Insights into the functions of living fiber-tracheids and very narrow vessels: evidence from the wood anatomy of Lobostemon (Boraginaceae)

The genus Lobostemon (Boraginaceae) comprises 28 shrubby species endemic to the Cape Floristic Region of South Africa. We investigated mature wood structure in 11 species and examined correlations between wood traits, plant stature, and climate to clarify the adaptive roles of living fibre-tracheids, vessel dimorphism, and related features. Vestured intervessel pits proved characteristic of Lobostemon . Contrary to the view that fibre-tracheids with living protoplasts are juvenile (Frankiewicz et al., 2024), we found that the ground tissue of mature wood is largely composed of these elements. Their bordered pits may facilitate the release of stored water into the transpiration stream, thereby enhancing hydraulic capacitance. Grouping of wider vessels increased with aridity and continentality, suggesting a role in sustaining water transport during seasonal drought by forming subsidiary pathways around embolized vessels. By contrast, the percentage of solitary narrow vessels positively associated with milder hot seasons, suggesting their function in capillary water storage and lateral transfer between vessels and fibre-tracheids, also supporting hydraulic capacitance. Increased plant stature in drier habitats likely reflects the need for greater stem storage capacity, consistent with an avoidance strategy that allows Lobostemon species to withstand seasonal drought.