The bark anatomy of Ningxiaites specialis from the Permian of China

Abstract

Bark is an important functional structure in woody plants. However, fossil tree axes are commonly decorticated. The development of bark during the evolutionary history of fossil plants thus remains poorly understood. Here, we describe exceptionally well-preserved extraxylary tissues of the Lopingian (Late Permian) conifer Ningxiaites specialis Feng, including vascular cambium and bark, from the Sunjiagou Formation (Changhsingian) of northern China. The vascular cambium bears one or two layers of cambial cells. The bark comprises secondary phloem and periderm. The secondary phloem consists of rays, axial parenchyma and sieve cells. The rays of the secondary phloem are uniseriate and continuous from the rays of the xylem. They are more frequently present in the inner zone of the secondary phloem. Axial parenchyma cells are vertically aligned and appear more regularly distributed in the outer zone of the secondary phloem. Elliptical or subcircular sieve areas are placed on the radial walls of the sieve cells. The periderm located outside the secondary phloem is composed of imbricate flattened cork cells. The cork cells show suberised cell walls and are generally filled with dark contents. Remains of the secondary phloem present between layers of periderm indicate the formation of rhytidome-type bark. This is the first detailed report of the bark anatomy of a conifer from the upper Palaeozoic of Cathaysia, and shed light on the early diversity of bark structure during the evolutionary history of conifers.