From cavities to canals: evidence of developmental continuity in Myrsine guianensis (Primulaceae).

Secretory cavities and canals are traditionally treated as distinct anatomical categories and widely employed as diagnostic characters in taxonomic studies. However, their coexistence or the presence of intermediate forms in the same individual or organ points to a more intricate developmental relationship. The mechanisms underlying their formation and structural differentiation, particularly the boundaries between cavities and canals, remain insufficiently understood. In this study, we selected Myrsine guianensis (Aubl.) Kuntze (Primulaceae) because it displays globose and elliptical cavities alongside linear, canal-like secretory spaces. This condition raises a fundamental question: do these structures represent two discrete types of secretory spaces, or are they transitional forms along a developmental continuum? To address this question, we investigated the secretory spaces from a developmental point of view using light and transmission electron microscopy. Secretory cavities originated from the fundamental meristem through a schizolysigenous process, giving rise to a lumen lined by a uniseriate secretory epithelium and surrounded by one or two layers of sheath cells. Epithelial cells exhibited ultrastructural features indicative of intense metabolic activity associated with oil-resin synthesis. Secretion involved pronounced cell-wall remodeling, transient periplasmic spaces, and the replacement of senescent epithelial cells. A shift from resin- and phenolic-rich secretion to predominantly oil-rich secretion coincided with epithelial senescence and the recruitment of sheath cells, thereby sustaining secretory activity. The fusion of adjacent cavities, epithelial reorganization, and progressive lumen expansion produced transitional forms between globose and elongated structures, supporting the existence of a developmental cavity-canal continuum. This structural plasticity challenges the view of secretory spaces as discrete anatomical entities and suggests that spatial constraints during morphogenesis contribute to their diversification.