158 L. M. BLACK 



propagative category. This term will therefore be used to designate both these 

 viruses and their kind of transmission. 



It is impossible to deal adequately with the propagative plant viruses 

 without also considering evidence concerning another type of relationship in 

 which the plant virus is taken in through the mouth parts, accumulated 

 internally, apparently without multiplication, passed through the insect 

 tissues, and introduced into plants again via the mouth parts of the carrier. 

 The most thoroughly studied virus of this kind is that causing sugar beet 

 curly top. In this paper such viruses and the relationship they exhibit wiU 

 be called circulative because of their remarkable ability to complete this 

 circle of penetration through the membranes of the body of the vector 

 without multiplication. It is realized that the propagative viruses have both 

 circulative and propagative properties and that the terms are not mutually 

 exclusive. However, they are not cumbersome and they do emphasize the 

 distinctive feature of each class. Other names for circulative transmission have 

 been proposed earlier. The circulative viruses belong in Huff's mechanical 

 category. However, the term "mechanical" seems inadequate both descrip- 

 tively and as a means of distinguishing such transmission from other very 

 different mechanical types. Freitag (1936, p. 340) described the vectors of 

 curly top as "internal mechanical carriers." The term "vector-latent" of Day 

 and Irzykiewicz (1954, p. 270) apparently would include both circulative and 

 propagative viruses. 



In order to give the reader some orientation, other kinds of relationships 

 between plant viruses and their vectors wiU be mentioned here but will not 

 be discussed. The most important of the other kinds of relationship is that 

 shown by the nonpersistent, aphid-borne viruses, which constitute the 

 largest group of plant viruses. Recently, Bradley and Ganong (1955, 1957) 

 demonstrated that the transmitted particles of a nmnber of these viruses are 

 carried upon the tip of the stylets — more exactly, the distal 15 fi. The non- 

 persistent, aphid-borne viruses are obviously noncirculative, but there are 

 some aphid-borne viruses whose position between the two extremes of 

 propagative and nonpersistent is uncertain and whose manner of transmission 

 is not clearly understood. Beetles, with biting mouth parts, transmit a number 

 of viruses that cannot be transmitted by the sucking mouth parts of aphids. 

 There is evidence that regurgitation of virus-containing juice may be 

 important in virus transmission by such beetles. It has been generally con- 

 sidered that transmission by beetles is purely mechanical, but the demonstra- 

 tion that virus may occur in the blood, that transmission may occur 20 days 

 after the end of acquisition of virus and even after over-wintering of the 

 beetles, indicates that, in some cases at least, it is possibly circulative 

 (Freitag, 1956). Simple mechanical transmission is exemplified by grasshopper 

 dispersal of potato spindle tuber or tobacco mosaic virus (Goss, 1931; Walters, 



