Comparative morphology and anatomy of Backebergia militaris (Echinocereeae–Cactaceae) cephalium

Abstract

The morphology and anatomy of the Backebergia militaris cephalium is characterized in comparison with its vegetative branch and with apical and lateral cephalia of other species. Our working hypothesis was that the B. militaris cephalium is more similar to the apical or lateral cephalia of other taxa than to its own vegetative branch. Our results revealed that the differences between the vegetative branch and the cephalium are primarily morphological: there is a change in the phyllotaxy, there are no interareolar spaces, making the ribs indistinguishable and abundant trichomes and bristles are produced in the areoles. In addition, a comparison between the vegetative branch and the cephalium shows differences in the epidermal cell size, thickness of the hypodermis and abundance of cortical bundles and mucilage cells. Unlike Melocactus intortus apical cephalia, B. militaris retains stomata in the epidermis and chlorenchyma, allowing the cephalium near the apical meristem to perform photosynthesis. The development of periderm in each areole after flowering and fruiting is a distinctive and defining feature of lateral and apical cephalia. In addition to early periderm development, the necrosis of cortical and pith parenchyma and fibers in secondary xylem and phloem in B. militaris distinguishes it from other cephalia described to date.