Stem anatomy and ontogeny of vascular variants in Ipomoea asarifolia (Desr.) Roem. & Schult. (Convolvulaceae)

An increase in stem thickness and ontogeny of successive cambia, intraxylary cambium, and inter‐and intraxylary phloem is elaborated for the first time in Ipomoea asarifolia (Desr.) Roem. & Schult. (Convolvulaceae). As the plants grew older, several small segments of cambium were initiated simultaneously external to the phloem formed by regular vascular cambium. Ontogenetically, these cambial segments were initiated from two different tissues, i.e. from the pericyclic parenchyma and dilating ray parenchyma cells. Products of both cambial segments showed unique orientation, and they may be identified based on the orientation of the xylem and phloem elements produced by them. Cambial segments initiating from the pericyclic parenchyma began with repeated divisions in one to a few cells to form oval to circular patches of the meristematic cells. This resulted in the formation of oval to elliptic vascular cylinders. The vascular cylinders were later interconnected by forming cambial segments that exclusively formed thin‐walled parenchyma on either side. In the second type, dilating ray cells underwent swelling followed by repeated anticlinal divisions and produced xylem on one side and phloem on the other side of the cambium; thus, these vascular cylinders were oriented tangentially. The secondary xylem consisted of dimorphic vessels, fibres, axial and ray parenchyma, and interxylary phloem. In the thick stems, intraxylary cambium was initiated from the marginal pith cells situated on the inner face of the protoxylem. Initially, this cambium was unifacial and deposited only intraxylary phloem, but subsequently, it became bidirectional and began to deposit secondary xylem centrifugally. The presence of variant secondary growth is correlated with climbing habits and family characteristics of the Convolvulaceae.