Abstract
A study on achene macro- and micromorphological characters of the following five Polish species of the <em>Senecio jacobaea</em> group was carried out by LM and SEM: <em>S. aquaticus</em> (var. <em>aquaticus</em> and var. <em>barbareifolius</em>), <em>S. erucifolius</em> (subsp. <em>erucifolius</em> and subsp. <em>tenuifolius</em>), <em>S. jacobaea</em>, <em>S. paludosus</em>, and <em>S. subalpinus</em>. The achenes differed in several characters, but the most distinctive differences concerned their indument, especially the length and morphology of hairs. The study clearly shows that two morphologically similar species, <em>S. jacobaea</em> and <em>S. erucifolius</em>, which are often confused can be easily distinguished based on the morphological characters of achenes. Additionally, we showed differences between the achenes of <em>S. erucifolius</em> subsp<em>. erucifolius</em> and subsp. <em>tenuifolius</em>, the two taxa which were not distinguished in floristic studies and herbarium collections from Poland. The key to determine Polish species of the <em>Senecio jacobaea</em> group, based on the morphological characters of achenes, is also given.
Highlights
Senecio L. (Asteraceae; Senecioneae) is a large and taxonomically difficult genus comprising between 1000 and 3000 species (Jeffrey et al 1977; Nordenstam, 1978; Bremer, 1994; Vincent, 1996)
The main macro- and micromorphological characters of the examined achenes are summarized in Table 2, while the selected macrophotographs and SEM microphotographs of the fruit showing its general view, the achene surface and pappus details are presented in Plates I-III
In S. aquaticus, no significant differences were found at the species level, so further we present the results only for nominal taxa
Summary
Senecio L. (Asteraceae; Senecioneae) is a large and taxonomically difficult genus comprising between 1000 and 3000 species (Jeffrey et al 1977; Nordenstam , 1978; Bremer , 1994; Vincent , 1996). (Asteraceae; Senecioneae) is a large and taxonomically difficult genus comprising between 1000 and 3000 species (Jeffrey et al 1977; Nordenstam , 1978; Bremer , 1994; Vincent , 1996). The section Jacobaea comprises from 3 to 15 species (Pelser et al 2002). Such different views of the species composition of the section resulted from a very general section description by different authors (e.g. Chater and Walters , 1976; Jeffrey , 1992; Shishkin , 1995) which could be applicable to many species from other sections of Senecio. Its phylogenetic position as well as the relationships within and among Senecio sections were recently examined by Pelser et al (2002, 2003, 2004, 2007)
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