The Relationships of the Euphorbieae (Euphorbiaceae) Author(S): M
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The Relationships of the Euphorbieae (Euphorbiaceae) Author(s): M. G. Gilbert Source: Annals of the Missouri Botanical Garden, Vol. 81, No. 2 (1994), pp. 283-288 Published by: Missouri Botanical Garden Press Stable URL: http://www.jstor.org/stable/2992098 . Accessed: 19/01/2015 03:28 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Missouri Botanical Garden Press is collaborating with JSTOR to digitize, preserve and extend access to Annals of the Missouri Botanical Garden. http://www.jstor.org This content downloaded from 210.72.93.185 on Mon, 19 Jan 2015 03:28:45 AM All use subject to JSTOR Terms and Conditions THE RELATIONSHIPS OF M. G. Gilbert2 THE EUPHORBIEAE (EUPHORBIACEAE)' ABSTRACT The Euphorbieae and Hippomaneae, though both usually placed in subfamily Euphorbioideae, differ in many inflorescence details and can be linked unambiguously only by one cryptic character: the rod-shaped starch grains in the latex. While the Hippomaneae have inflorescences very similar to those of many other members of the Euphor- biaceae, the Euphorbieaehave an inflorescence so specialized that one seems to be forced to relate it to other members of the family through a hypothetical ancestor with a synflorescence of axillary bisexual cymes more primitive than most extant taxa possibly other than Jatropha. A primary aim of this symposium is to review between the Old World and New World species of critically Webster's (1994) classification of the Eu- Euphorbia? How do the subgenera Euphorbia and phorbiaceae. This paper is an attempt to share a Lacanthis (sensu Gilbert, 1987) relate to the rest speculation arising from misgivings over the sup- of the genus? Such an analysis, in my view, could posed homogeneity of the subfamily Euphorbioi- do much to clarify these relationships and thus the deae. This is not a presentation of the results of taxonomy of the tribe as a whole. It would demand prolonged and detailed research but rather spec- a much greater knowledge of the possible origins ulations arising while producing a routine Flora of the Euphorbieae than is available. In a group account of the Euphorbiaceae for the Flora of so morphologically isolated, speculation is needed. Ethiopia (31 genera, 209 species), coupled with The Euphorbieae and Hippomaneae are includ- a long-standing interest in Euphorbia (the largest ed within the subfamily Euphorbioideae along with genus within the Ethiopian flora), particularly the three other tribes (Stomatocalyceae, Pachystro- succulent species. A Flora writer should delve into mateae, and Hureae) that have usually been as- the larger scale taxonomy of those families that sociated with the Hippomaneae (Webster, 1994), must be covered, but is rarely allowed the time to primarily on the basis of their caustic milky latex carry out anything more than superficial investi- from nonarticulate laticifers, frequently glandular gations on taxa not actually included in the Flora. bracts, and often highly reduced flowers that always Thus the following ideas must not be regarded as lack petals. This juxtapositioning has not always anything more than simple-minded speculations. been the case: Mueller Argoviensis (1866) and Determining the relationships of a group as spe- Bentham (1878) placed these groups at opposite cialized as the Euphorbieae used to be of little ends in their sequences. There are indeed major concern to a Flora writer. The matter became a differences in inflorescence morphology between little more relevant with the advent of cladistics, the two groups such that I felt forced to consider where the methodology demands a working hy- the possibility that the similarities were the product pothesis of probable sister groups and character of convergence. One unusual cryptic character, polarization. Sooner or later such methods should very characteristic, rod-shaped starch grains found be applied to the Euphorbiaceae in general, and in the latex, is a good contender for a svnapo- ideally to the Euphorbieae in particular, where morphy, which suggests that they do have a com- there are inconsistencies in current generic delim- mon origin. This in turn led to an attempt to re- itation. How, for instance, would a cladistic analysis construct a possible ancestral inflorescence type relate Chamaesyce, very widely recognized as a from which the modern plants could have evolved distinct genus, to the various sections within Eu- most parsimoniously. phorbia subg. Agaloma and the rest of Euphor- The rod-shaped starch grains have not been bia? There are many other interesting problems reported in other members of the Euphorbiaceae, waiting to be tackled: What is the relationship although many taxa appear never to have been I The author was employed by the Ethiopian Flora Project, funded by the Swedish Agency for Research Cooperation with Developing Countries (SAREC), at the time this paper was prepared. 2 Flora of China Project, Missouri Botanical Garden; address for correspondence: Department of Botany, Natural History Museum, Cromwell Road, London SW7 5BD, U.K. ANN. MISSOURI BOT. GARD. 81: 283-288. 1994. This content downloaded from 210.72.93.185 on Mon, 19 Jan 2015 03:28:45 AM All use subject to JSTOR Terms and Conditions 284 Annals of the Missouri Botanical Garden surveyed (Rudall, 1987), and their discovery else- lanthus but also various red-flowered Euphorbia where in the family would be a serious challenge species, pollinated by birds. In most members of to the taxonomic integrity of the Euphorbioideae. the Hippomaneae the styles are undivided, long The toxicity of the latex has also been mentioned and tapered, and are usually characteristically cir- as a linking character but the major compounds cinately coiled when young. There are not many involved-diterpene esters occur also in subfam- observations of pollination. The pendent inflores- ily Crotonoideae (Beutler et al., 1989). The pollen cences and long stigmas of some species suggest of the Euphorbioideae is rather uniform (Punt, that wind pollination is a possibility, but other gen- 1987), but the pollen type is apparently plesiomor- era have well-developed nectaries within the inflo- phic; thus the similarity should be treated with rescences indicating some form of animal pollina- caution. Morphological evidence is even less clear- tion, perhaps most often by small unspecialized cut. There are superficial similarities in that most insects (Bawa et al., 1985), but in at least one taxa in both groups have a sparse to nonexistent case, Mabea occidentalis Benth. (Steiner, 1983), indumentum of simple hairs but there are many predominantly by bats. Thus the significance of the exceptions in both major tribes, such as species of striking differences in the styles is not clear. An- Chamaesyce and Agaloma within Euphorbia and other possibly significant difference is the frequen- members of the Mabeinae and many species of cy of succulence within the Euphorbieae where it Stillingia within the Hippomaneae, and such ev- has clearly evolved several times independently, idence can only be regarded as essentially negative: whereas succulence appears to be almost absent in it does not disprove the possibility of a relationship. the Hippomaneae, recorded only from a few Bra- The tendency toward very reduced flowers, rare zilian species of Stillingia (Rogers, 1951). in the family as a whole, has also been invoked to The contrast in inflorescence organization be- support a common grouping. It is possible, theo- tween the Hippomaneae and the Euphorbieae seems retically, to select a reduction series for the flowers so great that one must consider whether the sirn- within the Hippomaneae from genera such as Ma- ilarities between them could be the result of con- bea, which have well-defined perianths and male vergence rather than an indication of common flowers with many stamens, through to Dalember- ancestry. If the most reasonable (parsimonious) tia, in which the male flower is reduced to a single hypothetical common ancestor has to have char- sepal and a single stamen and the female flower acters such that it would have to be placed within has only vestigial remnants of the perianth. This one of the other subfamilies, notably the Crotonoi- sequence could be regarded as continued by the deae, rather than the Euphorbioideae, the Eu- Euphorbieae, where the male flower is always re- phorbioideae would have to be regarded as a grade duced to a single naked anther and the female rather than a lade. flower to a naked ovary. There is evidence of a There seems to be little or no case for ques- female perianth in many genera within the Eu- tioning the homogeneity of the tribe Euphorbieae phorbieae, including some species of Euphorbia, as it is clearly defined by the very peculiar inflo- but the only clear indication of a male perianth, rescence the cyathium distinct from all other beyond an articulation between pedicel and fila- inflorescence types seen within the family. Lin- ment, is the vestigial male perianth seen in An- naeus and other very early authors, plus one of thostema and Dichostemma. There has been a the two giants of nineteenth-century Euphorbi- temptation to suggest that this sequence gives a aceae taxonomy, Baillon (1874), thought that the true guide to relationships, but this must be ques- cyathium was a hermaphroditic flower. However, tioned could the Hippomaneae really have given the alternative theory that it was an inflorescence rise directly to the Euphorbieae? was mentioned by Lamarck as early as 1788 and The major difference between the two groups is is now unquestioned, though there is still disagree- the inflorescence structure, which is discussed at ment in the exact interpretation of its organization.