Stem anatomy: development of included and intraxylary phloem in Barleria species (Acanthaceae) occurring in India

Abstract

Abstract Barleria is known for its different growth forms such as herbs, shrubs, climbers, and rarely trees that show a wide range of variation in wood structure. We investigated the Indian species of Barleria and observed that all are characterized by the presence of included and intraxylary phloem. Included phloem is the strand of sieve elements embedded within the secondary xylem and is believed to perform the same general function as regular phloem (i.e. photosynthate transport). Internal/intraxylary phloem comprises sieve elements that are formed on the periphery of the pith. In the present investigation, a complete study of the stem anatomy of 36 taxa (29 species, one subspecies, two varieties, and four forms) of Indian Barleria species along with two outgroups (Crossandra infundibuliformis and Petalidium barlerioides) was carried out. This is the first comprehensive study on Indian Barleria and could be used for identification of species in the genus. Differences in the outline of stem and pith, the composition of the secondary xylem, variation in the number of the sieve elements within the included phloem islands, septation in fibres, fibre cell wall thickening, and ray types can be used for species delimitation. The present study revealed that depending on species, vessel elements were angular or oval, having a short tail at one end and a long tail on the other end, a short tail on both ends, a long tail on both ends, or without a tail. Among the species investigated, no consistency was observed in the secondary xylem rays. They were mostly uni- to biseriate in some species while in the remaining species they were uni- to multiseriate.