One of my favorite species is Ptilotus rotundifolius (F.Muell.) F.Muell, the royal mulla mulla, which is found in the Pilbara, Gascoyne and Murchison bioregions of Western Australia. This long-lived shrub can reach > 2 m high. The specific epithet (rotundifolius) refers to the large and broad leaves (> 3 cm wide), which are covered in the a dense, velvety indumentum. These leaves make it one of the most cuddly Ptilotus species. The species was originally described by Mueller (1862) in the genus Trichinium, which was later included within Ptilotus by Mueller (1868).
Ptilotus rotundifolius is closely related to P. marduguru Benl (Benl, 1980; Hammer et al., 2015), a rare species from Southesk Tablelands in the Great Sandy Desert, Western Australia. These two species do not overlap in distribution and can be easily differentiated based on P. rotundifolius having pink flowers and spiciform inflorescences > 30 mm wide. Ptilotus marduguru has cream-green flowers and inflorescences that are < 21 mm wide. Like P. marduguru and other closely related species (e.g. P. auriculifolius (A.Cunn. ex Moq.) F.Muell.), P. rotundifolius has 5 fertile stamens and a centrally placed style on the ovary. These characters were found to be associated with the basal clades of Ptilotus (Hammer et al., 2015) and are probably pleisiomorphic within the genus.
The pink flowers and densely wooly indumentum on the interior surface of the sepals (sometimes “tepals”) of P. rotundifolius are shared with more distantly related species such as P. drummondii (Moq.) F.Muell., P. exaltatus Nees and Ptilotus manglesii (Lindl.) F.Muell., all of which have been observed to be pollinated by native Australian bees (pers. obs.). These characters may be indicative of a bee pollination syndrome in the genus. The close relatives to P. rotundifolius that have green flowers (i.e. P. marduguru and P. auriculifolius) may be pollinated by moths like more distantly related green-flowering species, such as P. macrocephalus (R.Br.) Poir. and P. polystachyus (Gaudich.) F.Muell. (pers. obs.). Shifts in pollination syndromes from bees to moths (or vice versa) may be an important driver of speciation throughout the genus.
- Benl, G. (1980). Five new taxa of Ptilotus (Amaranthaceae) from Western Australia. Nuytsia 3: 157-161. https://florabase.dpaw.wa.gov.au/nuytsia/article/53
- Hammer, T., Davis, R. & Thiele, K. (2015). A molecular framework phylogeny for Ptilotus (Amaranthaceae): Evidence for the rapid diversification of an arid Australian genus. Taxon 64(2): 272–285. http://dx.doi.org/10.12705/642.6
- Mueller, F.J.H von (1862). Fragmenta Phytographiae Australiae. Vol. 3, p. 122. (Government Printer: Melbourne)
- Mueller, F.J.H von (1868). Fragmenta Phytographiae Australiae. Vol. 6, p. 230. (Government Printer: Melbourne)