How the neotropical liana Machaerium multifoliolatum (Fabaceae) develop their distinctive flattened stems?

Abstract

Vascular cambial variants have evolved many times in the evolutionary history of seed plants, and different types have been described which are more frequent in lianas than in trees. Many species showing cambial variants have also flattened stems, although this stem conformation can be achieved through diverse anatomical ontogenies including a single cambium. Considering the variation in the origin and development of these stem systems across the angiosperms, the aim of this work was to study the structure and ontogeny in stems of the liana M. multifoliolatum, in order to show how they develop their distinctive flattened stems. For this purpose, stem samples from different developmental stages were collected and analyzed using light microscopy. The stems of M. multifoliolatum initiates the secondary growth with the formation of a regular cambium which function in the usual way for some time. Subsequently, new cambia arise successively developing secondary tissues in two opposite sides of the stem. The development of the new vascular system is due to the differentiation of new cambia within the nonconducting secondary phloem. The new cambia produce small strands of secondary xylem and phloem that never form a complete ring. Thus, the formation of eccentric semi-rings of successive cambia in two opposite sides results in the flattened stems, with new cambia arising several times during stem ontogeny. Besides the establishment of the cambial variant, other major anatomical characteristics are the presence of stratified secondary phloem and secondary xylem divided into self-supporting and lianescent phase. Given the similarity of these micromorphological aspects in the wood of lianas within Fabaceae and species from more distant related groups, our observations evidence the likely contribution of cambial variants and modifications in vascular cells that make lianas better adapted to climb on themselves or anything around.