358 



POPULATIONS 



the investigators seemed particularly at- 

 tracted to the theoretical explanation, es- 

 pecially since they had little confidence in 

 the phenomenon. This attitude persisted for 

 about five years until Yocom in 1928 re- 

 ported results with Oxytricha that he in- 

 terpreted as allelocatalysis. He found that 

 the rate of fission of infusorians cultured 

 singly in four-drop cultures was 10 per 

 cent greater after twenty-four hours than 

 that of single forms in ten-drop cultures. 

 In other words, by reducing the volume the 

 reproductive rate had been accelerated. 



During the next year Petersen (1929) 

 obtained results with Paramecium that 

 showed that in a small volume of medium 

 there was no diflFerence in fission rate be- 

 tween single and grouped organisms. How- 

 ever, in a larger volume the grouped forms 

 divided significantly faster than did the 

 singles. The importance of this work is its 

 demonstration that reproductive rate could 

 be varied through the manipulation of a 

 known experimental variable. Petersen did 

 not subscribe to the explanation advanced 

 by Robertson. 



Johnson (1933), using Oxytricha, made 

 an important contribution. After confirming 

 Petersen's findings, Johnson proceeded to 

 analyze further the factors involved when 

 the volume of medium was changed. In 

 sum, he showed that the bacterial flora of 

 the cultures is important in controlling the 

 fission rate. A series of cultures was set up 

 in which the bacterial population varied 

 from a high to a low concentration, and 

 into each of these a single infusorian was 

 introduced. Under these conditions the rate 

 of fission was significantly highest in inter- 

 mediate bacterial concentrations and fell off 

 as the bacterial density either increased or 

 decreased. This is convincing evidence for 

 the existence of an optimal bacterial density 

 for maximal reproductivity. Johnson showed 

 further that if paired, as well as single, or- 

 ganisms were introduced into the various 

 bacterial concentrations, the former repro- 

 duced faster in the supraoptimal bacterial 

 densities. This is probably related to the 

 point that the paired Oxytricha control and 

 reduce, during early population growth, the 

 bacteria more effectively than do the iso- 

 lated forms. 



Johnson's work fails to corroborate 

 the theory of allelocatalysis, although it 

 again shows that the type of results JFound 

 by Robertson are experimentally possible. 



As Alice (1934a, p. 18) says; "The work 

 leaves unsolved the question of the relative 

 importance of bacterial as opposed to some 

 other sort of biological (i.e., populational) 

 conditioning through the production of 

 excretory matter or of some X-substance. It 

 does indicate that the introduction of a 

 second infusorian may reduce the supraop- 

 timal numbers of bacteria towards the opti- 

 mum; that the introduced protozoan may 

 introduce more suitable bacteria, or less 

 suitable ones; in short, his work has made 

 it necessary that in future experimental at- 

 tacks on the problem the bacterial flora 

 shall be under control." 



Two interesting affirmative reports on the 

 subject appeared in 1938. One, a paper on 

 the soil amoeba, Mayorella, by Reich, 

 showed that when the amoebae are grown 

 as mass cultures free of bacteria, those 

 with small initial populations have a lower 

 division rate than do those with larger ini- 

 tial populations. Although no explanation 

 is advanced by the author, these findings 

 clearly support the validity of the phenom- 

 enon. 



The other papers are those of Mast and 

 Pace (1938, 1946), and Pace (1944). 

 These investigators worked with the flag- 

 ellate, Chilomonas paramecium, which they 

 cultured in a sterile solution containing 

 only relatively simple chemicals in known 

 proportions. Their experiments were set up 

 so that density and volume relations were 

 varied by design, and the rate of multipli- 

 cation was assayed. Their general conclu- 

 sion is best summarized in their own 

 words: 



"The results . . . demonstrate that the rate 

 of reproduction in C. paramecium varies di- 

 rectly with the density of population under 

 some conditions and inversely under others, 

 and that these results are not dependent upon 

 variation in the amount, the kind, or the avail- 

 ability of food but upon the concentration of 

 an unknown substance (X) produced by the 

 chilomonads. However, the fact that such 

 substance is produced by Chilomonas obviously 

 does not prove that similar substance is pro- 

 duced by other cells and it therefore does not 

 prove that Robertson's conclusion is valid; but 

 it nevertheless does lend some support to this 

 conclusion. The results presented in the pre- 

 ceding section of this paper seem to prove that 

 Chilomonas not only produces growth-promot- 

 ing substance in accord with Robertson's views 

 but that if this substance is concentrated, it 

 retards and inhibits growth and kills the 

 organisms" (1938, p. 379). 



