Ostrya, and Ostryopsis), and three tribes: Betuleae (Alnus and Betula), Coryleae (Corylus), and Carpineae (Carpinus, Ostrya, and Ostryopsis). The results agree with those from molecular phylogenetic studies, and suggest that micro-morphological characters are phylogenetically informative in Betulaceae, and reproductive organs of Betulaceae have evolved in the direction of reduction in bracts and tepals.

中文翻译：

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桦木科植物比较花和花序器官发生：对系统发育关系的影响

桦木科有简单的花，但有复杂的花序。最近使用分子数据的系统发育分析产生了强大的桦木科系统发育树。在这项研究中，我们在分子系统发育的背景下评估了生殖器官的比较器官发生数据的系统发育价值。用扫描电子显微镜记录了桦木科所有 6 个属的 21 个物种的花和花序发育过程。每个雌蕊聚伞花序有1个初级苞片，2个次级苞片，6属2～3朵花；只有在Alnus的背面有两个三级苞片。所有属的雌蕊花，但桤木有花被原基。两片花被提前停止发育，导致桦木成熟花中缺少花被；而花被片从花冠属、Ostryopsis、Carpinus和Ostrya的雌蕊基部共同的圆形原基起始，发育的形状不规则、裂片数不稳定的花被片贴生于雌蕊顶端。在雄蕊器官发生中，每个聚伞花序在所有属中都有一个初级苞片和三朵花；两个次生苞片仅存在于桤木、桦木和 Corylus 中。除了桤木和桦木之外，雄花没有花被片，并且在属之间的雄蕊数量上表现出很大的差异。每个雌花和雄蕊聚伞花序中的二级和三级苞片的数量，以及雌花和雄花中花被片的存在在所有属中都得到了澄清。微形态特征用于推断桦木科属和科的系统发育关系。我们的分析支持两个亚科的划分：桦木亚科（Alnus 和 Betula）和 Coryloideae（Corylus、Carpinus、Ostrya 和 Ostryopsis），以及三个部落：Betuleae（Alnus 和 Betula）、Coryleae（Corylus）和 Carpineae（Carpinus、Ostrya） , 和 Ostryopsis)。结果与分子系统发育研究的结果一致，表明桦木科的微形态特征具有系统发育信息，并且桦木科生殖器官向苞片和花被片减少的方向进化。