AN anatomist's VIEW OF VIRUS DISEASES 85 



modesmata, that is, the cytoplasmic strands supposedly interconnecting the 

 protoplasts of adjacent cells across the dividing walls, are suggested as the 

 pathways (Bennett, 1956). We are not yet sure, however, that plasmodesmata 

 are actually continuous structures from protoplast to protoplast. They may 

 be only processes or extensions of single protoplasts independent of those 

 derived from adjacent protoplasts (Lambertz, 1954). Furthermore, even if 

 the plasmodesmata were continuous from cell to cell, the virus particles seen 

 with the electron microscope usually appear to be much too large for passing 

 through these strands. One plausible suggestion is that viruses move not as 

 complete particles but as fractions of these, fractions that are capable of 

 forming complete particles in the presence of the host cytoplasm in a newly 

 invaded cell (Zech, 1952). 



The cell-to-cell movement is relatively slow, 2 to 4 mm. a day (Bennett, 

 1956; Zech, 1952). When the virus, which is introduced into the leaf through 

 the epidermis, reaches the elongated parenchyma cells located along the 

 vascular bundles, its rate of movement suddenly increases to as much as 8 

 mm. per hour (Zech, 1952). The virus enters the vascular bundles of the 

 inoculated leaf, reaches the stem, then, usually, moves toward the root and 

 upward again into the youngest, growing parts of the shoot. The movement 

 toward the root may be even more rapid than along the vascular bundles 

 in the leaf. During this downward movement the virus appears to be confined 

 to the vascular bundles. Later it issues from the vascular bundles and spreads 

 in the surrounding parenchyma tissue. The so-called systemic infection is thus 

 accomplished. 



The direction and rate of virus spread during the systemic invasion clearly 

 relate this movement to the translocation of food materials in the phloem 

 (Bennett, 1940a, 1956). The interesting experiments designed to test whether 

 mosaic viruses move with the water in the xylem are too many to review 

 here (see Bennett, 1940b; Esau, 1938). Suffice it to say that most workers 

 now agree that the rapid long-distance transport of mosaic viruses occurs in 

 the phloem; and that this rapid movement is combined with a slow spread 

 from cell to cell in parenchyma tissues. This combination assures a thorough 

 invasion of the host tissues by the virus. 



If we agree that the rapid phase of virus movement occurs in the phloem, 

 the question is still to be answered regarding what particular phloem cells 

 are concerned with this movement. Students of translocation in the phloem 

 generally assume that the rapid transfer of food materials occurs in the sieve 

 elements because of the specialized characteristics of these cells. The same 

 assumption is made regarding the transport of viruses. However, the mecha- 

 nism that could make possible a rapid movement of materials in the sieve 

 elements has not yet been revealed (Currier and coworkers, 1955). The prob- 

 lem involves the relation between the cytoplasm and vacuole in the sieve ele- 

 ments, the possible significance of the enucleate state of the elements, and 



