A biosystematic study of Panicum section Diffusa (Poaceae) in North America

Abstract

A taxonomic study was made of Panicum section Diffusa in North America. The following taxa were included: P. diffusum, P. bergii, P. capillarioides, P. filipes, P. furvum, P. ghiesbreghtii, P. hallii, P. hirsutum, P. lepidulum, P. pilcomayense, and P. tamaulipense. Panicum tamaulipense was described as a new species, and a new combination, P. hallii var. filipes, was made. The description of P. furvum was emended. Distributional range of P. diffusum was extended, and its confusion with P. hallii, P. filipes, and P. pilcomayense was clarified. Anatomical studies did not provide any new characters to delimit taxa. Culms had hollow internodes with three peripheral rings of vascular bundles. Transverse sections of the leaf were typically panicoid with radiating chlorenchyma, usually two bundle sheaths per vascular bundle, and bulliform cells in groups of 3-7. The epidermis was panicoid, with macro- and microhairs, stomata usually in two rows between the veins, and with low dome-shaped subsidiary cells, the short cells dumbbell or cross shaped, or occasionally nodular, and short cells in rows of 2 - many. Previous chromosome counts of 2n=18 for P. hallii var. hallii and P. hallii var. filipes, and 2n=36 for P. pilcomayense were confirmed. New chromosome counts reported: 2n=36 for P. diffusum and P. hirsutum. All meiotic divisions observed were regular and studies of megasporogenesis indicated the taxa to be sexual. Embryo sac development of the Adoxa type was recorded as the second record of this type for grasses. Measurements of stomatal size indicated a possible correlation with ploidy level..