Development of external vascular cylinders (neoformations) in stems and roots of Chiococca alba (L.) Hitchc. (Rubiaceae)

Abstract

Chiococca alba (L.) Hitchc. (Rubiaceae) is a shrub or liana with a cambial variant comprised by a central cylinder and several peripheral cylinders. Various terms such as successive cambia, secondary strands of xylem and phloem and polystelic stem have been used in the literature to describe such variant. The origin of the vascular bundles has been attributed to the stem and root cortex and/or to the phloem. Considering these divergences in characterization, the present study aimed to elucidate the origin and development of this cambial variant. Secondary vascular cylinders (neoformations) peripheral to a central cylinder were observed at three meters from the stem apex. Circular vascular cambia were formed in the cortex and in the dilated rays of the secondary phloem. The new cambia produced secondary phloem externally and secondary xylem internally. The cambial variant was observed in roots and stems of the examined species. This variant can be categorized as “external vascular cylinders”, a case of multiple cambia, following a recently proposed classification.