348 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



better term would be "a delay in the development of infective power 

 within the insect." This delay may be as long as 10 days in some 

 cases. 



It is not possible to deal at length with the question of the insect 

 relationships of plant viruses, but space permits touching upon some 

 recent interesting work on this subject. Storey [15], working upon 

 the leafhopper which transmits the streak disease of maize in East 

 Africa, has found that there exist two distinct races of this insect, 

 one race which can transmit the virus and one race which is unable 

 to do so; these races are termed active and inactive, respectively. 

 There is no visible difference between the inactive and active races, 

 and both are of the same species. Further, Storey has shown that if 

 a puncture is made with a fine needle in the wall of the gut or 

 alimentary canal of the inactive insect, the insect then becomes 

 capable of transmitting the virus. It would appear from this that 

 there may exist some factor or factors connected with the structure 

 of the wall of the alimentary canal in inactive insects which prevents 

 the virus from passing through into the blood and so reaching the 

 salivary glands whence it is injected into the plant. 



The next point concerns the mechanism of movement of the virus 

 in the plant. Since most viruses rapidly become systemic in their 

 hosts, there is evidently an efficient means of transport about the 

 plant. It has been shown by Bennett [1], Caldwell [2], and others 

 that if the phloem in a portion of the stem of a plant is destroyed 

 by steaming, the virus cannot pass over this bridge of dead tissue. 

 In other words, the virus is moving in the pliloem but not in the xylem. 

 The disease will develop normally in whichever half of the plant is 

 inoculated, but the virus will not pass from the upper to the lower 

 nor from the lower to the upper half, across the bridge of dead tissue. 



The general movement of a virus about the infected plant has been 

 well demonstrated by Samuel [9]. His experiments show that there 

 is no movement of tobacco mosaic virus from the inoculated leaf for 

 a period of 3-4 days. The virus then passes out of the inoculated leaf 

 and travels rapidly to the roots of the plant; about a day later it 

 travels with equal rapidity to the top of the plant. In pot plants 

 the more mature leaves become successively invaded from the top 

 downward and from the bottom upward until the plant is completely 

 invaded by the virus. 



The movement of the virus in the plant thus seems to be of two 

 kinds: first, a very slow cell-to-cell movement via the connecting 

 protoplasmic bridges until the phloem stream is reached, when the 

 main and most rapid movement about the plant begins. Further con- 

 firmation of this is afforded by some experiments with a newly dis- 

 covered virus known as tobacco necrosis [13]. This virus produces 

 only necrotic symptoms and thus etches out, as it were, its own move- 



