PLANT VIRUS PROBLEM— SMITH 



351 



teresting point, tobacco seedlings which by available methods of 

 inoculation have been shown to be virus-free, yet contain the virus 

 in their roots in quite large quantities some 5 weeks later. The fol- 

 lowing experiment illustrates this. Seed from a AVhite Burley tobacco 

 plant grown in the insect-proof glasshouse was sown in sterilized 

 sand in a "Cellophane" cage in the glasshouse. From the resulting 

 seedlings a number of small plants were chosen and all the roots 



Bacillus 



p 



rodic|»osus 



750 mp 



(diameter) 



Psittacosis 



Vaccinia 



Rabies 



Rous sarcoma 



fowl plague 



Pohalo virus x 



Bacheriophages 25- 



Ibbacco mosaic 



Hcemocycnin 



Yellow fever 



Fooh and mouth disease 



Oxyhoemoglobin 



Figure 1. — Table showing particle sizes of representative animal and plant viruses with 

 those of some bacteria and protein molecules for comparison. (1 mu= 1,000,000th 

 millimeter.) 



cut off except that one root was left on each plant. The roots of each 

 plant thus removed were ground up and the resulting paste inocu- 

 lated separately to three or four cowpeas, a plant which is extremely 

 sensitive to the virus. The tobacco plants were then repotted in 

 sterilized soil and allowed to grow on ; from this number 48 plants, 



