Botanica Pacifica. A journal of plant science and conservation 2025. Preprint
Article first published online: 30 OCT 2025 | DOI: 10.17581/bp.2025.14211
Bark structure in woody plants with herbaceous ancestry: a case of Lobostemon (Boraginaceae)
Azwinndini Matshinga 1
&
Alexei Oskolski 1,2
1 Department of Botany and Plant Biotechnology, University of Johannesburg, Johannesburg, South Africa
2 Komarov Botanical Institute, St. Petersburg, Russia
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.
Матшинга А., Оскольский А.А. Структура коры у вторично древесных растений на примере рода Lobostemon (Boraginaceae). Изучена структура коры Lobostemon curvifolius и L. trigonus (Lithospermeae, Boraginaceae), эндемичных кустарников Капского флористического региона, производных от травянистых предков. Lobostemon сходен с Echium и другими близкими травянистыми родами по наличию однорядной эпидермы с трихомами, кортикальной колленхимы, эндодермы, отсутствию склеренхимы в перицикле и флоэме, наличию непрерывного прокамбиального кольца. Эволюционный переход к кустарниковой жизненной форме у Lobostemon был сопряжен с количественными модификациями признаков коры у родственных травянистых групп; появление второй перидермы у L. trigonus представляет собой наиболее важное качественное новшество, сопряженное с древесностью. Гомогенная вторичная флоэма вероятно представляет собой продолжение круга первичной флоэмы, возникшее в результате постепенной трансформации кольца прокамбия в сосудистый камбий. Непрерывность перидермы у Lobostemon поддерживается путем тангентального увеличиения длины феллогена, а у L. trigonus также путем образования нерегулярных трещин в феллеме и формирования второй перидермы. Феллема и феллодерма напоминают паренхиму по своему строению, что тоже может рассматриваться как сохранение признаков травянистости в коре кустарникового стебля.
Keywords: Echium, secondary woodiness, Cape Floristic Region, procambium, bark abscission, bark dilatation, вторичная древесность, Капская флористическая область, прокамбий, сбрасывание коры, дилатация коры
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