Desmids and red algae. Darkfield illuminated polarised light micrograph of Batrachospermum sp. red algae (branch-like, background) and two Pleurotaenium nodosum desmid daughter cells (diagonal, centre) just after dividing. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Magnification: x200 when printed 10 centimetres wide.