1. The decrease in the rate of growth of a population of yeast cells, which results in the maintenance of an equilibrium crop level, is shown to be due to substances excreted into the culture medium by the growing cells. These toxic substances tend to destroy the young buds, because the percentage of budding cells is about the same at the time of most rapid growth and at the time of the growth equilibrium.
2. Alcohol is the product which primarily causes the decline of the growth rate. For the strain of yeast used, under the particular conditions of these experiments, a concentration of alcohol of about 1 mg. per cc. is associated with the beginning of the decrease of the growth rate.
3. The increasing acidity of the medium, due to CO2, pyruvic acid, and other organic acids, is also a retarding influence. It is a secondary factor, however, as the greatest increase of the acidity of the medium occurs after pyruvic acid, probably a by-product of alcoholic fermentation, appears.
4. When the medium is maintained effectively constant, by preventing the accumulation of these toxic products, the yeast grows at a constant rate and the yeast growth is potentially unlimited. The limit of growth found in actual experiments is due only to the size of the test-tubes and to the relative efficiency of the method used in keeping the medium effectively constant. The necessity of maintaining a constant rate of growth in studies on the relations of yeasts to vitamines and other products is stressed.