24 I. INTRODUCTION TO VIRUSES 



tissues though able to infect another proper tissue may also be possible. 



At any rate, as mentioned above, host cells of a high resistance 

 may be infected only with a great number of virus particles, a conclu- 

 sion which can also be reached with plant viruses. Thus, the minimum 

 infective quantity of tobacco mosaic virus particles is said to be usually 

 of the order of 10~" g, which can be reduced to 10~" to 10"^' g, if very 

 sensitive host plants are adopted (55). However, even in this latter 

 case the miniimum quantity is equivalent to the virus particles more 

 than 10^, so that there seems no doubt that an extremely large number 

 of virus particles are required for the establishment of the infection. 



The active unit of plant viruses such as tobacco mosaic virus is 

 commonly estimated from the number of local lesions produced on the 

 leaf on which a given quantity of a virus sample has been rubbed. The 

 numbers of lesions, however, are known to vary with each plant, and 

 even with one and the same plant various results are obtained with 

 different leaves. On this account, in the routine method a test sample 

 and the standard are applied symmetrically on the same leaf. It is 

 only natural that the discordance becomes greater if the species of 

 plants are not the same. For example, potato virus Y can affect 

 tobacco plant in. such a high dilution of 1/10,000, but fails to infect 

 potato plant unless the dilution is lower than 1/500 (55). 



