The development of the ovule, fruit and seed of Xyris spp. was studied to assess the embryological characteristics of potential taxonomic usefulness. All of the studied species have (1) orthotropous, bitegmic and tenuinucellate ovules, with a micropyle formed by both the endostoma and exostoma; (2) a cuticle in the ovules and seeds between the nucellus/endosperm and the inner integument and between the inner and outer integuments; (3) helobial, starchy endosperm; (4) a reduced, campanulate and undifferentiated embryo; (5) a seed coat formed by a tanniferous endotegmen, endotesta with thick-walled cells and exotesta with thin-walled cells; and (6) a micropylar operculum formed from inner and outer integuments. The pericarp is composed of a mesocarp with cells containing starch grains and an endocarp and exocarp formed by cells with U-shaped thickened walls. The studied species differ in the embryo sac development, which can be of the Polygonum or Allium type, and in the pericarp, which can have larger cells in either endocarp or exocarp. The Allium-type embryo sac development was observed only in Xyris spp. within Xyridaceae. Xyris also differs from the other genera of Xyridaceae by the presence of orthotropous ovules and a seed coat formed by endotegmen, endotesta and exotesta, in agreement with the division of the family into Xyridoideae and Abolbodoideae.
The floral anatomy and development of species belonging to all sections of Xyris (Xyris, Nematopus and Pomatoxyris) were investigated comparatively to elucidate gynoecium evolution in the genus, because placentation has been one of the key characters in recognizing its taxonomic sections. In species of section Pomatoxyris (axile placentation), the tricarpellate syncarpous ovary consists of a fertile synascidiate zone, whereas the symplicate zone is fertile only in its lower portion. In species of section Xyris (parietal placentation), the ovary has a short almost sterile synascidiate zone, whereas the more extensive symplicate zone is fertile. In species of section Nematopus (free-central or basal placentation), there is an extremely short and sterile trilocular, completely septate zone at the ovary base. Such differences are related to different patterns of gynoecium development. The ancestral character state reconstruction of placentation types shows that axile placentation is the most likely ancestral condition in Xyris, from which parietal and free-central placentation were derived by shortening of the septa during the evolution of the genus. Basal placentation evolved from free-central placentation and appeared independently several times in section Nematopus. Because the phylogenetic position of X. ptariana and X. teinosperma (section Nematopus) is unknown, either the axile placentation in these species is a reversal or highlights the need for a revised infrageneric classification.
The seed micromorphology was studied in eight species of Xyris (Xyridaceae) with taxonomic purposes. The results show that the presence of longitudinal endotegmic ridges in the seed coat is a pattern for the genus and that the shape of these ridges differentiates among the species. The following characteristics are also useful to identify the species: shape and size of the seed, number of cell rows between the ridges, and the striation pattern of the seed coat. Based on these characteristics, a standard terminology is proposed to describe the seed coat in species of the genus. An identification key for the studied species is also provided.
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