Spore morphology of <i>Adiantum</i> species from the Indian sub-continent using LM and FESEM: Palaeoecological analysis and Phylogenetic delineation

The palynology of fern spores has proven to be an immensely useful tool in the identification and phylogenetic characterization of various fern taxa. A diverse range of spore morphologies have been studied yet, nevertheless, the investigation of 21 species of <i>Adiantum</i> under Light Microscopy (LM) and Field Emission Electron Microscopy (FESEM) is a maiden attempt from India to develop a spore micro-morphometric dataset that complies with taxonomic traits, fossil counterparts, and evolutionary changes. All the studied spores of <i>Adiantum</i> species possess significant differences in lete character (aperture, primary character); exospores ornamentation (secondary character); and spore shape (tertiary character). The sexine pattern varies from psilate to verrucate, scabrate to granulate, and rugulate to fossulate (eurypalynous type). The perine is a characteristic feature of the fern spores as evident in <i>A. myriosorum, A. zollingeri,</i> and <i>A. philippense,</i> suggesting the diversity among different <i>Adiantum</i> spores. The multivariate statistical analysis indicates three major clusters based on spore morphological characteristics. The evolutionary outcome of this micro-morphometric analysis showed that the spore of <i>A. formosum</i> is composed of the most primitive characters (such as unconstricted lete arm, smooth exospore, and non-perinate sclerine), whereas the spores of <i>A. zollingeri</i> and <i>A. myriosorum</i> comprised advanced characters (such as constricted lete arms, rugulate exospores, and perinate sclerine). This extensive spore morphological examination would provide a comprehensive dataset for the identification of fossil spores (up to the species level) and taxonomic characterization of <i>Adiantum</i> species in India and its comparison with other species growing around the world.