Fig. 17.—Daily variations in the numbers of active individuals of Oicomonas termo (Ehrenb.) in artificial culture media kept at a constant temperature of 20° C. A, in hay infusion; B, in egg albumen.
X-axis: Days
Y-axis: Thousands
It was thought that an explanation of this phenomenon might be found in alternate excystation and encystation, since the latter is a constituent part of the animals’ life history (see [p. 73]). This, however, does not hold, for the cyst curve is not the inverse of that of the active; and, moreover, statistical treatment demonstrated that cyst formation is wholly unperiodic in character.
An explanation must therefore be sought in the changes in the organisms during the active period of their life, and the deduction can be drawn that, increased active numbers tend to be followed by death, conjugation, or both, while decreases in the active numbers are followed by rises in total numbers, i.e., reproduction, and this rhythmically.
This somewhat surprising conclusion appears to hold, in a lesser degree, for other soil protozoa, and is of sufficient importance to warrant further research. The direction in which this is being pursued is by a study of the reproductive rates of pure cultures of certain ciliates and flagellates under varying external conditions. Space does not admit of adequate discussion of this problem, but the results already obtained justify the view that such lines of work will elucidate some of the baffling problems of soil micro-biology.
Soil Reaction.
The development of the artificial fertiliser industry has in many ways revolutionised farm practice, with the inevitable result that new problems have arisen, not the least of which are biological in character.
If, as seems to be indubitable, the micro-organisms of the soil are of importance to soil fertility, it is necessary for us to know in what way this population is affected by the application of fertilisers, and a start has been made by investigating the effects of hydrogen ion concentration on soil protozoa. Much has already been written concerning this question, but almost entirely on results obtained in artificial cultures. It is always dangerous to argue from the artificial to the natural environment of organisms and particularly so in respect to the soil. Also, as Collett has shown, the toxic effects of acids are probably not entirely a function of the hydrogen ion concentration, but that the molecules of certain acids are in themselves toxic, an action which can, however, be diminished by the antagonistic powers of many substances such as NaCl.