Structure and development of the secondary xylem in ten species of Passiflora from Taiwan

Structure and development of the secondary xylem in ten species of Passiflora from Taiwan

Studies on secondary growth of the genus Passiflora (Passifloraceae) in Taiwan are scarce. This study aimed to investigate the patterns of cambial variants in 10 Passiflora species. Stem diameters were measured to determine structural and developmental differences in the secondary xylem of Passiflora species. Cambial variants observed included irregular conformations, furrowed xylem, external secondary vascular cylinders, and successive cambia. An external secondary vascular cylinder was evaluated to distinguish Passiflora coccinea from Passiflora vitifolia. Passiflora edulis stems were thick and cylindrical, with two rings of successive cambia. Passiflora suberosa subsp. litoralis had fissured xylems with shallow phloem wedges in small stems and exhibited circular and cylindrical structures with more secondary rays in large stems. Passiflora laurifolia and Passiflora quadrangularis had secretory tissues around the cortex, where the stem of the former was regular in conformation, and that of the latter was four-ridged. Among the species studied, P. quadrangularis exhibited aggregated rays in stem xylem cylinders. All investigated species had a diffuse-porous secondary xylem and vasicentric paratracheal parenchyma, except for P. suberosa litoralis, which had semi-ring-porous and indistinct growing rings. The cambial variants of the genus Passiflora described in the present study could increase the significances of this study in relation to ecological and functional contexts and it will provide the primary information for future morphological studies.