Abstract
<p>Podostemaceae live in swift-running rivers with stony beds, mainly in the Tropics. This article is a comparative study of three Marathrum spp. (M. rubrum, M. schiedeanum, and M. tenue) and the monotypic genus Vanroyenella (with V. plumosa). The study is based on material from Mexico. Marathrum rubrum and V. plumosa are Mexican species, whereas the other two species have wider ranges in Central America. Developmental features of Marathrum and Vanroyenella are described and compared with other NewWorld Podostemoideae. Green prostrate roots with asymmetric caps are fixed to the rock by adhesive hairs. Endogenous shoot buds are formed along the roots. They grow into thalloid (dorsiventrally flattened) stems that serve as holdfasts with adhesive hairs. Many compound leaves are dithecous; i.e., they have two sheaths, arranged in the same plane as the primary pinnae. These dithecous leaves can be called “mother leaves” because they give rise to daughter leaves in both their right and left sheath. One of the two sheaths of a dithecous leaf may be also occupied by a fasciculate inflorescence with one to 13 flowers that develop and open one by one. The close relationship of Marathrum and Vanroyenella (as suggested by molecular data) is corroborated by fundamental morphological similarities. One of the seemingly unique features of Vanroyenella is the feather-like construction of the leaves, with filamentous segments arising directly from the rachis. Essential features of pinnate leaf development, however, are shared with Marathrum spp.</p>
