VIRUSES — STANLEY 263 



nature of a material, but is important chiefly because it makes it 

 possible to obtain certain solubility and X-ray data which would 

 otherwise be unobtainable. 



There is not sufficient space for a detailed discussion of the 

 chemical properties of all the preparations of purified viruses and, 

 in order to provide you with an idea of their general chemical 

 properties, I shall devote most of the text to the two viruses which 

 have been extensively investigated from this standpoint, namely, 

 tobacco mosaic and tomato bushy stunt viruses. These are typical 

 viruses with respect to the essential and recognized characteristics of 

 a virus; yet it must be admitted that each has certain special prop- 

 erties which make it an unusually favorable material for experi- 

 mental work. Thus, tobacco mosaic virus is among the most stable of 

 all viruses and reaches a concentration in Turkish tobacco plants 

 which is far greater than that reached by most viruses in their re- 

 spective hosts even under the most favorable conditions, and bushy 

 stunt is the only virus that has been obtained in the form of large 

 rhombic dodecahedric crystals (pi. 1, fig. 2). However, there is no 

 more reason for regarding these viruses as atypical because of such 

 special properties than for regarding vaccine virus as atypical be- 

 cause of its unusually large size, or foot-and-mouth disease virus as 

 atypical because it is the smallest of all viruses. Tobacco mosaic and 

 bushy stunt are plant diseases, and it has been argued that the viruses 

 of plants differ fundam,entally from those of animals and, hence, that 

 information gleaned from studies on plant viruses has but little sig- 

 nificance in connection with animal viruses. This argument was 

 based chiefly on the failure of plant viruses to grow in animals and 

 of animal viruses to grow in plants. However, because there is no 

 difference in the fundamental virus properties, I have always con- 

 sidered this to be an erroneous viewpoint. Within the past few 

 years, Fukushi secured strong evidence that rice dwarf disease virus 

 multiplies in its insect vector, and Kunkel and more recently Black 

 have obtained experimental evidence which demonstrates beyond a 

 reasonable doubt that aster yellows virus can multiply in its insect 

 vector. The growth of a plant virus in an animal provides further 

 evidence in support of the conclusion that there is no fundamental 

 difference between the viruses of plants and those of animals. Dif- 

 ferent viruses must of necessity differ in certain of their properties, 

 and a composite picture of viruses as a group will not be obtained 

 until many viruses have been studied. 



Tobacco mosaic virus appears to be a conjugated protein contain- 

 ing about 95 percent protein and 5 percent nucleic acid. The latter 

 has been found to contain uridylic acid, guanine, cytosine, and ade- 

 nine, and to give a test for a pentose but not for a desoxypentose and, 

 hence, appears to be of the yeast rather than of the thymus nucleic 



