86 ESAU 



finally the nature of the interconnections between the elements. These con- 

 nections resemble plasmodesmata but are usually wider. 



Curiously enough, inclusion bodies that may be found in all kinds of living 

 cells in tobacco affected with mosaic have not been found in the sieve elements 

 (Esau, 1941; Zech, 1952). Since the sieve elements lack nuclei, they are per- 

 haps incapable of reacting to virus infection or of forming complete virus 

 particles from their precursors. 



In contrast to the mosaic viruses, those inducing yellows and leaf curl 

 diseases are more highly specialized with regard to agencies and methods of 

 transmission and transport in the plant. Usually only one or few species of 

 insects are able to transmit such viruses. Furthermore, the phloem-limited 

 viruses, as far as they have been studied in this respect, must be introduced 

 by the insect into the phloem tissue itself before an infection results. Micro- 

 scopic preparations of plant parts containing feeding punctures of the insect 

 carrier of the curly top virus, the sugar-beet leafhopper, showed that the insect 

 was making every effort to reach the phloem tissue with its mouth parts 

 (Lackey, 1953). It has been shown also that, if the insect fails to reach the 

 phloem, infection is not likely to result (Fife and Frampton, 1936). 



As to the movement of the curly top virus in the plant, the data accumulated 

 by Bennett (see Bennett, 1940b) strongly suggest that the movement of the 

 virus and that of the food materials transported in the phloem are closely 

 correlated. A good example is furnished by experiments with triple-crowned 

 sugar-beet plants obtained by splitting each plant longitudinally in three parts 

 but leaving the lowermost part of the fleshy root intact (Bennett, 1937). After 

 a while each of the three parts developed a new crown of leaves. The curly 

 top virus was introduced into one of the three crowns (crown I in fig. 11). 

 Another crown was either defoliated or protected from light for 5 days after 

 the first crown was inoculated with the virus (crown II in fig. 11). The third 

 crown was left untreated (crown III in fig. 11). The inoculated crown de- 

 veloped symptoms of the disease. Shortly thereafter the shaded (or defoliated) 

 crown developed the symptoms also, but the untreated one remained free of 

 symptoms for 3 months or more. When, instead of waiting for the symptoms 

 to appear, Bennett tested the experimental plants for the presence of virus, he 

 found that the virus had moved into the darkened or defoliated crown in 24 

 to 48 hours after the first crown was inoculated. The most convincing ex- 

 planation of these results is that the darkened or defoliated crown was de- 

 pendent on the other parts of the plant for the supply of carbohydrates since 

 without light or leaves it could not form its own photosjmthates. In receiving 

 the carbohydrates from the common root part it was also supplied with the 

 virus that moved toward the storage root with the carbohydrates from the 

 inoculated crown (fig. 11). The untreated crown was, of course, forming 

 carbohydrates which moved toward the root. It could receive the virus 



