The genus Eleocharis (Cyperaceae, monocotyledons) is characterized by bladeless leaves, which are leaves having only leaf-sheaths. To study the evolutionary process through which Eleocharis species lost their leaf blades, we analyzed the outer morphological and anatomical characters of the representative Eleocharis species, plus species from nine phylogenetically related genera. From the analysis, we recognized eight characters and we optimized their character states on a recent molecular phylogenetic tree. As a result, we recognized five characteristics shared by Eleocharis species as follows: (1) the most apical internode is more than seven times longer than the next apical internode; (2) bladeless leaves having only leaf sheaths; (3) transversely septate aerenchyma in culms; (4) densely and peripherally located, slender, and square timber-shaped fiber bundles in culms; and (5) palisade chlorenchyma in culms. In these characteristics, (2) and (4) are synapomorphies of Eleocharis. These two apomorphic characteristics seemed to be adaptations for inhabiting running waters.