The genus Taraxacum Wigg. (Asteraceae) forms a polyploid complex within which there are strong links between the ploidy level and the mode of reproduction. Diploids are obligate sexual, whereas polyploids are usually apomictic. The paper reports on a comparative study of the ovary and especially the ovule anatomy in the diploid dandelion T. linearisquameum and the triploid T. gentile. Observations with light and electron microscopy revealed no essential differences in the anatomy of both the ovary and ovule in the examined species. Dandelion ovules are anatropous, unitegmic and tenuinucellate. In both sexual and apomictic species, a zonal differentiation of the integument is characteristic of the ovule. In the integumentary layers situated next to the endothelium, the cell walls are extremely thick and PAS positive. Data obtained from TEM indicate that these special walls have an open spongy structure and their cytoplasm shows evidence of gradual degeneration. Increased deposition of wall material in the integumentary cells surrounding the endothelium takes place especially around the chalazal pole of the embryo sac as well as around the central cell. In contrast, the integumentary cells surrounding the micropylar region have thin walls and exhibit a high metabolic activity. The role of the thick-walled integumentary layers in the dandelion ovule is discussed. We also consider whether this may be a feature of taxonomic importance.

A description of Taraxacum zajacii J. & P. Marciniuk, a new species of T. sect. Palustria in Poland is given. Taraxacum zajacii is a pentaploid (2n = 40). Morphologically, the new species is closest to the T. subalpinum/T. neterophilum group.Taraxacum sect. Palustria includes 131 mostly apomictic species, markedly different in their karyology. Sexually reproducing diploids are represented in the section by only two rare Mediterranean species, Taraxacum raii and T. tenuifolium. The apomictic taxa form a series of polyploids, from common triploids (2n = 24) and tetraploids (2n = 32) to rare pentaploids (2n = 40) and very rare hexaploids (2n = 48) represented by only two species, which are T. ranunculus and T. flos-lacus (Kirschner & Štěpánek 1998, Tikhomirov 2003, Aquaro et al. 2008. In Poland, T. sect. Palustria includes 23 exclusively apomictic species with 15 triploids, six tetraploids and two pentaploids (Marciniuk et al. 2010b, Marciniuk 2012.A large population of a distinct taxon which, after morphological and karyological studies, appeared to be a new species was found in 2008. The description of this species is given in this paper.Morphometric analyses were made on herbarium materials collected in the field and on cultured live plants. In total, 100 individuals and herbarium specimens were analysed.For karyological studies, seeds collected from the cultured plants were germinated on moistened filter paper in Petri dishes. Three-to four-day-old seedlings were incubated in 8-hydroxychinoline for 4 h at room temperature. Then they were rinsed in distilled water and fixed in 96% ethanol/ glacial acetic acid (3:1) for 24 h. The fixed material was stained in 2% acetic orcein for 3-4 days at room temperature. The stained seedlings were rinsed in 45% acetic acid, then heated to a boiling point over a flame. For slide preparation, root tip meristems were cut off and squashed in a drop of 45% acetic acid, dry-iced, air-dried and mounted in Entellan. The chromosomes were counted during the mitotic metaphase and photographed using a Nikon Eclipse 80i microscope equipped with a monochrome CCD camera.

An evolutionary trend to reduce “unnecessary costs” associated with the sexual reproduction of their amphimictic ancestors, which may result in greater reproductive success, has been observed among the obligatory apomicts. However, in the case of the female gametophyte, knowledge about this trend in apomicts is not sufficient because most of the ultrastructural studies of the female gametophyte have dealt with amphimictic angiosperms. In this paper, we tested the hypothesis that, in contrast to amphimictic plants, synergids in apomictic embryo sacs do not form a filiform apparatus. We compared the synergid structure in two dandelions from sect. Palustria: the amphimictic diploid Taraxacum tenuifolium and the apomictic tetraploid, male-sterile Taraxacum brandenburgicum. Synergids in both species possessed a filiform apparatus. In T. brandenburgicum, both synergids persisted for a long time without any degeneration, in spite of the presence of an embryo and endosperm. We propose that the persistent synergids in apomicts may play a role in the transport of nutrients to the embryo.

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The genus Taraxacum is taxonomically complicated due to apomixis. Therefore, there are significant differences in knowledge of regional Taraxacum-Floras among European countries. Similarly, some taxa are well characterised with well-known distribution, whereas some ones are known e.g. from a single locality. Taraxacum scanicum of section Erythrosperma is one of the most common microspecies of this section in Europe. Recently, this taxon was split into three microspecies. In this study we map the distribution of three described taxa of this group in Poland, i.e. T. scanicum s.s., T. prunicolor and T. cristatum. The survey showed that T. scanicum has a similar distribution as T. prunicolor in Poland. This is in contrast with the hitherto known distribution of both taxa. T. cristatum was found at a single locality that is quite distant from the distribution in southern central Europe. We also provide with new morphological characteristics of achenes and pollen for T. scanicum s.s. and T. prunicolor

Measurements of the pollen size in 5 species of Taraxacum sect. Palustria at three levels of ploidy: 2n = 3x = 24 (T. paucilobum), 2n = 4x = 32 (T. vindobonense, T. trilobifolium), 2n = 5x = 40 (T. mendax) and one taxon of unknown number of chromosomes 2n = ? (T. portentosum) are presented in this paper. Obtained results indicate a lack of distinct positive correlation between the pollen size and ploidy in the studied group of plants. Distinct relationship was, however, found between ploidy and the range of pollen size and shape variability. Most variable were the pollen grains of triploid T. paucilobum and the least -those in pentaploid T. mendax. Ranges of pollen variability in tetraploid T. trilobifolium and T. vindobonense and in T. portentosum of unknown number of chromosomes showed intermediate values.