The Geography of Phytochemical Races

6.2 The Hawaiian Islands 255
6.2.4 Metrosideros (Myrtaceae)
Metrosideros consists of 50 species, about half of which comprise each of the two subgenera,
subgen. Mearnsia, which occurs in New Zealand, New Caledonia, and New
Guinea, but does not fi gure in the present example, and subgen. Metrosideros, whose
members occur from New Zealand and its sub-Antarctic islands north to the Bonin
Islands, east to Pitcairn, and north to the Hawaiian Islands. New Zealand appears to be
the center of diversity, and is the source of the only known fossil pollen of the genus,
aged at late Paleocene/early Eocene (Mildenhall, 1980). As S. D. Wright et al. (2000)
point out in the introduction to their work, members of subgen. Metrosideros are well
equipped for long-distance travel; seeds are small and light, viable for at least 6 hours
at −30°C, and can tolerate saltwater for at least a month.
Five species of Metrosideros are endemic on the Hawaiian Islands, one of which,
M. polymorpha Gaud., is the very common ‘ohi‘a lehua. The variation implied by
the specifi c epithet has been accommodated by the recognition of eight varieties.
A survey of collections of M. polymorpha (four varieties by our reckoning) from
four islands revealed a remarkable level of homogeneity with few exceptions. The
only signifi cant differences noted lay with the collections from Molokai where a
higher level of B-ring hydroxylation was observed (Bohm and Yang, unpublished
observations). The usual problems plague this sort of study, a limited sample size
(both in numbers of sites and numbers of individuals), lack of information on
how local environmental conditions may infl uence pigment profi le, and the lack
of detailed information on other species, notably M. colina (Forster and Forster)
A. Gray from the Marquesas, which has been shown by DNA studies to be closely
related to the Hawaiian taxon (S. D. Wright et al., 2000, 2001).
6.2.5 Silversword alliance (Asteraceae)
The “silversword alliance” (Carr, 1985) consists of the three endemic Hawaiian
genera, Argyroxiphium DC, Dubautia Gaudich., and Wilkesia A. Gray. The silversword
alliance plus 13 genera from western North America comprise subtribe
Madiinae (Asteraceae; Helenieae). The North American members of the subtribe
are commonly referred to as tarweeds owing to the resinous exudate that occurs on
aerial parts of most species of the subtribe.
The term “silversword” refers the sword-like, white, tomentose leaves that characterize
both subspecies of Argyroxiphium sandwicensis DC. Argyroxiphium consists
of fi ve species, one of which is thought to be extinct (A. virescens Hillebr.),
limited to higher elevation sites on the islands of Hawaii and Maui. Wilkesia consists
of two species, one fairly common and one very rare, restricted to drier areas
of western Kauai. Dubautia is the largest of the three with 21 species (33 taxa in all)
(Carr, 1985) that grow in a wide range of habitats throughout the islands. In addition
to detailed morphological analysis that resulted in the fl oristic treatment, there have