Abstract
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 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 parenchymalike phellem and phelloderm further reflect persistence of herbaceous traits in the woody stem.