Abstract
Aglaothamnion rigidulum is described as a new species from southern Kwazulu-Natal, South Africa. The main distinguishing characters are the robust nature of the filaments, the dense cortication in the lower part of the thallus, the alternating distichous branching pattern and the unbranched determinate laterals which curve upwards and overtop the apical cell. In addition to a detailed description of the vegetative anatomy and reproductive structures, attention is drawn to unusual changes of the branching pattern associated with the formation of procarps near the apices of indeterminate axes, and the subsequent formation of an involucrum protecting the developing gonimoblast. The plane of branching changes by 90° at the fertile axial cell, and the determinate laterals 2–3 cells below and above the fertile axial cell are strongly incurved and branch once or twice subdichotomously, thereby forming a protective involucrum enclosing the developing gonimoblast from all sides. This aspect of the morphology has not been well documented in either Aglaothamnion or Callithamnion, but may well occur in several other species belonging to the Callithamnieae.
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