GROWTH 531 



other chapters. The critical periods of binary fission, conjugation, and 

 intercellular reorganization are important to the study of growth, and 

 further information on these phenomena will facilitate our understand- 

 ing of growth. 



Protozoan Successions: Nonlaboratory 



The variety of Protozoa and the numbers of each vary in time, and 

 the abundance of individuals is usually inversely correlated with the 

 diversity of kinds. At Geneva, Roux (1901) found the largest variety 

 of species in January and in October and found that in the same loca- 

 tions there was considerable variation at the corresponding time in two 

 successive years. 



The sequence of Protozoa on sewage filtration beds (New Jersey) 

 was followed by Crozier (1923) and Crozier and Harris (1923). A 

 maximum number of rhizopods was found in August and of ciliates in 

 May-June and November-December. Paramecium had a sharp maxi- 

 mum in December-January, Vortkella in late December and in May, 

 and Colpoda in the first third of the year. The sequence was attributed 

 to the amount of anaerobiosis and to the formation and sloughing of 

 the film. In this environment the abundance was directly correlated with 

 the diversity of types. 



Noland (1925) found the sequence of Protozoa related to the tem- 

 perature, oxygen, and carbon-dioxide concentrations in natural ponds. 

 The hydrogen-ion concentration was not believed to be a controlling 

 factor. Most of the Protozoa found were not those usually studied in the 

 laboratory, but when samples were transferred to the laboratory, Col- 

 poda cucullus, Glaucoma pyriformis, and Paramecium caudatum ap- 

 peared, showing that these animals may thrive better in the laboratory 

 than in natural habitats. 



Changes in the concentration of Protozoa in a Philadelphia pond 

 were followed for a year by Wang (1928), who measured also the 

 temperature, oxygen concentration, pH, and relative amount of dis- 

 solved gases. The surface forms showed the greatest variation, which 

 was believed due to the dissolved oxygen, depending on temperature 

 and on the activity of the plants. A marked increase of acidity could be 

 a limiting condition. The maximum number of forms was found in 

 September-October. Since the amount of sunlight was greatest at this 



