AN ANATOMIST S VIEW OF VIRUS DISEASES 9 1 



tion between certain yellows viruses to the phloem tissue of the hosts. More- 

 over, we are also justified in pointing to the sieve elements as the possible 

 conduits for the movement of the viruses through this tissue. Can one go 

 equally far in depicting the relation of the Pierce's disease virus to the 

 xylem, the tissue which this virus affects primarily? In this disease, as in 

 curly top, the insect carrier — also a leafhopper — gives us a clue regarding the 



HEALTHY 



DISEASED 



Xylem elements 



Sieve elements Hyperplasia 



Fig. 17 and 18. Effect of curly top upon sugar-beet phloem. Cross sections of 

 vascular bundles from a healthy (lig. 17) and a diseased (fig. 18) leaf. Each bundle 

 has three mature sieve elements. Near the outermost sieve element in fig. 18 the 

 cells have increased in size (hypertrophy) and then divided so that an excessive 

 number of cells developed in this area (hyperplasia). The broken lines indicate re- 

 cently formed walls, the dense stippling a cell with disorganized contents. (Adapted 

 jrom Amer. Jour. Bot.: fig. 17 from Vol. 21, p. 641; fig. 18 jrom Vol. 22, p. 155.) 



place of entry of the virus into the plant. Whereas the sugar-beet leafhopper 

 seeks out the phloem in search of food (lig. 19), the grape leafhopper in at- 

 tacking the plant aims to reach the xylem (fig. 20). In a count of over one 

 hundred punctures that were made by carriers of the Pierce's disease virus in 

 grape canes and alfalfa stems, 88 per cent ended in the xylem (Houston and 

 coworkers, 1947). That these insects actually withdraw water from the plant 

 is graphically illustrated by the insect itself as it ejects large amounts of water 

 while working on the plant. This predilection for the xylem contents seems 

 puzzling. If it has to do with obtaining food, the sugar-beet leafhopper appears 

 to be a far more efficient insect since it utilizes the phloem sap, which is rich 

 in foodstuffs, for this purpose. 



Experiments were designed to force the grape leafhopper to work on tissues 

 other than the xylem (Houston and coworkers, 1947). The bark on a grape- 

 vine cane was partially separated from the wood, and a metal shield inserted 



