Premise of research. Although the term “pseudobulb” has been used in the literature on Orchidaceae for almost two centuries, no definition establishes unambiguous criteria to distinguish such a succulent shoot from a “normal” stem. It is currently impossible to decide whether variation is continuous rather than categorical. Methodology. We studied three morphological characteristics (succulence index, deviation from spherical shape, and proportion of lignified tissue in cross sections) in stems of 89 orchid species. Pivotal results. Shape as such did not allow us to separate groups: variation was continuous in our data set. However, a cluster analysis of all three traits separated species with normal stems from those with thickened stems. Within the latter group, two subgroups emerged: a relatively homogeneous subgroup of species with rather globose, mostly heteroblastic pseudobulbs and a morphologically and anatomically much more diverse second subgroup of species with homoblastic stems. A second part of the study assessed ontogenetic changes in stem shape. In the majority of cases, stem shape changed substantially from sometimes almost spherical in small juveniles to elongated in larger individuals. This pattern, although much more pronounced in species with normal stems, was also found in certain species with pseudobulbs. While water storage capacity in the smallest plants is maximized relative to the transpiring surface, the increased absolute water storage capacity of larger plants relaxes the selective pressure to depart more and more from a spherical shape. Conclusions. Although some ambiguity remains, the results of our quantitative approach confirm the usefulness of the pseudobulb concept. However, we identify a large, quite heterogeneous group of species with intermediate traits between those of normal stems and pseudobulbs.

中文翻译：

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什么是假灯泡？– 走向量化定义

研究的前提。尽管“假鳞茎”一词已在兰科文献中使用了近两个世纪，但没有任何定义建立明确的标准来区分这种多汁的枝条和“正常”的茎。目前无法确定变异是连续的还是分类的。方法。我们研究了 89 种兰花茎的三个形态特征（多汁指数、球形偏差和横截面木质化组织的比例）。关键结果。这样的形状不允许我们将组分开：在我们的数据集中变化是连续的。然而，对所有三个特征的聚类分析将具有正常茎的物种与具有加厚茎的物种区分开来。在后一组中，出现了两个子组：一个相对同质的物种亚群，具有相当球状的，主要是异胚鳞茎，以及形态和解剖学上更加多样化的具有同胚茎的物种的第二亚群。该研究的第二部分评估了茎形状的个体遗传变化。在大多数情况下，茎的形状发生了巨大变化，从有时在小型幼鱼中几乎呈球形变为在较大个体中伸长。这种模式虽然在具有正常茎的物种中更为明显，但在某些具有假鳞茎的物种中也被发现。虽然最小植物的储水能力相对于蒸腾表面最大化，但较大植物增加的绝对储水能力降低了选择压力，使其越来越远离球形。结论。虽然有些暧昧，我们定量方法的结果证实了假灯泡概念的有用性。然而，我们确定了一个大的、相当异质的物种群，其具有介于正常茎和假鳞茎之间的中间特征。