PLANT VIRUS PROBLEM— SMITH 349 



ment through the plant. Photographs have been taken at 2-day in- 

 tervals of the path followed by the virus in the leaves of cowpea 

 [Vigna sinensis). The first six photographs show merely a gradual 

 increase in size of the lesion at the point where the virus has entered 

 the leaf. As soon, however, as the virus enters the phloem it begins 

 to travel rapidly through the leaf, moving in 48 hours over a much 

 greater distance than in the whole of the preceding 12 days' slow 

 cell-to-cell movement. 



On another aspect of the subject two interesting discoveries have 

 recently been made : firstly, it has been found that some plant viruses 

 exist in a number of closely allied strains, and, secondly, it has been 

 shown that infection with one strain of a virus will immunize a plant 

 from infection with another strain of tliat virus. Space Avill not 

 suffice to allow of a discussion as to whether these strains actually 

 arise by mutation from existing strains, but the evidence rather indi- 

 cates that this is the case. 



The immunity conferred upon a plant by a virus strain against 

 other strains of the same virus is of the nonsterile type. There is 

 apparently no question of the production of antibodies, and it is 

 the presence of the first virus which inhibits the entrance of the 

 second strain. This type of immunity is well shown in the case of 

 potato virus X [8], tobacco [6] and cucumber mosaic viruses [7] 

 and by the virus of tomato streak [11]. All these viruses exist 

 in strains and the "green" and "yellow" strains of the tobacco or 

 cucumber viruses are particularly suitable for this kind of experi- 

 ment. ^ If a healthy tobacco plant and one systemically infected 

 with a "green" strain of tobacco mosaic are inoculated with a "yel- 

 low" strain, the healthy plant develops the yellow spots character- 

 istic of this virus, while the plant already infected with the "green" 

 strain is protected against invasion by the "yellow" strain. A sim- 

 ilar protective action is exerted in the case of a plant infected with a 

 "yellow" strain against invasion by the "green" strain. It should 

 perhaps be emphasized that the presence of one virus in a plant 

 is no bar to the entrance of a second virus of a different type; the 

 cross immunity holds good only for like viruses and virus strains. 

 This kind of immunity therefore is likely to prove a useful tool 

 in the work of classifying viruses and in distinguishing like from 

 unlike viruses in those cases where diagnosis by symptoms alone is 

 unreliable. 



A possible practical application of this type of immunity lies 

 in the protection of a crop from infection with a severe virus by 

 previous artificial infection with a mild strain of the same virus. 

 Here, however, lie a number of pitfalls, chief of which is the un- 

 fortunate liability of certain viruses, even when in a mild form, 

 to give rise, jointly with another virus of a different type, to a 



