GROWTH ASPECTS OF TLANT VIRUS INFECTIONS 



623 



and the aster yellows virus, which are limited to the phloem, cause 

 hypertrophy and hyperplasia of the phloem and pericycle, resulting in 

 numerous short sieve elements. Derangements in phloem and peri- 

 cychc tissues undoubtedly account in part for certain teratological ef- 



Figure 2. Adventitious root 

 and shoot development in the 

 carrot (left) as a result of in- 

 fection by the aster-yellows 

 virus. A healthy plant is at 

 the right. (Photo courtesy of 

 R. H. Larson.) 



Figure 3. Potato tuber, show- 

 ing phloem net necrosis as a 

 primary effect of leafroll in- 

 fection. (Photo courtesy of 

 R. H. Larson. ) 



