Petalidium glutinosum (Engler, 1888) Clarke, 1899

Swanepoel, Wessel, Manzitto-Tripp, Erin A., Dexter, Kyle G. & Van Wyk, Abraham E., 2024, Petalidium etendekaense (Acanthaceae), a new species from Namibia, with notes on the taxonomic identity of P. glutinosum, Phytotaxa 636 (1), pp. 35-47 : 36-37

Identity of Petalidium glutinosum

In the second half of the 19th century, some herbarium specimens collected by the South African botanist Rudolf Marloth were sent by him to Germany, where Adolf Engler and others documented them in two instalments published in 1888 under the title “Plantae Marlothianae”. Among these specimens was the collection Marloth 1435, which was gathered in May 1886 in northwestern Namibia. Engler (1888) described it as a new species, Pseudobarleria glutinosa Engler (1888: 259) . The pollen morphology of this species was studied and illustrated by Lindau (1893: t. 1–13). Subsequently, Clarke (1899: 90) transferred the species to Petalidium with the new combination Petalidium glutinosum .

Upon arriving in Germany, Marloth’s herbarium specimens, which included the aforementioned gathering, were taken up in the private herbarium of Engler. From 1884 to 1889, Engler worked in Breslau, where he focused on the Marloth material along with other projects. In 1889, when Engler relocated to Berlin, his private herbarium became part of the Berlin Herbarium ( Urban 1916, J. Paule, pers. comm.). In the protologue of Pseudobarleria glutinosa, Engler (1888) mentioned only the Marloth gathering but did not specify a particular specimen or the institution housing it. This suggests that all existing duplicates of Marloth 1435 should be regarded as syntypes ( McNeill 2014). We have encountered duplicates of this gathering in Herbs BOL, K, SAM in NBG, and PRE. However, in Herb. B, where one would expect to find material examined by Engler, no specimens of this gathering are extant, leading to the assumption that the original specimen(s) were likely destroyed in the fire resulting from a bombing raid during World War II in 1943. Nonetheless, the duplicate in Kew bears a printed label of “Herbarium A. Engler” with the original handwriting on the label, including the name “ Pseudobarleria glutinosa Engl. ”, matching that of Adolf Engler (confirmed by N. Kilian, pers. comm.). Engler must have received more than one specimen of this gathering from Marloth and exchanged this specific duplicate with Kew, a practice that was quite common at the time ( Kaiser 2022). We designate the Marloth 1435 sheet at Kew as the lectotype for the name Pseudobarleria glutinosa . We have chosen this particular specimen because it is of good quality, contains all relevant information, and is also the only one among the duplicates that was certainly examined by Engler as it originated from his private herbarium and bears his handwriting.

In her revision of Petalidium in South Africa and Namibia (then still South West Africa), Obermeijer (1936) upheld P. glutinosum as a separate species. However, she did not provide a new description but instead referenced additional herbarium specimens and referred to Engler’s (1888) account, which described the flowers as “dusky orange”. In the subsequent comprehensive revision of Petalidium in Namibia, Meyer (1968) regarded P. glutinosum as a synonym of P. variabile , a perspective that has since been adopted by all researchers studying the group. Here, however, we follow Obermeijer (1936) in recognizing P. glutinosum as a species distinct from P. variabile ( Table 1 ).

In 1955, Hermann Merxmüller and Richard Hainz described the new species P. pilosibracteolatum (as “ pilosibracteolatum ”) in Suessenguth & Merxmüller (1955). They speculated that this species could potentially be a stable hybrid between P. variabile and P. setosum C.B.Clarke ex Schinz (1916: 434) . However, according to the authors of P. pilosibracteolatum , the statement “very common everywhere” (“sehr verbreitet, überall”) made by the collectors of the original material makes such a possibility rather unlikely in our opinion. After extensive fieldwork and conducting a thorough morphological analysis of the original material, we have concluded that P. pilosibracteolatum is conspecific with P. glutinosum . The presence of long, whitish, simple trichomes on the bracteoles and the green colour of the bracteoles are particularly significant diagnostic features (instead of grey-white bracteoles in P. variabile , which turn brown or greenish brown in herbarium material). While examining the leaves of the type gathering of P. pilosibracteolatum (E. & H. Walter 1261 in Herb. WIND), it was noted that they are densely strigulose. Such trichomes appear to be absent in the available images of Marloth 1435. However, the bracteoles of Marloth 1435 exhibit the distinctive long simple trichomes, similar to those in E. & H. Walter 1261. The apparent absence of strigulose trichomes in Marloth’s collection might be attributed to relatively young shoots or leaves transitioning to a glabrescent state, a variability commonly observed in living plants. Additionally, it is important to mention that the type collections of both P. pilosibracteolatum and P. glutinosum are from the vicinity of Usakos, a botanically well-known area. From this region, only a single entity is known, possessing the specific combination of morphological characters observed in these two collections. Based on priority, the correct name for this entity is P. glutinosum . Therefore, we here place P. pilosibracteolatum into synonymy with P. glutinosum .

Petalidium glutinosum (Engl.) Clarke (1899: 92) .

Type: —[ NAMIBIA.] Namaland [Damaraland], [in lapidosus], Usakos, alt. 900 m [850 m], May 1886, Marloth 1435 (lectotype K000394073 scan! designated here; isolectotypes BOL138559 photo!, PRE0118630-0!, SAM0039812 -0 in NBG scan!) .

≡ Pseudobarleria glutinosa Engler (1888: 259) . Type: as above.

= Petalidium pilosibracteolatum (as “ pilosi-bracteolatum ”) Merxm. & Hainz in Suessenguth & Merxmüller (1955: 69), syn. nov. Type: Südwest-Afrika [ Namibia], Damaraland, farm Nudis [Dr. R. Seydel ], [im Gamikaubibrivier], Distr. Karibib, “sehr verbreitet, überall”, [13-02]-1953, H & E Walter 1261 (holotype M0109674 scan!; isotype WIND!).