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Petalidium is a fascinating genus with over 93% of the species occurring in the arid or hyper arid parts of southwestern Africa. Many species have the same distribution ranges as iconic species like Welwitschia mirabilis (Welwitschiaceae) and the peculiar caudex Adenia pechuelii (Passifloraceae). Seasonal and erratic rainfall with extreme events and droughts are not unusual in this area, however the taxa do not exhibit forms easily identified with aridity like succulence, rodlike stems, underground storage or are pachycauls.

There are several reasons why resolving species delimitations are challenging and it is not surprising that a satisfactory study of the systematic has not yet been completed. This webage records what is known about the Petalidium genus and the aim is to provide a starting point and stimulus for further research.

The taxonomy has not been neglected, but it was fragmented with regional accounts for Angola in Portuguese, Namibia in German, and some in English. There is also limited uniformity between authors regarding the morphological traits selected or terminology used.

The three disjunct regions in which the taxa occur are shown in the first map on the left and the second map illustrates how many species are found west of the escarpment.

Forty six Petalidium species are accepted here, however this is an ongoing study and changes are expected. In total, almost 100 names have been published or suggested as well as numerous forms. The information is assembled from comprehensive plant systematic research, years of field work and visits to herbaria in Southern Africa and Europe. Images and more information on each species is and will be included in Flora of Namibia [http://herbaria.plants.ox.ac.uk/bol/namibia].

Inappropriate species delineation can have negative impacts on fundamental and applied research. Relevant taxonomy based on criteria that help recognition of species and the way they have adapted to the harsh conditions is essential and much needed. Much can also be gained by looking at genera in other families with similar distribution patterns as they share characteristics of Petalidium species. Many of these are also difficult to identify and distinguish, as individuals of a species can be variable.

Priorities for future research include identifying traits of taxonomic value (indumentum, sterile branchlets, areolate pattern of veins in dried specimens), adaptation strategies and identifications of specimens from years of outstanding rainfalls and dry seasons.

Checklists, species maps, etymology and references are found in the tabs above and these will be updated as progress in made

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Overall distribution range is shown in the first map

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The concentration of species west of the escarpment (black line) is shown above

Work on this database is not funded and data may not be sold or used for commercial purposes. The images in https://herbaria.plants.ox.ac.uk/bol/namibia are the property of the author and Herta Kolberg.

Please cite as: Craven, P. 2024. Genus Petalidium (Acanthaceae) https://herbaria.plants.ox.ac.uk/bol/petalidium

For further information, please contact petalidium@gmail.com

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© Craven P. 2024.

The database is developed and published online using BRAHMS.