An investigation in certain species of Hormidium and related genera, of some of the characters commonly used in their identification

Clonal cultures of algae identified as Hormidium, Ulothrix or Stichococcus species show variation in characters commonly used in distinguishing between the genera. Both chloroplast size and shape, and the amount of fragmentation vary with the cultural conditions and appear to depend on the nutrient supply. Pyrenoids were generally clearer in culture than under natural conditions. Culture made the detection of two pyrenoids per cell possible in one clone and the development of clearly visible pyrenoids appears to depend on nutrition. The character of absence of pyrenoids for separating the genus Stichococcus from Hormidium is therefore considered of doubtful value. Reproduction by motile cells could not be obtained although the methods suggested by Klebs and others were used. Clonal cultures of Horidium, Ulothrix, and Stichococcus show variation in the characters commonly used in the identification of the species. Cell measurements atter by as much as +/-20% when the algae are grown in a different culture solution. The formation of a silky film is a useful character only if considered in conjunction with the culture solution. Constriction of the filaments and wall thickness also vary when the algae are grown in different culture solutions. The lack of precision which marks the boundaries of the genera Hormidium, Ulothrix and Stichococcus is emphasized for by suitable culture algae can be pushed over the boundaries. Clones of algae identified as Uronema species retained the special characters of the genus under all conditions under which they were studied. Reproduction by motile cells occured readily and the terminal cells were always acuminate. The validity of the genus is supported.