A study of the diatom genus Pinnularia in Iowa

A total of 163 diatom samples were taken from various habitats and locations in Iowa. The habitats sampled included lakes, ponds, rivers, streams, sloughs, fens, and bogs. Of the thirty Pinnularia taxa found in these samples, three belonged to the Brevistriatae, two to the Distantes, two to the Parallelistriatae, six to the Capitatae, ten to the Divergentes, three to the Maiores, and four to the Complexae. Three of the taxa could not be identified to species;Variation in valve length, breadth, and shape, structure of the axial and central areas, striae number, striae divergence, raphe structure, and structure of the apical raphe fissures was examined within each taxon. All of the taxa were examined with the light microscope and a few were also examined with the transmission electron microscope or the scanning electron microscope. Most of the species showed considerable variation in valve size and shape, size and shape of the axial and central areas, and degree and direction of radiation of the striae. The striae number and raphe structure, particularly the structure of the apical raphe fissures, were found to be, in most cases, reasonably stable. The variability observed in some of the species was sufficient to suggest that these species probably intergrade with other closely related species