92 ESAU 



between the phloem and the xylem. When an infective leafhopper was con- 

 fined to the partially isolated bark the insect failed to transmit the virus. 

 When, on the other hand, it was given access to the xylem, indeed even when 

 it was restricted to the xylem, it introduced the virus into the plant. These 

 studies clearly show that the Pierce's disease virus must be placed into the 

 xylem for a successful inoculation of the plant with the disease. The previously 

 mentioned observation that the primary symptoms of Pierce's disease develop 

 in the xylem ties in well with the experience on transmission of the virus. 



Fig. 19 and 20. Interpreting the feeding habits of the insect carriers of viruses. — 

 Fig. 19. The sugar-beet leafhopper obtains the food from the phloem tissue and 

 with it picks up the phloem-limited virus of the curly top disease and transmits it 

 to other plants again by puncturing the phloem. — Fig. 20. The grape leafhopper 

 punctures the xylem, pumps a large amount of water through its body, and with 

 it picks up the virus of the Pierce's disease. It transmits this virus to other plants 

 again by puncturing the xylem. {From orighmls by Milton Shenkojsky.) 



With regard to the movement of the Pierce's disease virus within the plant, 

 some experiments have indicated a relatively rapid upward transport in alfalfa 

 stems, so that movement with the water may be assumed; but the evidence 

 is still too scanty to be conclusive. It could be that the virus is carried with 

 the water under certain conditions, but it probably also moves in the paren- 

 chymatous components of the xylem tissue which form an interconnected 

 system. In any event, it would be difficult to visualize the virus living and 

 multiplying in the water-conducting cells, for viruses are known to depend 

 on living cytoplasm for their multiplication; and mature conducting tracheary 

 elements of the wood are devoid of living contents. One may question also 

 the ability of a virus to leave a living parenchyma cell and enter a nonliving 

 cell or go in the opposite direction, from a nonliving into an unbroken living 

 cell. Some studies with mosaic viruses suggest that these viruses are unable to 



