88 ESAU 



and 13). In the stem of N. glauca intervening between the two scions the 

 phloem and the associated tissues were removed; that is, the stem was 

 "ringed" (iig. 12). (The rather woody stem of N. glauca lends itself nicely 

 for this operation.) Since Nicotiana has internal as well as external phloem 

 (fig. 16), it was necessary to remove the tissues outside and inside the xylem. 

 Thus the water-conducting tissue alone remained as a connection between the 

 two grafts (fig. 14). The virus was introduced into the upper scion and in- 

 duced the development of symptoms here. None appeared on the lower scion 

 because the virus failed to pass the xylem bridge (fig. 12). For further check- 

 ing, a small strip of phloem was left associated with the xylem in another 

 experiment (fig. 15). The virus moved through this strip and caused an in- 

 fection of the lower scion (fig. 13). 



If in contrast to the curly top virus a mosaic virus can easily move through 

 parenchyma cells, the ringing of the stem should form no barrier to the 

 passage of a mosaic virus. The latter could utilize the connected system of 

 parenchyma cells present in the xylem. Indeed, in most experiments involving 

 mosaic viruses and ringed stems, the viruses succeeded in passing the xylem 

 bridge, although they were noticeably delayed in their progress (Bennett, 

 1940a). The delay must have resulted from the absence of phloem in the 

 ringed part of the stem — an additional evidence that the rapid transport of 

 a mosaic virus occurs in this particular tissue. 



Virus-plant relations have been explored also by utilizing the intimate asso- 

 ciation between the host and one of its plant parasites, the dodder (Bennett, 

 1944). Viruses were transmitted from one plant to another by inducing the 

 same dodder plant to attach itself to two host plants, one diseased and the 

 other healthy. These studies have indicated that the dodder plant itself ac- 

 quires the virus in a manner suggesting that the virus moves from the host 

 into the parasite with the food from the phloem. On the other hand, the entry 

 of virus from the dodder into the still noninfected host appears to occur 

 against the prevailing direction of food movement. Of course, one would not 

 expect all viruses to be equally dependent on the phloem tissue for the move- 

 ment between the dodder and the host. It is likely that the mosaic types of 

 viruses would utilize their ability to move through parenchyma cells, whereas 

 the yellows viruses would be restricted to the phloem in their passage between 

 dodder and host. 



The anatomic relation between the host plant and the dodder is certainly of 

 interest in connection with virus transmission through the dodder plant. When 

 the dodder stem comes in contact with a host plant it sends out an outgrowth, 

 the so-called haustorium, that becomes firmly appressed to the surface of the 

 host. The haustorium is really a modified adventitious root. Cells on the mar- 

 gin of the haustorium, which are in contact with the host surface, elongate 

 and penetrate the tissue of the host and absorb some of the host cells along 

 their way. These hypha-like cells are so numerous that soon they form a 



