12 



PHYSIOLOGICAL TRIGGERS 



ference in properties between intracellular and extracellular virus will be 

 brought up again with reference to some other observations. 



In these experiments, a susceptible cell was presented with but a single 

 infectious unit. Nevertheless, when susceptible cells were simultaneously pre- 

 sented with more than one particle, the survival curve for infected centers still 

 retained its exponential features. Even with as many as seven infectious parti- 

 cles per susceptible site, no multiplicity of infection was observed. Consequently, 

 one is led to the conclusion that the initial host-virus complex is a highly 

 specific association; for either the host cell contains only a single attachment 

 site for the virus, or an exclusion mechanism operates to keep other particles 



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20 



Fig. 3. Increase in lesion area 

 and infectivity with time after 

 inoculation. 



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out. Thus, one and only one virus particle initiates the infection, and then under- 

 goes three phases of development before it begins to multiply. 



There is a limitation imposed on the further exploitation of the Luria-Laterjet 

 technique to the investigation of the plant virus system because of the spread of 

 the virus to adjacent cells. After further multiplication of the virus it would be 

 difficult to tell whether one were inactivating many particles in a single cell, or 

 destroying virus particles now contained in numerous cells. However, addi- 

 tional information concerning TMV reproduction has been obtained from study- 

 ing the kinetics of lesion spread. 



By the time a lesion is apparent to the naked eye, hundreds of leaf cells have 

 been invaded, have produced their complement of virus particles, and have died. 

 The necrotic area, in the form of an ellipse, grows with time in an orderly 



