22 S. G. WILDMAN 



WhUe it is true that many measurements of virus synthesis taking place 

 in the systemic system have been made, most of such measurements throw 

 very httle hght on the processes involved in virus multipHcation at the 

 cellular level of leaf organization. The problem of locating those cells in the 

 systemic host that are directly engaged in virus acti\aty is made even more 

 difficult by the absence of necrotic lesions to serve as indicators of previous 

 virus activity. Holmes (1931) has developed a staining method to reveal 

 starch concentrations at the foci of infection, but this method has not been 

 exploited to the extent of providing information on the intracellular de- 

 velopment of virus. In time, slightly chlorotic areas which are considered to 

 correspond to areas of virus activity will appear on the inoculated leaf. The 

 most important progress toward elucidating virus behavior in the systemic 

 host stems from the direct observations of infected leaf hair cells by Zech. 



B. Direct Observation of Virus Activity in Hair Cells 



Zech (1952) has succeeded in infecting individual leaf hairs of the systemic 

 host of TMV, N. tabacum, and then observing changes induced in these ceEs 

 by virus. He has employed both phase and ultraviolet hght microscopy. 

 Much of his work is concerned with the systemic spread of virus infection 

 from one organ to another, but these interesting observations will be omitted 

 from this discuf s'on. 



Zech's observ^ations, germane to this discussion, can be summarized as 

 follows: (1) Large leaf hairs, consisting of 6 ceDs arranged in a row, the basal 

 cell being an extension of the epidermis, are selected for infection and micro- 

 scopic observation. The proximal cell is cut in the presence of virus contained 

 in gelatin. (2) About 5 hours after infection, an accelerated rate of proto- 

 plasmic streaming is noticed in the first intact cell nearest to the point of 

 infection. Within 10 hours, an increased rate of protoplasmic streaming is 

 present in all of the hair cells. (3) About 60 hours after infection, a sweUing 

 of the protoplasm is detected, but the swelling occurs first in the basal cell, 

 5 cells removed from the point of infection. (4) About 74 hours after infection, 

 disorganization of chloroplasts is observed in the basal ceU, but disorganized 

 chloroplasts do not appear in the cell next to the infection until around 100 

 hours have elapsed. (5) Amorphous masses, previously known in TMV 

 literature as X-bodies,° appeared in the basal ceU at 80 hours, but none 



^ As X-bodies and crystalline inclusions make their appearance, enormous numbers of 

 rod-shaped particles can also be visualized by electron microscopy. Steere (1957) has 

 isolated and sectioned crystals removed from leaf hairs and demonstrated a herrmgbone 

 pattern of orientation of the rod-shaped particles. Nixon (1956) has sectioned leaf hairs 

 and displayed astronomical nmnbers of virus particles in situ. Matsui (1956) has pre- 

 sented interesting electron microscopic observations of ultrathin-sectioned tissue 

 infected with TMV. 



