AN ANATOMIST S VIEW OF VIRUS DISEASES 



8i 



for infectivity of different parts of diseased plants show that not only the 

 symptoms but the virus itself is thus ubiquitously distributed within the plant. 

 Indeed, some mosaics invade the tissue from which seeds develop and are 

 thus transmitted to a new plant through the seed (see Bennett, 1956). Why 

 some mosaics are transmitted through the seed and others are not is an in- 

 triguing problem that is yet to be solved. Some authors think that certain 

 viruses are unable to invade the generative cells, others suggest that viruses 

 are inactivated in the floral parts long before the seeds are formed (see Esau, 

 1948b). 



Striated material 



Fig. 6. Cell with inclusion bodies characteristic of tobacco mosaic. The bodies are 

 of two kinds: an amoeboid x-body and a body of crystalline striated material. The 

 relative size of the bodies may be judged from their comparison with the nucleus. 



In contrast to the mosaic viruses, those of the yellows group — viruses in- 

 ducing one or more of such symptoms as dwarfing, general yellowing, and 

 leaf curling — do not appear to be so indiscriminately dispersed in the plant. 

 For example, the viruses of the curly top disease (first described as a disease 

 causing a curling of the leaves, the top, in the sugar beet) and aster yellows 

 disease (first recognized as a disease causing a yellowing of foliage in asters) 

 induce primary pathologic changes in the phloem tissue (Artsch wager and 

 Starrett, 1936; Esau, 1933, 1941; Girolami, 1955). Some derangements may 

 occur in other tissues as well, but these abnormalities are obviously secondary 

 and can be explained as resulting from the initial disturbance in the food- 

 conducting tissue, the phloem. The high degree of localization of the curly 

 top virus in the phloem tissue has been clearly demonstrated by the high 

 concentration of the virus in the phloem exudate and the relative freedom 

 from the virus of tissues other than the phloem (Bennett and Esau, 1936). 



Thus, with the help of anatomical studies, we have learned to discern 

 between the phloem-limited viruses of the yellows group and the mosaic viruses 

 that occur in all or almost all living tissues of a given host plant. Relatively 

 recently a third possibility was recognized: the close association between the 

 virus and the water-conducting tissue, the xylem. The virus that induces the 



