Abstract
The Chusquea ramosissima informal group, including four named species, is classified within Chusquea subg. Chusquea based on morphological characters, but has not been rigorously studied in its entirety. The putative synapomorphies distinguishing the C. ramosissima informal group from the remainder of Chusquea subg. Chusquea are the presence of a pseudopetiolate culm leaf blade that remains green and synflorescences borne on a mix of longer leafy and shorter non-leafy subsidiary branches per node, features unknown in the rest of the subgenus or the genus as a whole. In addition, the species of the C. ramosissima informal group share a bud complement with a set of 1–4 laterally compressed, + falcate subsidiary buds on each side positioned facing each other, a central bud with parallel sides and a broadly triangular apex, and thin-textured spikelets. Because of this unique combination of characters, including two that are unique within Chusquea, and molecular support, we here formally describe Chusquea sect. Tenellae within Chusquea subg. Chusquea to accommodate this group. Five species, distributed in Argentina, Brazil, Bolivia, Paraguay and Uruguay, are included in the newly recognized Chusquea sect. Tenellae: C. fasciculata, C. ramosissima, C. tenella, C. tenuiglumis, and the newly described C. ovatifolia. This revision includes a comparison of sect. Tenellae with the five previously recognized sections of subg. Chusquea, detailed descriptions for the five species of sect. Tenellae, line illustrations or images for all species, maps of their distributions, and morphological keys for their identification. The names Chusquea fasciculata, C. tenuiglumis, C. tenuiglumis var. laxiuscula and C. tenuiglumis var. subcylindrica are lectotypified.