Diatoms are single-celled plants sometimes ranked as a division, sometimes as a class within a division of the plant world. Remarkable both for the large number of species and for the variety of habitats in which they exist, diatoms have attracted attention recently as indicators of environmental conditions. This book provides a general survey of the diatoms of the interior waters (thus excluding Lake Michigan and the border rivers) and selected subaerial habitats (soils, growths on tree trunks, damp spots) of Illinois. It features identification keys, descriptions of species, distribution maps, and a generous supply of illustrations. The work thoroughly documents current knowledge of diatoms and provides a tool for botanists who wish to carry the work further.