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Riparian environments are unique habitats highly susceptible to sudden flooding following heavy rainfall. In such conditions, the sterile in dimorphic fronds that remain aboveground for several months have adapted to thrive by developing linear and lanceolate laminae supported by robust yet flexible petioles. However, it remains unclear whether fertile fronds, which develop only for a short period of several days to weeks, exhibit similar adaptations. To address this question, we conducted morphological, anatomical, and mechanical analyses of the stipes of Osmunda lancea (Osmundaceae), which inhabits riversides, and its inland sister species O. japonica . Our results revealed that the stipes of O. lancea were more flexible than those of O. japonica , accompanied by a reduction in cell length within the sterome. Furthermore, the sterome of the stipe was found to be composed of thin-walled cells, enabling the formation of the stipe at a low carbon cost within a short period and facilitating spore dispersal.

The curve of a river bed creates a difference in the speed of water flow inside and outside this curve, indicating that plants growing along the river experience differential water-flow stresses during sudden floods caused by heavy rains. In this study, we conducted morphological, anatomical, and mechanical analyses using Osmunda x intermedia (Honda) Sugim. (Osmundaceae), a hybrid of Osmunda japonica Thunb. and the rheophytic O. lancea Thunb., growing inside and outside the river curve to elucidate the plant traits influenced by differential water-flow stresses. The external morphological analysis revealed that the O. x intermedia populations growing both inside and outside the river curve exhibited values intermediate between those of the parent species. However, the results of the anatomical and mechanical analyses of the petioles of the hybrid species did not necessarily reveal values intermediate between those of the parent species; however, in the hybrid species, the cell wall volume per unit volume was related to petiole strength, and the cell wall volume per unit volume of the hybrid population growing inside the river curve was significantly higher than that in the parent species or the hybrid population outside the river curve. In addition, the flexibility of petioles in the hybrid population growing outside the curve was associated with a lower cell wall density in the sterome than in that inside the curve, which may cause elastic bending that bends the cells further because of thinner cell walls. The results obtained in our study revealed that O. x intermedia adapts to different water-flow stresses through complex anatomical and mechanical changes that cannot be determined from external morphology alone.

Plants along rivers have narrow lanceolate leaves, flexible stems, and petioles to avoid the water flow stress caused by flooding. This study aimed to determine whether the adaptation of Adenophora triphylla (Thunb.) A.DC. var. japonica (Regel) H. hara (Campanulaceae) with narrow leaves in riverside habitats was achieved through phenotypic plasticity or genetic morphological changes, where we conducted comparative morphological analyses through cultivation experiments. Our cultivation experiments revealed that the morphology of radical leaves in the riverside population had significantly smaller and narrower laminas and shorter and thicker petioles than those in the inland population, and that the former had significantly more radical leaves than the latter, suggesting that the former brings the total leaf area closer to that of the latter by increasing the number of radical leaves. The cauline leaves were significantly thinner and smaller in the riverside population than in the inland population, and the stems of the former were significantly shorter and thicker than those of the latter. In addition, a significant difference was observed between the riverside and inland populations in the number of rosette leaf branches from the rhizome, with the former having significantly more rosette leaf branches. Our results reveal that populations of Ad. triphylla var. japonica with genetically distinct leaf and stem morphologies have become established along rivers, where flooded water imposes strong selective pressure. In these riverside populations, thicker and shorter petioles and stems appear to reduce bending moments without breaking, while narrower and smaller laminas of both radical and cauline leaves further contribute to this reduction. This study found that riverside populations of Adenophora triphylla have genetically fixed morphological traits—such as narrower leaves and thicker, shorter petioles and stems—that help resist water flow stress. These traits were maintained even under cultivation, suggesting adaptive evolution rather than mere plasticity.

During a revision of Radulaceae in Southeast Asia, an undescribed species of the genus Radula was discovered from Thailand and Brunei Darussalam. Morphological characters were examined from fresh and herbarium specimens using stereo and compound microscopes. Phylogenetic analyses were conducted based on six chloroplast markers ( atp B –rbc L, psb T –psb H, psb A –trn H, rps 4, trn G, and trn L–F) using Maximum Likelihood and Bayesian Inference approaches. Radula rheophila is described and illustrated as a new species from peninsular Thailand and Brunei Darussalam. The new species is characterized by female bracts usually in 2 or 2.5 pairs unequal pairs; longitudinally rectangular or ligulate leaf lobules when large, broadly transversely rhombic or rectangular when small, 1/3–2/5 the length of the lobe; ovate to broadly ovate leaf lobes with broadly obtuse to rounded apices and entire margins; finely botryoidal oil bodies (2–3 per cell); and stems with thickened exterior cortical walls and thin-walled interior cortical and medullary cells thickened at the corners by concave trigones. Phylogenetic analyses strongly support its placement within Radula subg. Odontoradula . The discovery of R. rheophila enhances understanding of morphological variation and phylogenetic relationships within Radula , highlighting the significance of integrative taxonomy for uncovering tropical bryophyte diversity.

The term rheophyte describes a biological group of flood‐tolerant plants that are confined to the beds of swift‐running streams and rivers in nature and grow up to flood level, but not beyond the reach of regularly occurring flash floods. Although over 35 yr have passed since the first global census of rheophytes, no updates have been recorded regarding the number of taxa in this biological group in seed plants. Therefore, the present work aimed to (1) review the main topics associated with rheophytism (e.g., morphological characteristics, genetic studies, geographic distribution, conservation, and evolutionary aspects); (2) provide an updated checklist of rheophytes distributed around the world considering the two main groups in seed plants (gymnosperms and angiosperms); (3) demonstrate the distribution of rheophytism in the angiosperm phylogeny; and (4) estimate the geographical distribution and richness of selected taxa on the world map for the first time. All data compiled for the present study originated from a search of peer‐reviewed articles, secondary literature (theses, dissertations, reports, books, and floras), and electronic facilities. We compiled a data set composed of four taxa in gymnosperms (Podocarpaceae family) and 1,368 taxa (including obligate, facultative, and unclassified rheophytes) distributed in 114 families and 508 genera in angiosperms. Most of the studied taxa belong to eudicotyledons (72.81%), while 1.46% belong to magnoliids, and 25.73% belong to monocotyledons. The families with the highest number of taxa in descending order are Podostemaceae, Araceae, Myrtaceae, Rubiaceae, Asteraceae, Apocynaceae, Arecaceae, Fabaceae, Phyllanthaceae, and Poaceae. Of the 114 families plotted in angiosperm phylogeny, at least 80 harbor obligate rheophytes. The geographical distribution of rheophytes in angiosperms, as expected based on the first census of this biological group, is mainly in the tropical and subtropical regions. The high richness of rheophytic taxa was mostly found in southern Mexico, southern China, Borneo, and northern and eastern Australia. In contrast, the geographical distribution of rheophytes in gymnosperms is restricted to New Caledonia and Tasmania. The present study will help to advance knowledge regarding the diversity of rheophytes in angiosperms and gymnosperms while drawing attention to this biological group, which has often been overlooked. There are no copyright restrictions. Please cite this data paper when the data are used in publications and teaching events.

Premise The drivers of isolation between sympatric populations of long‐lived and highly dispersible conspecific plants are not well understood. In the Hawaiian Islands, the landscape‐dominant tree, Metrosideros polymorpha, displays extraordinary phenotypic differences among sympatric varieties despite high dispersibility of its pollen and seeds, thereby presenting a unique opportunity to investigate how disruptive selection alone can maintain incipient forms. Stenophyllous M. polymorpha var. newellii is a recently evolved tree endemic to the waterways of eastern Hawai'i Island that shows striking neutral genetic differentiation from its ancestor, wet‐forest M. polymorpha var. glaberrima, despite sympatry of these forms. We looked for evidence for, and drivers of, differential local adaptation of these varieties across the range of M. polymorpha var. newellii. Methods For paired populations of these varieties, we compared seedling performance under contrasting light conditions and a strong water current characteristic of the riparian zone. We also conducted a reciprocal transplant experiment and contrasted adult leaf anatomy. Results Results suggest that the riparian zone is harsh and that selection involving the mechanical stress of rushing water, and secondarily, light, led to significant reciprocal immigrant inviability in adjacent forest and riparian environments. The strongest adaptive divergence between varieties was seen in leaves and seedlings from the site with the sharpest ecotone, coincident with the strongest genetic isolation of M. polymorpha var. newellii observed previously. Conclusions These findings suggest that disruptive selection across a sharp ecotone contributes to the maintenance of an incipient riparian ecotype from within a continuous population of a long‐lived and highly dispersible tree species.

Strobilanthes schomburgkii (Craib) J.R.I.Wood is a plant native to Thailand's humid tropics. It is discovered to be newly naturalized in Bandung, Cirebon, and Sumedang, West Java. It occurs in riparian habitats along the Winong River and Ciherang River, and in a ditch in Pasir Impun, which resemble its native habitat. The discovery represents the first record of S. schomburgkii as a naturalized species in Indonesia. Considering its recent establishment, regular monitoring and further study of its population dynamics and potential impacts on native vegetation are recommended to anticipate and prevent its possible spread.

Ctenium bennae Xanthos is described and illustrated as the only rheophytic species in the genus. The new species is known from a single waterfall on the Benna Plateau, at the border between Forecariah and Kindia Prefectures in Guinea Conakry. Ctenium bennae is here assessed as Near Threatened according to the categories and criteria of IUCN.

Orthotrichum, sensu stricto, is a genus of about 90 species that are mostly adapted to xerophytic environments, occurring on a variety of substrate types, including tree trunks and rock surfaces. However, three species—O. euryphyllum, O. rivulare, and O. sprucei—inhabit seasonally submerged substrates along streams and other water bodies. The morphological similarity and peculiar ecological preferences of these three species have been recognized taxonomically by several infrageneric treatments, in particular as subgenus Rivularium, assuming that the acquisition of the rheophytic lifestyle as a single event. We examined with sequence data from nuclear and plastid markers with the hypothesis that these three species form a monophyletic group. Adaptation to the rheophytic habitat arose at least twice in Orthotrichum and very likely additional times in the subfamily. The subgenus Rivularium, as previously recognized, is not monophyletic and the three rheophytic species should be placed in the type subgenus.