Development of ovule, fruit and seed of Xyris (Xyridaceae, Poales) and taxonomic considerations
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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.