See page 661, under R. fictolacteum. R. rex, in the former narrow sense, was often referred to by growers as a good form of R. fictolacteum, which had come into gardens many years earlier. But nomenclaturally R. rex is the senior species, of which R. fictolacteum now becomes a subspecies.
subsp. fictolacteum (Balf.f.) Chamberlain – This differs little from subsp. rex, the leading character of which is the fawn indumentum of the leaves, against brown or rust-coloured in subsp. fictolacteum (there is also a slight difference in the component hairs). The two subspecies intergrade, though the former has on the whole a more easterly distribution (Rev. 2, p. 255 and Map 76). In both the corolla is white, usually flushed with pink.
subsp. arizelum (Balf.f. & Forr.) Chamberlain – See R. arizelum, p. 601. The leading characters used in the Edinburgh revision to separate this from the preceding two subspecies are the pale yellow corolla and the relatively broader leaves. However, Dr Chamberlain remarks that many specimens collected in the wild, and belonging to R. rex sens. lat., cannot be referred with certainty to either subsp. arizelum or subsp. fictolacteum. Some of the undetermined specimens are intermediate between the two subspecies. Others have the yellow flowers of typical subsp. arizelum but the narrower leaves of subsp. fictolacteum; while others have white or pink flowers with the foliage of subsp. arizelum. To the latter category belongs the type of R. arizelum var. rubicosum Cowan & Davidian (see page 601). A number of the unplaceable specimens were collected in the region ranging from north-west Burma to south-east Tibet. In this connection it is of interest that Ludlow and Sherriff 1380 from the latter area, originally referred to R. arizelum, is determined in the Edinburgh revision as R. falconeri, and that Cox & Hutchison 427 from the Assam Himalaya, mentioned under R. arizelum on page 601, is now considered to be a possible hybrid of R. falconeri subsp. eximium.