The mature fruit wall structures were investigated for 18 of about 20 Ixeris species (Compositae-Lactuceae). fourteen species had 10 costae, nearly the same size in transverse sections, and four species had 5-7 or 10-13 costae. Fruit-wall structures were divided into two main types: winged type (subdivided into Type I and Type II-A, B, C), and ribbed type (Type III). Type I has thick intercosta (I. repens); Type II-A, B and C have well-developed costae (I, polycephala, I. debilis, I. stolonifera, I. longirostrata, I. chinensis and I. tamagawaensis). The ribbed type (Type III) has the least conspicuous costae (I. dentata and 10 other species of sect. Ixeridium). In the winged type, costae are formed by cell divisions in the initial stage of costal tissue development, but its development was less conspicuous in the intercostal tissue of I. repens of Type I. In the ribbed type, costae are formed by the disintegration of intercostal tissue, rather than by cell divisions in costal tissue throughout pre- and post-fertilization stages. Based on the different fruit-wall types and ontogenetic processes, as well as on karyological data, we suggest that 11 species belonging to the sect. Ixeridium, with ribbed achenes, and large chromosomes with a basic number of x=7, can be distinguished from the remaining species, which have winged achenes and small chromosomes and a basic number of x=8. New evidence concerning fruit wall structures obtained in the present study does not support earlier sectional classifications, except for sect. Chorisis, to which a single species, I, repens, is referred. Our results suggest that the species referred to sect. Ixeridium, with ribbed achenes and large chromosomes with a basic number of x=7, represent a distinct phylogenetic group and, thus, should be placed in a separate genus from Ixeris s. str.
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