PROCESS OF INFECTION AND VIRUS SYNTHESIS 9 



point for starting the infection when the leaf is rubbed with virus is another 

 question. I have found that it is possible to destroy most of the larger leaf 

 hairs by rubbing the leaf in the absence of virus. When the leaf is rubbed 

 24 hrs. later with virus, the same number of lesions appeared as on 

 control leaves which did not receive the rubbing pretreatment. Consequently, 

 it is doubtful if the larger leaf hairs participate in starting the infective process 

 when the normal teclmique of rubbing the leaf with virus is used. 



The N. glutinosa leaf contains areas which are more difficult to infect than 

 other areas. Utihzing a grid with 5 mm. X 5 mm. squares placed over a leaf, 

 an analysis of the distribution of lesions has been made (Rappaport and 

 Wildman, mipublished). Where the total number of lesions per leaf is in the 

 range of about 10 to 100, there is an apparent correspondence between the 

 number of observed lesions per 25 mm^ of area compared to the number 

 predicted from a Poisson distribution, suggesting that lesions are distributed 

 in a random mamier. However, as the total number of lesions rises, the 

 departure of the observed classes from the predicted increases enormously. 

 For example, where approximately 1,000 lesions per leaf were produced by 

 brush inoculation of the virus, the Poisson formula predicts the probabihty 

 that an area will not contain a lesion as 1 m 10,000, whereas the observed 

 frequency was closer to 1 in 100. Thus, we are forced to the conclusion 

 that certain areas of the leaf are more resistant to infection than others. 



As far as the very first act of infection is concerned, it is still impossible 

 to say whether the susceptible site is produced only in a particular type of 

 cell, or whether the potentiahty is present in all cells, but realized only in a 

 few because of the variability in ease with which susceptible sites can be 

 produced. 



E. Nature of Virus Attachment at Susceptible Site 



A simple experimental observation indicates that when virus makes con- 

 tact witha susceptible site, the union is essentially instantaneous and irrevers- 

 ible. In our experiments, we have never been able to reduce the number of 

 lesions by vigorously washing the leaf immediately after inoculation by 

 rubbing. Washing the leaf after rubbing with virus has long been a standard 

 practice because of the report (Holmes, 1929) that the number of lesions is 

 increased by washing. We have not been able to observe this effect. Rather, 

 the same number of lesions appear, irrespective of whether the leaves are 

 washed after rubbing or not. If the wash water is collected and tested for 

 infectivity, lesions will be produced, but the amount of infectivity in the 

 wash water is greatly reduced from that of the inoculum. The fact that in- 

 fectious virus is obtained in the wash liquid suggests that such virus merely 

 failed to make contact with the limited number of susceptible sites created 



