PHILBRICK, C. THOMAS1*, ALEJANDRO NOVELO R.2, and DONALD H. LES3. 1Department of Biological and Environmental Sciences, Western Connecticut State University, Danbury, CT 06810, USA; 2Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, México, D.F., 04510, México; 3Ecology & Evolutionary Biology, U-42, 75 North Eagleville Road, University of Connecticut, Storrs, CT 06269-3042, USA. - Taxonomy and Systematics of Podostemum (Podostemaceae).
A revision of Podostemum (Podostemaceae) is presented based on
morphological analyses of extensive field collections. A. Michaux
established the genus Podostemum in 1803 with the description
of P. ceratophyllum from eastern North America. Numerous
species have subsequently been described from Mexico and southern
South America. (Several Indian species had previously been included in
Podostemum, although they are now placed in the Indian
Zeylanidium. Podostemum is a New World genus.) Seventeen
specific and subspecific taxa were recognized by P. Van Royen (1954)
in Podostemum as part of his treatment of New World
Podostemaceae. We interpret nine species and varieties recognized by
Van Royen as environmental forms of the seven species recognized in
our treatment: P. ceratophyllum Michx., P. müllerii
Warming, P. comatum Hicken, P. distichum (v. Chamiso)
Weddell, P. irgangii Philbrick & Novelo, P. ricciiforme
Liebmann, P. fruticulosum (Tulasne & Weddell) Weddell. We
recognize species based primarily on stipule form (number of lobes,
orientation) and to a lesser degree on leaf structure. Podostemum
distichum is highly polymorphic in leaf and stipule form; five
previously recognized species are interpreted as environmental forms
of P. distichum. Phylogenetic analyses based on morphological
characters indicate that Podostemum is paraphyletic. P.
mülleri is monophyletic with species of Crenias. In
contrast, ITS sequence data indicates a different placement of P.
mülleri. Morphological data also place P. ceratophyllum and
P. comatum as unresolved at a basal location in the topology,
while P. distichumand the newly described P. irgangii
are sister-species.
Key words: phylogeny, Podostemaceae, Podostemum, riverweeds, systematics, taxonomy