Physiology 475 



extended curves have been traced for Paramecium bursaria (326), Poly- 

 toma (467), Astasia longa (506), and Tetrahymena pyriformis (190, 263). 

 The histories of such populations show essentially the same phases as 

 those reported for bacteria. 



Initial stationary phase. The occurrence of this phase in cultures of 

 T. pyriformis is related to the age of the inoculum. No stationary phase 

 follows inoculation of fresh media from cultures in logarithmic growth, 

 but with older inocula, length of this phase varies with age of the stock 

 (439). A similar relationship has been observed in Chilomonas Para- 

 mecium (393). The responsible factors remain unknown, although or- 

 ganisms in old cultures show lower reproductive ability than those in 

 young cultures. This difference in C. Paramecium has been attributed to 

 storage of an "X-substance" in excessive amounts. After inoculation of 

 fresh medium with old flagellates, fission is delayed until the excessive 

 X-substance diffuses into the medium (393). Another possibility is that 

 the activity of important enzymes is impaired in old cultures, perhaps by 

 progressive vitamin or mineral deficiency or by the accumulation of in- 

 jurious substances. Upon inoculation of fresh medium, the regeneration 

 or reactivation of essential enzymes would have to precede growth. A 

 change to a radically different medium might demand the development 

 of "adaptive" enzymes before growth could occur, or perhaps the environ- 

 mental selection of types adapted to growth in the new medium. If the 

 fresh medium contains mainly complex foods, some preliminary digestion 

 might be a prerequisite for growth. Under certain conditions, changes in 

 the organisms must be the major factor, since no relationship between 

 length of the initial stationary phase and size of the inoculum has been 

 observed in Tetrahymena pyriformis (439). 



The lag phase. The occurrence of lag has been explained in various 

 ways. According to one view, favorable changes are produced by the or- 

 ganisms in a "biological conditioning" of the medium. This possibility 

 is supported by the stimulatory effects of old culture fluid added to fresh 

 media for Chilomonas Paramecium (393, 397), Colpidium striatum (378), 

 and Tetrahymena pyriformis (187, 273) in pure cultures. An analogous 

 "conditioning" has been noted in bacterized cultures of ciliates (260, 

 337). Another explanation is that the inoculated organisms are still re- 

 covering from damage suffered in the stock culture. Therefore, lag is a 

 period of physiological recovery leading to the accumulation of metabolic 

 intermediates essential for synthesis of protoplasm. During lag in cultures 

 of Tetrahyjnena pyriformis there is marked phosphatase activity, with 

 liberation of inorganic phosphate into the medium (123). The condition 

 of the organisms evidently is the important factor in some cases, since 

 inocula from cultures of T. pyriformis in the logarithmic phase show no 

 lag while those from older stock cultures usually do (439). Such a rela- 



