Physiology 477 



meciiim in an acetate and inorganic-salt medium. As the medium becomes 

 increasingly alkaline, growth ceases and death of the flagellates soon fol- 

 lows (73, 222). Periodic addition of acetic, hydrochloric, or lactic acid 

 increases maximal density two- to four-fold (222). 



Duration of the stationary phase may depend upon a variety of factors. 

 The thiamine content of the medium is important for T. pyriformis (190, 

 552), and the pH of the mediinn is a limiting factor for C. Paramecium 

 (222). It is somewhat uncertain just how the population is maintained 

 during this phase. Fission may continue at a rate which balances the 

 losses from death, or the life of individual organisms may be prolonged. 



Phases of death. Little is known about this phase in protozoan popu- 

 lations. Morphological changes often accompany the decline in popula- 

 tion, and a gradual decrease in individual size to about half the maxi- 

 mum, observed in the maximal stationary phase, has been traced in T. 

 pyriformis (421). Death may be accelerated by a sharp drop in pH, re- 

 lated to thiamine deficiency in a medium containing sugar (552). For 

 some species the decline in numbers is described by a fairly smooth curve; 

 in other cases, the curve is more or less irregular. Populations of Para- 

 mecium hursaria show a steady decline over a period of three weeks or 

 more in certain media (326). Populations of T. pyriformis, in a casein- 

 peptone medium, have decreased in two major steps separated by a 

 period of several weeks in which the population remains almost constant. 

 Following the second step, in which most of the ciliates die, a small popu- 

 lation may persist at least six months longer (190). The longevity of 

 such small populations is related to the available thiamine. T. pyriformis 

 lives for about four months in a certain gelatin medium, while added 

 thiamine extends life of the populations to 11-12 months. With peptone 

 culture fluid which has previously supported growth, supplementary 

 thiamine extends life of the cultures from a maximum of one week to a 

 minimum of at least nine months (185). 



Size of the inoculum in relation to growth. There are three possible 

 relationships between the initial density of popvdation and the rate of 

 growth. (1) The rate of growth may be independent of the initial density 

 under a given set of conditions. (2) The growth-rate may be higher with 

 large than with small inocula. (3) The growth-rate may vary inversely 

 with initial density of popidation. 



A relationship of the first type has been noted in pure cultures of T. 

 pyriformis (439). With optimal bacterial concentrations, a similar rela- 

 tionship has been observed in species-pure cultures of Stylonychia pusta- 

 lata (16). 



A relationship of the second type involves the so-called allelo catalytic 

 effect of Robertson (488, 489, 490, 492, 493). According to Robertson's 

 views, fission is stimulated by a nuclear autocatlyst which is liberated only 

 during fission. Once fission has occurred, the autocatalyst which reaches 



