What Is a Pseudobulb? Toward a Quantitative Definition

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.