Podostemum from South America (P. comatum, P. distichum, P. irgangii, P. muelleri, P. ovatum, P. rutifolium subsp. rutifolium, P. scaturiginum, P. weddellianum) and the North American P. ceratophyllum. Fluorescence microscopy was used to determine whether pollen tubes were present in the stigmas of unopened flowers from 109 collections. Preanthesis cleistogamy was documented in P. weddellianum, representing the second report of this phenomenon in the genus (previously documented in P. ceratophyllum). The 12 collections with preanthesis cleistogamy represent populations from throughout the geographic range of P. weddellianum. It is hypothesized that preanthesis cleistogamy evolved twice within Podostemum, once in P. ceratophyllum and once in P. weddellianum.