Direct observations on the occurrence and spatial distribution of planktonic bacteria were made by using a membrane filter technique. The membrane filters were treated to allow direct microscopic examination of bacteria on their surfaces. Deviations between direct counts on the membranes and plate (colony) counts of bacteria depended upon availability of organic matter in the natural waters studied, and can be accounted for by clumping effects and the occurrence of very small forms not visible on the membrane filter surfaces. Indirect evidence for these ‘dwarf’ forms was provided by later development of colonies with cells of normal size, after the membrane filter was placed on a nutrient medium. This response suggests that the organisms are zymogenous forms. Pure cultures of and were used in some experiments to demonstrate the relationship between the clumping effect and concentration of dissolved nutrients in water. A concentration of 0·5 mg. peptone/l. in a tap-water medium caused accumulations of bacteria around clean chitin particles; accumulations of bacteria did not develop at the higher concentrations tested. This behaviour is explained by the local differences in concentrations of nutrients in the medium. It correlates with the observations on natural populations.


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