96 WALLACE 



studied in relation to cross-protection is that commonly referred to as 

 "recovery." With some viruses, infected plants gradually pass from 

 an acute or severe symptom stage to a chronic, mildly affected condi- 

 tion. In the latter stage the infected plants may at times show no 

 symptoms although the virus is present in them. Such recovered plants 

 generally manifest no additional symptoms when reinoculated with 

 the same or related virus strains. 



4. Acquired immunity 



The two types of behavior mentioned above under the section on 

 "cross-protection" have been described by some workers as "acquired 

 immunity." The use of the term "acquired immunity" for these reac- 

 tions has been vigorously criticized by many workers. Certainly these 

 reactions differ widely from acquired immunity as known in the field 

 of animal immunology. Strong arguments have been advanced that 

 these plant reactions are not immunological. 



Incomplete recovery, persistence of the virus in recovered plants, 

 and the lack of a circulatory system in plants comparable to that of 

 animals are the chief arguments advanced that the acquired resistance 

 found in plants that recover from severe sjrmptom stages of virus infec- 

 tion is not immunological. It has been suggested that the mild or 

 chronic symptoms are merely a second stage of symptomatology that 

 follows after the viruses reach the growing point and are able to invade 

 the tissues uniformly in the early stages of differentiation. 



The behavior of plants that recover from the curly top virus seems 

 to differ from other reported studies and more nearly approaches true 

 immunological reactions.^ Briefly these reactions are as follows: 



A. Turkish tobacco plants commonly recover from severe symp- 

 toms of curly top and are resistant to reinoculation. 



B. Commercial varieties of tomato do not recover. 



C. Recovered tobacco plants contain virus unchanged in virulence. 



D. Healthy tobacco plants infected by grafting with recovered 

 tobacco plants develop mild symptoms. 



E. Healthy tomato plans develop severe symptoms when infected 

 by means of insect transfer from recovered tobacco, but develop mild 

 symptoms when infected by grafting with recovered tobacco. 



F. The mildly-affected tomato plants contain virulent virus. 



^Wallace, James M. Acquired immunity from curly top in tobacco and tomato. 

 Jour. Agr. Res., 69:5, 187-214. 1944. 



