Light micrograph of Hydrodictyon reticulatum (Gr. hydro, water & Gr. dictyon, net), showing one cell of a middle-aged colony. Hydrodictyon cells are multinucleate, shown here by staining with Acridine Orange causing yellow-green fluorescent response of the nuclei under blue light excitation. The single reticulated chloroplast shows red fluorescence, the several pyrenoids show red fluorescence with a yellow border. Culture material of the Culture Collection of Algae at the University of Cologne. Microscopic contrast method: Fluorescence contrast, blue excitation. Magnification 380x when printed at 10 centimetres width.