Abstract:In order to understand the taxonomic significance of caryopsis characters in Avena L. (Poaceae), the micromorphology of 27 species caryopsis were observed under stereoscopic microscopy and scanning electron microscopy, and its taxonomic significances were analyzed combining with molecular phylogenetic reconstruction. The results showed that there were three shapes of caryopses in Avena, including fusiform, oblanceolate and elliptic, and three spermoderm sculpturing patterns, including striate, ribbed and reticulate. Caryopsis shapes, spermoderm sculpturing patterns and stylopodium persistence patterns showed limited taxonomic significance at the infrageneric level, and caryopsis size and macrohair density were of important diagnostic characters at the interspecific level, while caryopsis ventral face, compression, and embryo proportion had limited diagnostic significance at the interspecific level in Avena. Avena macrostachya Balansa ex Coss. & Durieu should be treated as a member of Avena due to its caryopsis characters of fusiform and striate sculpturing pattern falling within the variation scope of caryopsis micromorphological characters of Avena. The differences of caryopsis size, shapes,spermoderm sculpturing patterns between A. nuda L. and A. sativa L. supported that A. nuda should be treated as a separate species. The wide variation ranges of caryopsis size, macrohair density and embryo proportion indicated that they were the adaptive consequences of wide variation range of climate condition across the geographical distribution region of Avena. The caryopses of concave ventral face have a relatively small bulk, which may have aided rapid development and maturation, a rhythm that is of particular value in short growth period in temperate and cold distribution region for Avena.