44 MORPHOLOGIC VARIATION 



in the culture also somehow changes the cells so that they are noi 

 immediately able to respond to fresh medium by growth; whether 

 this change is of the nature of an injury, and just what is the cause, 

 is not clear. 



The latter view is practically that of Buchanan, namely that 

 the cells of a culture which has ceased growing are changed into 

 "resting" cells physiologically analogous to spores or seeds, and that 

 when introduced into fresh media these must "germinate" before 

 they can grow. The lag phase is the time required for this germina- 

 tion. Buchanan's theory postulates, therefore, a functional difference 

 between growing cells and resting cells, and it is interesting to note 

 that such physiological differences have been demonstrated by Sher- 

 man and Albus (1923), and to anticipate somewhat by stating that 

 I have found corresponding morphological differences. Sherman and 

 Albus (1924a) also explained lag on this basis, observing that the 

 increased sensitivity of "physiologically young" cells could be demon- 

 strated before the maximum growth rate had developed. It is also 

 interesting to note in this connection that Durbin explained the 

 latent period in the growth curves of regenerating tadpoles' tales 

 as being due in part at least to the time required for the adult cells 

 of the stump to transform to embryonic cells capable of growing. 



Buchanan first divided the lag phase into two parts, an initial 

 dormant period and the true lag phase or period of accelerating 

 growth. The former may be quite prolonged, as has been observed 

 by Burke and his co-workers, who believe that such prolonged 

 periods of dormancy, especially observed with spores, have nothing to 

 do with the ordinary lag phase. But I believe that the difference is 

 only one of degree, and that the short period of dormancy commonly 

 observed is the same sort of a phenomenon. During this period 

 of dormancy some of the cells may actually die, and as will be 

 seen later, they may also show morphological changes of the same 

 sort as they did in the parent culture. This period of initial dor- 

 mancy, then, is not entirely occupied by a transformation, either 

 physiological or morphological, to a new type of growing cell. It 

 may perhaps best be explained in the terms of Tenfold, as being due 

 to an "inertia", i.e., to the bacteria continuing to react for a time 

 in the new medium to a stimulus received in the old. 



