Wood and bark anatomy of Lobostemon (Boraginaceae) : Ecological and evolutionary implications

Azwinndini Matshinga

The genus Lobostemon (Boraginaceae) comprises 28 shrubby species endemic to the Cape Floristic Region of South Africa representing an interesting case of secondary woody lineage originated from herbaceous ancestor. The structural diversity of stems within this genus has been poorly studied to date, This study is focused on anatomical exploration of wood and bark of in the species of Lobostemon with clarification of functional and adaptive roles of some structural traits as well as of the pathways of their structural transformations associated with the evolutionary shift from herbaceous to woody habit. 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 abundance of very narrow vessels per group correlated positively with humidity, indicating 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. The bark structure of Lobostemon curvifolius and L. trigonus (Lithospermeae, Boraginaceae), Cape endemic shrubs derived from herbaceous ancestors, is described. Lobostemon shares several traits with Echium and other herbaceous relatives (uniseriate epidermis with trichomes, cortical parenchyma and endodermis, absence of sclerification in pericycle and phloem, continuous procambial ring). The evolutionary 17 shift to a woody habit in Lobostemon mainly involved quantitative modifications of these bark features, while the development of a subsequent periderm in L. trigonus represents the most important qualitative change linked to shrub growth. Homogeneous secondary phloem appears as a continuation of primary phloem formed by the procambial ring, gradually transformed into vascular cambium. Periderm continuity during bark dilatation is maintained by tangential phellogen elongation, and in L. trigonus also by irregular phellem cracking with initiation of new periderms, but without outer bark abscission. The parenchyma--like phellem and phelloderm further reflect persistence of herbaceous traits in the woody stem.

Wood and bark anatomy of Lobostemon (Boraginaceae) : Ecological and evolutionary implications

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