SUMMARY: In mixed cultures where and W were in competition for the same dissolved food, the population size of both species (expressed in terms of total number of organisms), as well as the size and shape of individual organisms, were all affected by the presence of the other species. The size of the Chilomonas population was significantly larger in mixed than in single cultures up to the stationary phase, after which the Chilomonas rapidly decreased in numbers and finally died out. The Tetrahymena population was also larger in mixed cultures in the early stages, though by the beginning of the stationary phase this was not so. Only when the Chilomonas had died out did the Tetrahymena population again reach the size of that in single cultures. In terms of total volume of organisms, the growth of the Tetrahymena population was not as good initially as in single cultures. Later, when the Chilomonas population became very small, the total volume of the Tetrahymena population increased and finally reached the size of the population in single cultures.

and were both more slender in mixed than in single cultures, the differences being statistically significant. Variability of the organisms was greater in mixed cultures and changed with the age of the population. During population growth, the individual size of the Chilomonas and the Tetrahymena changed; but the time-course of change was different for the two species. While the size of the Chilomonas increased in the logarithmic phase and then decreased, the size of the Tetrahymena decreased in the logarithmic phase and increased in the phase of negative growth acceleration. In old cultures, the size of both species was smaller than at the beginning of the stationary phase.


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