Studies in the genus Dialium (Cassieae-Caesalpinioideae)
The present study is the first detailed worldwide revision of the whole genus Dialium. The genus has previously been revised and subdivided into infrageneric taxa (with enumeration of species of Africa and the Neotropics only) but their delimitations have been based solely on gross morphology. Therefore, in order to define the taxa, the present study reviewed the gross morphological characters and augmented them with data from anatomy, palynology, cytology and phytochernistry. This made possible a new classification which better reflects the relationships of the infrageneric taxa. The various characters are reviewed, covering gross morphology, indumentum and other epidermal characters of the leaflets, anatomy of the pulvinus, testa and wood, palynology, cytology and phytochernistry. The important taxonomic characters in Dialium are pointed out and used to define the genus and infrageneric taxa, as well as in recognising species and varieties. Some taxonomic characters are discussed in relation to geographical distribution in order to explain the present-day occurrence of infrageneric taxa east of, and one species west of, continental Africa. Evolutionary trends are explained based on these taxonomic characters. The genera in the subtribe Dialiinae are found to be closely inter-related. However, the majority of them are more closely related to each other than they are to Dialium itself. The genera Baudouinia, Eligmocarpus and Mendoravia are taxonomically rather remote from the rest of the Dialiinae and their position needs further evaluation. The subgenus Uittienia is found not to belong to Dialium and has to be reinstated as a monotypic genus but within the same subtribe. The thesis concludes with a formal taxonomic treatment of Dialium. The arrangement of the infrageneric taxa is such that it reflects their supposed taxonomic relationships. There are no new species described. Previously reduced species and infraspecific taxa are reviewed ana further reductions are made. As a result, only 27 species (4 with varieties) are now recognised out of the previously estimated 40 species.