364 A TEXTBOOK OF THEORETICAL BOTANY 



the diseases from year to year and become progressively weakened. This 

 is the reason why cuhivated strains of Potato have only a limited life and 

 must be replaced at interv'als by new^ seed hybrids which are free from virus. 

 Sometimes a plant may carry a virus without any external symptoms except 



Fig. 352. — Leaf-roll Virus affecting Potato 

 leaf. 



a general weakening, or it may carry a virus which is only pathogenic for 

 another species. One is led to wonder whether perhaps the mysterious 

 phenomenon of growing old is not due to the continuous effect during life 

 of such mild or hidden viruses in our system. 



Some viruses can only be transmitted by direct protoplasmic contact, as 

 in grafting and budding, but many are carried by sucking insects, especially 

 aphids, in whose bodies the virus may remain active for many days. The 

 superiority of Scotch Potato seed is due largely to the absence from Scotland 

 of Myzus persicae, an aphid w'hich is one of the chief carriers or vectors 

 of Potato viruses. 



Lastly it may be mentioned that the deliberate infection of certain crop 

 plants by symptomless viruses has been shown to prevent their infection by 

 virulent ones. Whether this is a practical measure or not, it is interesting 

 as the first case of artificial immunization among plants. 



Certain peculiar viruses infect even the Bacteria. These viruses, known 

 collectively as bacteriophages, enter and destroy the bacterial cells. They 

 are usually specific for particular bacterial species. Resistant individual 

 cells frequently survive attack by the virus and, from these, resistant strains 

 may arise which perpetuate the species. 



