The Flowering Plants of Hawaii
Part 47 Sterculeaceae-Theaceae-Thymelaeaceae-Tiliaceae
The Sterculiaceae, a pantropical family consisting of about 60 genera with 700 species, according to the Manual, are represented in the islands by Melochia umbellata, an attractive Asian species that has escaped from cultivation (see image), and by the pantropical, indigenous Waltheria indica (Waltheria.jpg). Waltheria is known by several names in Hawaiian, the most common one being `uhaloa. It was, and likely still is, used medicinally as a treatment for sore throat, and from personal conversations, as a remedy for headaches. The plant is widely distributed on all of the Hawaiian Islands, including Midway Atoll, and can be found growing in dry, often disturbed sites. I have seen it growing in gravelly parking lots. The photograph was taken at Pu`uhonua o Hönaunau National Historical Park.
Recent molecular data have lead workers to include (submerge is the technical term) Sterculiaceae within Malvaceae, the cotton family (Mabberley, p. 822). We maintain its traditional position here. See also the work of Soltis et al. (2005) for discussion of this, and other, major changes in relationships based upon molecular data.
The only member of the tea family, Theaceae, on the Hawaiian Islands is the endemic Eurya sandwicensis, änini or wänini in Hawaiian. This rare, small tree occurs in mesic to wet forests on all of the main islands except Läna`i, Ni`ihau, and Kaho`olawe. The illustration of this species (see image) comes from the Starr image collection.
Thymelaeaceae are a family of 850 species in 45 widely distributed genera but with concentrations in Australia and tropical Africa (Mabberley, 857-858). Many species are toxic to a greater or lesser degree. One of the more familiar members of the family, at least to North Americans, would be Daphne, widely planted for its hardiness and evergreen foliage. The only Hawaiian member of the family is the genus Wikstroemia, known in Hawaiian as `äkia or kauhi. There are a dozen endemic species in the islands, three of which are indicated as being rare, with one likely extinct (W. hanalei has not been collected since 1916). The Hawaiian name appears to have been an inclusive one, possibly indicating that differentiation among the various forms may not have been considered useful (or possible?). This may have been a prophetic appraisal of the situation as Bo Petersen of the Botanical Museum at the University of Göteberg in Sweden—who contributed the section on the genus for the Manual—informs us that specific delimitation is difficult. An indication of the differing opinion regarding the number of species can be seen in his recognition of 12 as opposed to 24 described by the Swedish botanist Skottsberg in 1972. The characteristic leaf arrangement and yellow flowers of typical Wikstroemia species are illustrated here (see images).
In an effort to clarify relationships within this group of plants, Stephanie Mayer, now in the Department of Ecology and Evolutionary Biology at the University of Colorado, performed a multivariate statistical analysis of a large set of vegetative, inflorescence, and floral characters—the technical term is morphometric analysis—gathered from 38 populations of Wikstroemia (Mayer 1991a). Owing to the scarcity of material of several species, examination was limited to eight of the most common ones as recognized by Peterson. The statistical approach is designed to lump together specimens that have the largest number of features in common, ideally bringing together those individuals that would be recognized as comprising a species. The outcome with the Wikstroemia data, however, was that no clear-cut groups emerged from the analysis. In essence, the variation was shown to be continuous. Further supporting the view that comparatively little differentiation has occurred during Wikstroemia’s time on the islands was the observation that viable offspring were formed in most crosses attempted between individuals from different populations (Mayer, 1991b). Locating natural hybrids in nature proved to be very difficult owing to the high degree of similarity, morphological and ecological, exhibited by likely parental plants.
Two hypotheses were put forward to account for these observations. The first suggests that the species of Wikstroemia are in the process of diverging, and if given enough time, may develop into entities that are more easily distinguished from one another. The second suggests that the present situation may, in fact, be stable, maintained by just enough gene flow among populations that no sharp demarcation between ‘species’ is possible. This idea troubles some workers because it makes it very difficult to put a label on any given specimen from the field. In the final analysis, until more definitive data emerge—DNA sequence data to the rescue?—the names assigned the various entities will likely be used, with the awareness that a “pure” species cannot be defined with any certainty. Biologists have discussed, debated, and fought over this question throughout the history of science. For all practical purposes, and that is what we are really dealing with in most situations, “species” are groups of individuals that we can identify—that is, to which we can assign a name—with greater or lesser levels of confidence, for the purpose of discussion. The name of a plant is simply a convenient means of communicating information.
Returning to the real world now, we see that `äkia proved to be a very useful plant. Preparations were used as a treatment for asthma, and as a laxative, and for stunning fish. In a more macabre use, preparations made from roots and bark were used as a means of executing criminals. In a more constructive mode, several species provided some of the strongest fibers of any island plants.
Tiliaceae, the linden family, are represented on the islands by two genera, Heliocarpus with one species, and Triumfetta with two; all three have become naturalized. Triumfetta semitriloba (see image), the Sacramento bur, is a native of warm habitats in Central and South America and the West Indies. It occupies dry, disturbed sites in the Hawaiian Islands. I have no image of Heliocarpus.