De noodzaak van planten verzamelen, met de ‘cryptische soort’ Beursjesganzenvoet (Oxybasis chenopodioides; Amaranthaceae) als Nederlands voorbeeld

This paper elucidates the indispensability of herbarium collections regarding the discovery of cryptic species and the knowledge of their plasticity, ecology and geography. In Table 1 fifteen species are listed that had been ‘hidden’ due to their resemblance to related species, and have been identified as members of the flora of the Netherlands after 1900. From these Oxybasis chenopodioides is chosen as an example, because it has remained ‘cryptic’ inasmuch as it is still insufficiently recognized. Macroscopic features that were considered characteristic of Oxybasis chenopodioides are reflected in several epithets on species or variety level: fleshiness (crassifolium), with almost smooth-edged leaves (paucidentatum), having an inflorescence resembling Dysphania botrys (botryoides). However, such features may also occur in other Oxybasis species. The distinguishing character state was not mentioned until 1841, when Karelin & Kirilov described the strong and peculiar dimorphy of hermaphroditic and female flowers in the same cluster. Because of the smallness of female flowers (reflected by the epithet minutiflora chosen by these authors) the plant should be viewed through a stereo microscope. In 1955 ‒1964 subsequent revisions by S. J. van Ooststroom and P. Aellen revealed a number of finds in the western half of the Netherlands, the majority of which date back to the 19th century (Table 2). Most of these finds were identified before as Chenopodium urbicum or as Blitum rubrum / Chenopodium rubrum. After van Ooststroom's and Aellen's revisions only few new specimens of Oxybasis chenopodioides have been collected in the Netherlands, all of them were gathered in the estuary in the southwest, i.e. in the Province of Zeeland and the former island of Goeree in the Province of Zuid-Holland (Fig. 10). A number of specimens of Oxybasis chenopodioides come from brackish habitats. Among others it was collected along the former Zuiderzee before this inland sea was closed in 1932 and became a freshwater lake. Some other finds relate to creeks and an old breakthrough vortex that remained saltish after being embanked. The richest actual site of O. chenopodioides has originated from reclamation of saliferous peat for salt-making. However, this species also occurred in non-saline habitats, viz. near villages in dune areas and in urban environment, notably at the edge of Amsterdam. In the middle of the 19th century its was twice collected in arable land. Therefore, its apparent dis­appearance from the northwestern part of the Netherlands still needs to be explained. Finally, it should be noted that Blitum chenopodioides, described by Linnaeus (1771), was considered a monstrous form of Blitum virgatum by A. J. Retzius according to Willdenow (1797). This remark has been overlooked by Aellen (1933) when choosing Blitum chenopodioides as a basionym for the present species.