Vol. 8, No. 4 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN October 5, 1953 



A TAXONOMIC REVISION OF THE GENUS MACROLOBIUM 

 (LEGUMINOSAE-CAESALPINIOIDEAE) 1 



Richard S. Cowan 



INTRODUCTION 



Macrolobium is one of nearly a hundred genera of the subfamily Caesalpini- 

 oideae (Leguminosae), and with about twenty other genera comprises the tribe 

 Amherstieae. The subfamily is exceptionally well developed in tropical America 

 and many of its South American representatives are greatly in need of study. Such 

 genera as Swartzia (tribe Tounateeae) and Cassia are quite confused taxonomi- 

 caily but are such immense groups that their study must be undertaken over a long 

 period. The same sort of complexity has existed, to a lesser extent, in Macro- 

 lobium, which is sufficiently smaller so that a more immediate solution of its 

 taxonomy appeared possible. In the course of routine identifications of the 

 legumes from Venezuela, the writer became especially interested in this genus be- 

 cause of its morphological diversity and he became convinced that it was in need 

 of critical investigation because of the difficulty encountered in naming these 

 collections. Such a revision could profitably be undertaken at the New York Bo- 

 tanical Garden because of the considerable collections of the genus, including 

 much type material, on deposit there. 



What Macrolobium lacks in numbers of species is compensated for in numbers 

 of individuals, for in some areas in Venezuela the author has observed riverine 

 vegetation in which the commonest trees were members of this genus. It is found 

 from northern Panama south to Peru on the western coast of South America and 

 to southern Brazil on the Atlantic Coast. It is predominantly a genus of lowland 

 riverine or savanna plants, but the species of section Stenosolen occur in the 

 foothills on both sides of the Andes. The lands annually inundated by the over- 

 flow of rivers during the rainy season are a frequent habitat, but many species 

 prefer the sandy savanna and sub-savanna areas. Certain taxa of the genus were 

 observed by the writer growing only in the vicinity of rapids, but this must surely 

 be an edaphic correlation. 



Concurrently with the study of the materials of the genus in the herbarium of 

 the New York Botanical Garden, loans were obtained from the major herbaria of 

 the world which contain appreciable numbers of South American collections. 

 These herbaria are listed below with the abbreviations used in the text, which 

 are, for the most part, taken from the list by Lanjouw (1952). 



A-Arnold Arboretum, Jamaica Plain, Massachusetts. 

 BM-British Museum (Natural History), London, England. 

 BGF-British Guiana Forest Department, Georgetown, British Guiana. 

 COL-Instituto de Ciencias Naturales, Bogota, Colombia. 

 F-Chicago Natural History Museum, Chicago, Illinois. 

 G-Conservatoire et Jardin Botanique, Geneva, Switzerland. 

 GH-Gray Herbarium of Harvard University, Cambridge, Massachusetts. 

 IAN-Instituto Agronomico do Norte, Belem, Brazil. 



K-Royal Botanic Gardens, Kew, England. 

 MO-Missouri Botanical Garden, St. Louis, Missouri. 



Submitted in partial fulfillment of the requirements for the degree of Doctor of Phi- 

 losophy, in the Faculty of Pure Science, Columbia University. 



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