350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



much more severe disease than is produced by either virus acting 

 separately. 



Mention must be made of a comparatively new method of ap- 

 proach to the plant virus problem, i. e., the discovery that the intra- 

 peritoneal injection of rabbits with plant virus extracts induces the 

 production of antibodies in these animals. These antibodies react 

 specifically with the antigen (virus sap) in some observable way. 

 Three types of immunologic reactions have been demonstrated, com- 

 plement-fixation, precipitation, and neutralization of the pathogenic 

 properties of the virus. Such neutralization is specific for each 

 virus; thus, tobacco mosaic virus is inactivated only by antitobacco 

 mosaic serum, and tobacco ringspot virus only by antitobacco ring- 

 spot serum and so on. The cross specificity is absolute, and the 

 addition to any of the viruses of a heterologous antiserum exhibits 

 no effect. This specificity, however, does not extend to distinctions 

 between virus strains even when the strains produce very different 

 symptoms in the host plants (Chester [3]). 



This new technique is likely, therefore, to prove another useful tool 

 in the difficult task of classifying and differentiating plant viruses. 



Since viruses are so often spoken of as filter-passing or ultra- 

 microscopic, and described by other adjectives referring to their 

 small size, it may be of interest to give a few details of the actual 

 magnitude of some viruses. The sizes of virus particles can be meas- 

 ured with fair accuracy by means of ultrafiltration through collodion 

 membranes, the pore size of which can be measured. These mem- 

 branes are prepared by a special technique devised by Dr. Elford 

 [4] of the National Institute of Medical Research at Hampstead, 

 and the process of their manufacture is too complicated to describe 

 here. It has been found by the application of this technique that 

 plant viruses vary very much in their particle size, ranging from 

 75 to 100 millimicrons for a potato virus down to 17-25 millimicrons 

 for a new tomato virus. The comparative chart shown in figure 1 will 

 give some idea of the range of size of different plant and animal 

 viruses. 



In conclusion it is proposed to give a short account of an in- 

 teresting new virus, because it well illustrates the kind of problem 

 with which the virus worker is sometimes faced. It has been found 

 at Cambridge [12] that a high proportion of the normal stock of 

 healthy tobacco plants carry a virus in the roots but not in the 

 stem and show no signs of disease during the whole of their life. 

 Under certain conditions, however, in the winter and early spring 

 the virus may pass up into the plant and develop disease symptoms 

 in the lower leaves. Unlike most other plant viruses, this virus does 

 not become systemic in the host. Further, and this is the most in- 



