IXTRODUCTION H 



These mechanisms either consist of pure proteins, like 

 the chromosomes, or their major part is such, as in the 

 case of the membrane. 



The first two and the hist of the mechanisms listed are 

 probably in<lependent of each other, while multiplication 

 cannot take place without synthesis, and synthesis can- 

 not take j)lace without energy, i.e., without enzyme action. 



A partial or even complete inactivation of mechanisms 

 II or V need not cause death if the synthesis mechanism 

 can replace them. Synthesis has been observed, under 

 abnormal conditions, to continue while multiplication 

 was lost, resulting in single cells of very great length. 

 These cells are considered dead by the bacteriologist; in 

 fact, they disintegrate soon after they have ceased to in- 

 crease. 



This consideration brings us to the problem of injury 

 to the cell. Boycott (1920) was led to believe from the 

 trend of the death curves obtained with bacteria that 

 they might be too simply organized to recover from in- 

 jury. How^ever, it seems that any organism that has the 

 ability to grow must have the ability to repair a certain 

 amount of injury. In multicellular organisms, recovery 

 is frequently not a repair of the injured cells, but is 

 accomplished through their replacement by new cells 

 which are produced by surrounding, uninjured tissues. 

 Similarly, in unicellular organisms, damaged protein 

 molecules may be replaced by new molecules produced 

 by the uninjured synthesis mechanism. Native proteins 

 are unstable, and the cell is continuously replacing inac- 

 tivated molecules. The same synthesis mechanism wiiich 

 originally formed all the cell constituents, and replaces 

 all mechanisms that deteriorate in the 'Svear and tear" 

 of life, may even work more rapidly in case of injury, 

 though within narrow limits. 



A special kind of injury is of common occurrence in ^.^^-a^f^-^^^^ 

 bacteria, namely, a temporary and reversible loss of re/\v51H?^^ 



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