ABSTRACT
It is impossible to find a naturally occurring biological system which behaves so simply, and it is difficult to make one experimentally, though it can be done. If a small population of yeast cells, or bacteria, is inoculated into a large mass of nutrient medium, allowed to grow for a time, and a new inoculum transferred to fresh medium after a fairly short period, the growth rate per unit mass may be kept constant indefmitely. The essential points are that neither lack of nutrient nor the presence of harmful excreta are allowed to inhibit the system. If frequent transfers are not made, one or other or both these are certain to occur, and the rate of growth will slacken, till the growing mass becomes stationary and eventually begins to decline when the death-rate of cells overtakes the rate of increase. Samples taken from such declining cultures usually take a little time to get going when transferred to fresh medium, so that in the 'typical' growth curve of a colony of cells (yeast, bacteria, tissue-cultures and the like) the logarithm of size when plotted against time, is not a straight line, but has the form shown in Fig. 13.1.
