476 Physiology 



tionship has been noted also in Cbilomonas Paramecium (222). The 

 duration of lag in bacterized cultures of ciliates also may increase, up to 

 a maximum, with age of the inoculum (491). 



In addition to changes in fission-rate, changes in individual size may 

 occur during lag. Inocula containing small T. pyriform,is show a gradual 

 increase to a mean size which is later maintained during the logarithmic 

 phase. If the inoculated ciliates are large, the size decreases to about the 

 same average as that reached by small ciliates (421). 



Phase of logarithmic groiuth. In late lag the rate of growth increases 

 to a maximum as the population enters the logarithmic phase. During 

 this period the average size of individual organisms as well as the growth- 

 rate may remain essentially constant, as in T. pyriformis (421). Length 

 of this phase is influenced by various factors, and within such a genus as 

 Leishmania (58), may vary with the species in a particular medium. The 

 initial concentration of food is a major influence in cultures of Astasia 

 klehsii (82), Glaucoma scintillans (272), and T. pyrijormis (19, 116,440), 

 although the rate of fission may be independent of food concentration 

 within wide limits (82, 440), Supplementary thiamine extends the loga- 

 rithmic phase for Chilomonas Paramecium in an acetate and ammo- 

 nium-N medium (73), and any essential vitamin, food, or metal presum- 

 ably could become a limiting factor during this phase of growth. A pure 

 culture also may accimiulate waste products or undergo other unfavorable 

 changes which bring the logarithmic phase to an end. 



Phase of negative growth acceleration. Such unfavorable changes as 

 depletion of the food supply or marked changes in pH of the medium 

 sooner or later become significant and the rate of fission decreases, as in 

 Euglena (235, 236). Lower rates of oxygen consumption in this phase 

 have been reported for Chilomonas Paramecium (221), Trypanosoma 

 cruzi (33), and Tetrahymena pyriformis (431). Changes in the respiratory 

 quotient for T. pyrifor7nis (431) also indicate qualitative changes in 

 oxidative metabolism. There is also a gradual increase in size of in- 

 dividual ciliates in populations of T. pyriformis (421). 



Phase of maximal density. Progressive changes in the culture medium 

 finally check increase in number and the population reaches its maximum. 

 Maximal density has been correlated with initial concentration of food 

 in Astasia klehsii (82), Glaucoma scintillans (272), Mayorella palestinensis 

 (473), Parainecium bursaria (326), and T. pyriformis (19, 116, 272, 440). 



For certain organisms at least, the vitamin supply may be a more critical 

 factor than the total amoimt of food. T. pyriformis shows almost no 

 growth in a filtered and autoclaved peptone medium which has previously 

 supjDorted a population of the same species. With added thiamine and 

 riboflavin, however, this used medium supports populations approximat- 

 ing those obtained 'with fresh peptone (181). Maximal density also may 

 be limited by adverse changes in pH, as noted for Chilomonas para- 



