Hood, J.L.A. and J.C. Semple. 2003. Pappus variation in Solidago (Asteraceae: Astereae). Sida 20(4): 1617–1630.
The pappus in the goldenrod genus Solidago (Asteraceae: Astereae) was examined in 75 species and compared with variation in 14 species of 12 related genera in subtribe Solidagininae in the narrow sense following Nesom (2000) and three species of asters of two genera basal to the North American Clade of the tribe. Solidago has been described as having a simple pappus consisting of one whorl of barbellate bristles with the exception of a few species in several sections, sometimes treated as distinct genera. Species of Solidago sect. Corymbosae (Oligoneuron) have long been recognized as having a biseriate pappus with clavate bristles similar to that of other genera of the subtribe. However, nearly all species of Solidago displayed some evidence indicating the pappus bristles occur in two more or less heteromorphic whorls of bristles, here designated as the primary outer whorl and the primary inner whorl. A few species also occasionally had a third, much shorter secondary outer whorl of a few bristles. In the most heteromorphic species, the primary outer whorl of bristles was slightly shorter and had tips that gradually tapered. The primary inner whorl of slightly longer bristles had distinctly clavate tips up to several times as broad as the bristle below the tip. As well, the bases of the primary outer whorl were clearly external to those of the primary inner clavate whorl in some species. In contrast, most species in several subsections of the genus exhibited little evidence of a biseriate pappus or clavate bristle tips. The pappus of each species was scored on several traits: 1) non-clavate to distinctly clavate bristle tips, 2) evidence of alternating non-clavate and clavate bristles or shorter and longer bristles, and 3) evidence of overlapping primary outer and inner whorls of bristles. One species of Solidago had an atypically short pappus; the bristles of S. sphacelata were less than half the length of the cypsela body. The biseriate pappus of Brintonia discoidea was usually tinted with anthocyanotic pigments unlike any species of Solidago; the species has been included in Solidago by some authors.