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Front cover illustration

The photograph, obtained with two superimposed pictures taken with white light and fluorescence, shows filaments of the cyanobacterium Anabaena sp. strain PCC 7120. In the absence of a source of combined nitrogen, two cell types are generated: heterocysts that fix molecular nitrogen, and vegetative cells that perform photosynthesis. Vegetative cells are highly fluorescent because of the high content of photosynthetic pigments and appear red in the picture, while heterocysts have lost most of these pigments and thus appear less fluorescent. The diameter of the cell is about 3-5 mm. Photograph supplied by Professor Cheng-Cai Zhang, Université de la Mediterranée (Aix-Marseille II), IBSM-CNRS, Marseille, France. See the paper by Li et al. on pp. 3257-3263.