AN anatomist's VIEW OF VIRUS DISEASES 89 



compact mass extending deep into the body of the host plant. Eventually the 

 extending hypha-like cells reach the vascular tissues of the host. 



From the standpoint of virus translocation we are particularly interested 



Fig. 12-16. Translocation of curly top virus in tobacco. — Fig. 12 and 13. Two 

 tobacco plants, each produced by grafting two scions of Nicotiana tabacum, one 

 above and one below, to a stock of N. glauca. The stem of the stock was "ringed"; 

 that is, bark was removed in a ring-Hke area. The pith and the internal phloem also 

 were removed through a small hole. In fig. 13 the ring was incomplete since some 

 of the bark with external phloem remained attached to the xylem. In each plant 

 the upper scion was inoculated with the virus. It developed symptoms. Since the 

 ringed stem part in fig. 12 contained no phloem, the virus was unable to pass into 

 the lower scion and the latter remained healthy. In fig. 13 the narrow phloem bridge 

 in the ringed part allowed the virus to pass into the lower scion and the latter 

 developed the symptoms. — Fig. 14 and 15 are cross sections of the ringed parts of 

 stem from fig. 12 and 13, respectively. Only xylem appears in fig. 14. A small 

 amount of bark with phloem (dotted) is attached to the xylem in fig. IS. — Fig. 16 

 shows a complete stem section with external and internal phloem (both indicated by 

 dots) associated with the xylem (indicated by hatching). {Fig. 12 atid 13 from origi- 

 nals by Milton Shenkojsky.) 



in the phloem development in the haustorium, but the picture of xylem de- 

 velopment, being better known, provides the necessary background. The 

 parasite cells in contact with the water-conducting, or tracheary, cells of the 

 host differentiate into equivalent kinds of elements, that is, tracheary elements. 

 In the meantime the haustorium develops vascular tissue in its back part, in 

 connection with the vascular tissue of the main body of the parasite. In this 

 manner a continuous strand of water-conducting cells is established that 



