Euphorbia in Southern Africa

Book excerpt: This book presents a new account of Euphorbia in southern Africa. Euphorbia is the second largest genus of plants in the world. Southern Africa enjoys a high diversity in Euphorbia and 170 species occur here naturally. Of these 170 species, 128 or 74% are endemic. Where most species of Euphorbia in the northern hemisphere are herbs or shrubs, most of those in southern African are succulent. These succulents range from small, almost geophytic forms where the tuber is larger than the above-ground parts to huge trees 6 to 15 m or more in height. Many of them are spiny. There are also small numbers of herbaceous species in southern Africa and many of these are also dealt with here. The last account of the succulent species for southern Africa was published in 1941 and much new data has accumulated since then. Our understanding of the relationships of the species in Euphorbia has also been greatly enhanced by recent analyses of DNA-data, which led to new and unexpected results. From this new information an entirely new classification was developed, in which Euphorbia was divided into four subgenera. This provides the taxonomic framework for the presentation of our species here. Around ten new species have been described and these are presented in detail for the first time. This monograph is made up of two volumes. Volume 1 contains an extensive introductory chapter with an overview of the genus in the region, emphasizing many of its important and distinctive features. This is followed by Chapter 2, which deals with subgenus Athymalus. Of the four subgenera, this one is by far the most diverse in southern Africa, with 80 species. Volume 2 contains Chapters 3 (subg. Chamaesyce, 34 species), 4 (subg. Esula, 11 species) and 5 (subg. Euphorbia, 45 species), as well as an additional Chapter 6 covering the remarkable diversity of subg. Euphorbia in Moçambique. Each of Chapters 2 to 5 includes a key to all the species, followed by an account of each of them. This account includes synonymy, a description, data on distribution and habitat, line-drawings of floral features and other diagnostic details, notes on how the species is distinguished from its closest relatives and a brief history of its discovery. Several colour photographs are included for each species, illustrating its habitat, vegetative habit and flowering features, demonstrating key points distinguishing it from others and often showing its variability. Euphorbia is an important component of the vegetation in many of the drier parts of southern Africa. This book is based on a thorough evaluation of the vast herbarium record for southern African members of Euphorbia, on the extensive field-work conducted in the region and the wide taxonomic experience of the author. It is believed that both the professional botanist and the layman will find much that is new and informative in this monograph.