Part 17 Euphorbiaceae
Euphorbiaceae, the spurge family, are very well represented in the Hawaiian flora ranging from common and widespread weedy species, to one of the most important Polynesian introductions. The family is a moderately large one, 313 genera with as many as 8,100 species (Mabberley, p. 273) (numbers according to the Manual are ca. 300 and 7,500, respectively). The family enjoys a cosmopolitan distribution. Perhaps the most familiar member of the family, and certainly the one most people use on a regular basis, is the genus Hevea. It is the latex of species of Hevea that yield the best quality of natural rubber.
An edible member of the family is Manihot esculenta from the root of which comes tapioca. A noxious weed, and the source of castor oil (a perennial childhood threat) from its seeds, is the castor bean Ricinus communis (see image below). Many cultivated ornamental euphorbs are widely available commercially, and increasingly popular because of their attractive foliage and floral displays. The best known decorative plant is likely Euphorbia pulcherrina, (see images) the familiar poinsettia widely grown as an ornamental plant for mid-winter rites and festivities in North America, among other places.
The most important member of the family that occurs in the Hawaiian Islands is Aleurites moluccana, the kukui (or kuikui) nut tree (see image). This typical young tree specimen can be seen growing in the Polynesian introductions section of Limahuli Garden. The flowers of kukui are pictured in (see image). It is also known commonly as candlenut, in reference to the use of nuts (see image), or their expressed oil, as candles; and as the tung tree. Tung, or tung tree, is also a name used for a related species of Aleurites (A. fordii) which is a native of China. Tung oil, from whichever source, is useful as a wood finish itself and as a component in some wood finishing products. Incidentally, A. fordii has escaped from cultivation in Florida where it has become an invasive pest.
On the brighter side of things, kukui is the official state tree of Hawaii in recognition of its importance to the founding peoples. There is very little of the tree that wasn't used; in addition to the use of the oil, the wood was used for a variety of purposes, dyes were made from roots and bark, some medicinal preparations were derived from various parts, meal from the nuts was used as a food, and the nuts are strung together to make attractive leis–these are readily available from almost any tourist establishment in the islands. The pale grey-green, reflective leaves make this tree easy to identify at a distance (see image) as can be seen in the view taken on the Kuilau Trail on Kaua`i.
Antidesma, a genus of some 170 Old World species, is represented in the Hawaiian Islands by two, A. platyphyllum (see image) and A. pulvinatum, both of which are endemics. Hawaiians used the wood of A. platyphyllum, which is moderately hard, as an anvil for beating tapa cloth. The berries provided a dye. The berries of these species also suggest that they are likely to be bird dispersed. Both species were known by the same names: hame, ha`ä, ha`ämaile, hamehame, mehame, and mehamehame.
The next genus to be described, Chamaesyce, will no longer appear in subsequent formal treatments of the Euphorbiaceae, all of its members having been transferred to Euphorbia. We will continue to use the historical name for purpose of this discussion, however. The Hawaiian Islands are home to 22 species of Chamaesyce, which overall has about 250 species, primarily in the New World. Of the species listed in the Manual, 15 are endemic of which several are rare and endangered. Hawaiian names for endemic members of the genus are `akoko, koko, `ekoko, and kökömälei. One of the most attractive of the endemic species is C. skottsbergii (see image) which occurs on calcareous substrates at lower elevation. The photograph was taken on the northern coast of Moloka`i, but this species also occurs on O`ahu, Maui, and Kaho`olawe. Chamaesyce celastroides (see image) is one of the most variable and widespread members of the genus. This variation has been recognized by formally naming eight varieties. Chamaesyce prostrata (see image), a naturalized species from the New World, occurs widely in the islands in low elevation, often disturbed areas. This plant was found growing beside the road on the southern coast of East Maui.
A recent study by Yang and Berry (2011) revealed a relationship between Hawaiian members of Chamaesyce (described as Euphorbia) and likely ancestors from warm areas of North America. Origin of the Hawaiian species was discussed in terms of long distance dispersal followed by frequent hybridizations leading to the current level of diversification.
The genus Claoxylon is an Old World tropical genus of about 75 species (Mabberley, p. 192) with representatives on the Society Islands and on the Hawaiian Islands. The only species on the Hawaiian Islands is C. sandwicense (see image), known as po`olä or, laukea (on Kaua`i). This species is most common on Kaua`i and O`ahu, much less so, tending towards rare, on the other islands.
Euphorbia itself is represented on the islands by at least five species only one of which is endemic. The endemic is E. haeleeleana (see image), which is marked as an endangered species. It occurs naturally in only a few sites on Kaua`i and on O`ahu. I have seen a small specimen growing in the Limahuli Garden. The specimen illustrated here, however, was photographed in the wild by Dr. Gerry Carr.
The next example, Euphorbia tirucalli (see image), was not listed in the body of the Manual because its naturalization had not been documented at the time of preparation of that volume; however, its status as a naturalized species is noted in the Supplement to the Manual. Examination of the image shows the thin pencil-like branches that give rise to pencil tree as one of its common names. Another common name, milk bush, arises from the white, milky latex that exudes from branches when they are injured (see image).
The latex is poisonous, as is the latex from many members of the family, and can cause serious burns on contact with the skin. There has been study of this species, which grows in arid conditions where few other plants can survive, as a potentially significant source of biofuel. It was originally planted on the islands as a natural cattle fence. I have seen individuals of milk bush growing on the southern coast of Kaua`i near the Spouting Horn parking lot, and an excellent example of milk bush as a fence, although cattle are no longer involved, can be seen along the road that leads into the McBride Garden of the NTBG.
Another cultivated member of Euphorbia that is not listed as naturalized, but can nonetheless be found in what appear to be well established colonies, is E. tithymaloides (see image). This species has also been recognized as a member of the genus Pedilanthus, which authors of the Manual note in their introduction to the family. Mabberley lists Pedilanthus as synonymous with Euphorbia (p. 640).
This plant, a native of the West Indies, occurs widely distributed in the Pacific Basin. It is known commonly by a variety of names including Japanese poinsettia, redbird cactus, shoe spurge, and slipper flower. I have seen this plant in decorative gardens as well as growing on sandy outcrops and headlands on the southern coast of Kaua`i where the photograph was taken.
The genus Macaranga is represented in the Hawaiian Islands by two naturalized species M. mappa (see images) and M. tenarius (image). Both are characterized by large leaves; one of the common names for M. tenarius is the parasol leaf tree. With their huge leaves and dense flowering heads these plants are not easily mistaken for many others. The genus consists of 280 Old World tropical species. Macaranga mappa is native to the Philippine Islands and occurs on O`ahu and on the Big Island, where it is commonly seen in a variety of low elevation habitats. It is common in the vicinity of Hilo.
The last member of Euphorbiaceae that we will look at is the common castor bean, Ricinus communis (see image) which is known in Hawaiian by a variety of names including pä`aila, ka`apehä, kamäkou, kolï, and lä`au `aila. Hawaiians rubbed leaves of Ricinus on their faces to relieve fever. Castor oil, expressed from the seeds, is a well known purgative, while proteins extracted from the seeds are among the most deadly poisons of plant origin known. Castor bean is considered a noxious weed in many parts of the world leading to active programs of eradication. In Australia, for example, removal of castor bean plants from one's property is required by law. I know of no such stringent requirement in the Hawaiian Islands, but by my reckoning, the plant is prospering exceptionally well. At the other extreme Ricinus is planted as a decorative plant in my home city, Vancouver.
Yang, Ya. and P. E. Berry. 2011. Phylogenetics of the Chamaesyce clade (Euphorbia, Euphorbiaceae): Reticulate evolution and long-distance dispersal in a prominent C-4 lineage. American Journal of Botany 98:1486-1503.
February 12, 2012