Platanus glabrata Fernald

The plant described here as Platanus glabrata has a particularly involved nomenclatural history, making the presence in cultivation of trees of this sort particularly hard to define. The taxon first seems to have been described by the German botanist Otto Kuntze in 1891, as one of nine New World Platanus which he chose to treat as varieties of the Old World P. orientalis. Kuntze had travelled widely and presumably had the opportunity to study most of these variants in the wild, although his type specimen for var. palmeri was a herbarium specimen at Kew labelled P. lindeniana (Kuntze 1891); Kuntze named his variety after the English-born American botanist Edward Palmer, who had collected this specimen in northern Mexico in 1880 (Palmer 1269; Nixon & Poole 2003). Kuntze’s eccentric taxonomy meant that few subsequent authors made use of this name, and in 1901 Merritt Fernald independently described Platanus glabrata, again with Palmer 1269 as the type specimen. In 1998 Kevin Nixon and Jackie Poole published the combination P. occidentalis var. palmeri (in French, within a paper by Daniel Geerinck which was otherwise devoted to trees grown in Belgium; it seems rather unlikely that P. palmeri itself has ever been cultivated in Europe; Geerinck 1998).

Material identified as Platanus occidentalis var. palmeri was sampled by Guido Grimm and Thomas Denk for their 2010 nuclear marker study of Platanus, and was found to sit about equidistant between typical P. occidentalis and P. mexicana (sensu Nixon and Poole); Grimm and Denk concluded that this entity (along with P. rzedowskii, described by Nixon and Poole in 2003) were best treated as part of a conglomerate of taxa intermediate between P. occidentalis and P. mexicana. They proposed (but did not publish) P. palmata as a species name, and this usage has since become general; however, if the taxon is treated as a species, Fernald’s P. glabrata has precedence (R. Govaerts pers. comm; Nixon & Poole 2003; Geerinck 1998; Grimm & Denk 2010). A few authorities continue to treat Fernald’s P. glabrata as synonymous with P. mexicana, a position which remains at odds with Nixon and Poole’s concept (adopted here) of P. mexicana as a tree with fruit-balls carried usually five to seven together and with leaves densely pubescent and whitish underneath.

The smaller leaf with five rather than three lobes which are also largely untoothed helps distinguish Platanus glabrata from P. occidentalis, the common species of the eastern United States, but in general appearance P. glabrata is certainly closer to P. occidentalis than it is to P. mexicana (sensu Nixon and Poole); like P. occidentalis, its leaves become green and almost hairless underneath, although some pubescence may persist under the veins; the leaves of P. mexicana remain densely pubescent and distinctively silvery underneath. (P. glabrata – ‘the plane-tree that loses its hairs’ – was named to differentiate this tree from the southern forms of Mexican Plane (‘P. lindeniana’), as which the collection Palmer 1269 was originally identified; the name does not imply that it is less pubescent than P. occidentalis.)

The features distinguishing Platanus glabrata also seem clinal from Texas into northern Mexico, while plants resembling P. glabrata occur sporadically within the population of P. occidentalis from eastern Texas into Oklahoma, Iowa, Arkansas and Louisiana, and even rarely in the eastern coastal states (Nixon & Poole 2003). The five lobes to the leaf are a feature shared with P. rzedowskii, another intermediate taxon with a slightly more southerly distribution in Mexico, and may indicate a degree of introgression with members of the ‘western clade’ of Platanus such as P. wrightii; both P. mexicana and P. occidentalis characteristically carry leaves with just three lobes (Nixon & Poole 2003).

Jacobson 1996 suggests that the Platanus cultivated in Texas include, as should be expected, trees referrable to P. glabrata; a 2002 accession also grows under this name in the significantly harsher climate of the Denver Botanic Gardens (North American Hardiness Zone 6a) (Royal Botanic Garden Edinburgh 2025).