GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 



625 



same proportion of the two tissues being maintained as in normal sec- 

 ondary thickening ( Knight and Tinsley, 1958 ) . 



Enations and veinal enlargements do not occur with all virvis dis- 

 eases, but they are not uncommon ( Figure 5 ) . Irregular enlargement of 

 veinal tissue is a diagnostic symptom produced by the beet curlytop 

 virus and the wound-tumor virus. Enations generally occur as leafy 

 outgrowths from leaf surfaces. On certain hosts, the tobacco-mosaic 

 virus causes the production of distinct phylloid structures which have 

 the same cellular anatomy as the leaves from which they arise. 



Necrosis is a very common effect of viruses and may occur in a 

 number of tissues. Viruses causing a derangement in phloem tissue 

 often cause "phloem necrosis" (see Figure 3), and necrotic streaking 

 along vascular tissue characterizes many mosaic viruses. Necrosis often 

 occurs in meristematic tissue and not in other tissue. In many infec- 

 tions, virus invasion of the host is restricted to a few cells around the 



Figure 4. Tumors on crown 

 and roots of Riimex acetosa L. 

 as a result of infection by the 

 wound-tumor virus. (Photo 

 courtesy of L. M. Black.) 



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