The neotropical genus Leucaena comprises 22 species, 6 infraspecific taxa, and two named hybrids. A morphological data set is analyzed cladistically to determine species relationships. The comparative morphology of species of Leucaena and closely related genera is investigated and presented. This includes detailed studies of bark anatomy, nyctinastic leaf movements, morphology of extrafloral nectaries, seedlings, inflorescences, pods, anther, and pollen morphology. Scanning Electron Microscopy was used to investigate micromorphology. Criteria for the assessment of primary homology, including the problems of character coding in relation to inapplicable data and continuous characters, are discussed. A total of 29 binary and multistate characters are analyzed. A preliminary analysis of sister group relationships amongst the genera of the informal Leucaena and Dichrostachys groups and two outgroup genera, Parkia and Xylia, revealed no support for these groups and suggested that Schleinitzia, the only other genus traditionally placed in the Leucaena group, may not be the sister group of Leucaena. Instead, three resolved clades are found: a monophyletic Leucaena; a clade containing Schleinitzia, Desmanthus, and Neptunia; and a clade containing the remaining genera of the Dichrostachys group, including Calliandropsis, Alantsilodendron, Gagnebina, and Dichrostachys. This analysis was used to select different combinations of outgroups for the species-level analysis. The 22 species of Leucaena are delimited using the phylogenetic species concept. The difficulties posed by hybrids for cladistic analysis and the evidence for hybridization in Leucaena are reviewed. Analysis of species relationships omitting the four known tetraploid species showed no support for division of Leucaena into two recently recognized sections. The morphological analysis showed broad concordance with results from cpDNA restriction fragment analysis. A combined analysis was not pursued here due to a number of limitations, including intraspecific polymorphism due to introgressive hybridization, associated with the cpDNA data. A full taxonomic account of Leucaena is presented; all taxa are illustrated and their distributions mapped. One hybrid, L. xmixtec, is described as new.
HUGHES, C. Monograph of leucaena (Leguminosae-Mimosoideae). Systematic botany monographs (1998) 55: 1-175.