Stigma form and surface in the tribe Boragineae (Boraginaceae): micromorphological diversity, relationships with pollen, and systematic relevance

A comparative analysis of stigma form in 62 species representing 15 genera of the tribe Boragineae by means of light and electron (transmission and sc anning) microscopy highlighted the structural diversity, peculiarity, and f unctional specialization of the receptive surface of the boraginaceous stig ma. Cluster analysis identified 13 stigma morphotypes, 6 of which were pecu liar to the genera Anchusella, Brunnera, Cynoglottis, Gastrocotyle, Pentagl ottis, and Trachystemon. Nonea showed the broadest intrageneric stigma poly morphism, with 6 types differing in terms of stigma form and papillar struc ture. Stigma and pollen variations were poorly associated in this complex g enus, while in other groups, the pollen-stigma typological correspondence i ndicated the taxonomic distinctiveness of some critical species, such as Ga strocotyle hispida (Forssk.) C.B. Clarke and Anchusella spp. Positive corre lations were found between pollen and papillar size, between interpapillar distance and pollen equatorial diameter, and between pollen volume and styl e length. Qualitative and quantitative pollen-stigma relationships suggeste d functional implications of an adaptive nature that were interpreted throu gh the recognition of three distinct stages of early interaction, i.e., cap ture, lodging, and hydration of the grains.