90 ESAU 



unites the xylem tissues of the host and parasite and enables the parasite to 

 take up water from the host. Haustorial cells also penetrate the phloem. Some 

 of these cells apply themselves closely to the sieve elements of the host with- 

 out destroying them. The contact between the parasite and host cells is very 

 intimate. In fact, hypha-like processes encircle the sieve elements of the host 

 like fingers of a hand. These hypha-like cells do not appear to develop the 

 specialized characteristics of sieve elements, but they are in continuity with 

 such elements in the older part of the haustorium. They appear to form a 

 bridge between the sieve elements of the host and those of the parasite. An 

 additional detail, that numerous plasmodesmata may be recognized in the 

 walls of the parasite cells located within the host tissues, suggests a possible 

 exchange of materials between parenchyma cells of host and parasite (pro- 

 vided, of course, that plasmodesmata are concerned with such an exchange). 

 Thus it would seem that various kinds of viruses, those limited to the phloem 

 or to the xylem or those occurring in various tissues at the same time, would 

 find suitable pathways for moving from the host into the dodder plant and 

 in the reverse direction. 



Another problem in virus transmission that has been illuminated by ana- 

 tomic research concerns the movement of viruses across a graft union. Ben- 

 nett (1943) has found that when virus-diseased scions of tobacco were 

 grafted to a healthy stock and left attached to the stock for periods of vary- 

 ing length, mosaic viruses were able to pass the graft union sooner than the 

 curly top virus. This difference is readily explained by the evidence that 

 phloem connection between the stock and the scion is established later than 

 the junction of parenchyma (Crafts, 1934). The union of parenchyma tissues 

 would suffice to transfer the mosaic viruses from scion to stock, whereas the 

 curly top virus requires a bridge of phloem. 



As with regard to mosaics, we assume that the curly top virus moves in the 

 sieve elements in its long-distance travel. We have no direct proof for this 

 assumption; only a circumstantial evidence. In young plant parts, recently 

 invaded by the curly top virus, the first visible symptoms have been recog- 

 nized at the level to which mature sieve elements were just reaching (they 

 differentiate from the more mature regions of the plant toward the younger) ; 

 and the degenerative changes were initiated in parenchyma cells located next 

 to the sieve elements (fig. 18; Esau, 1935, 1941). It seemed as though an 

 injurious substance issued from the mature sieve elements and affected the 

 surrounding cells. Some of these cells became hypertrophied or degenerated 

 completely; others formed a hyperplastic tissue by dividing repeatedly (com- 

 pare fig. 18 with fig. 17). The injurious substance might have been the virus 

 (or its precursor) that was transported in the sieve elements. A similar locali- 

 zation of the first symptoms in the vicinity of the youngest mature sieve ele- 

 ments was observed in the aster yellows disease (Girolami, 1955). 



Thus we have an impressive collection of data that indicates a close rela- 



