Stem anatomy and ontogeny of phloem wedges in some species of Gymnema (Apocynaceae)

The anatomy of climbing plants shows a fascinating arrangement of vascular tissues, which may vary from species to species. In the present study, stem anatomy of Gymnema cuspidatum, G. latifolium, G. montanum and G. sylvestre (Apocynaceae) and the occurrence of vascular variants are described here for the first time. They were compared histologically to understand the structural variations in the secondary xylem and ontogeny of vascular variants if any within the species. All four species increase their stem diameter by a single ring of vascular cambium like most of the eudicots. In thick stems, periderm forms distinct ridges and furrows in all the species except G. sylvestre, in which it was minimal. Differential activity of the cambium resulted in the formation of distinct and deep phloem wedges in G. latifolium and G. sylvestre while in other species they were either shallow or almost absent. The secondary xylem was diffuse-porous with distinct growth rings in G. cuspidatum and G. montanum while they were indistinct in G. latifolium and G. sylvestre. Besides regular xylem elements, the presence of perforated ray cells and radially oriented laticifers in the xylem rays was observed frequently in all the investigated samples. The formation of the intraxylary primary phloem was observed concomitant to the regular protoxylem and protophloem at the pith margin. Subsequently, an additional intraxylary phloem was developed from the adjacent pith cells and by the activity of intraxylary/medullary cambium at the pith margin. In thick stems, parenchymatous cells situated between the protoxylem and intraxylary protophloem led to the initiation of intraxylary cambium. In conclusion, three species showed distinct ridges and furrows due to the formation of periderm except G. sylvestre, phloem ray parenchyma showed the formation of sclerenchyma only in the thick stems of G. sylvestre. The formation of intraxylary phloem in all four species may serve as an additional way for the transport of organic nutrients.