74 life's beginning on the earth 



tobacco mosaic is the best known example. In this disease, 

 which attacks by preference the leaves of tobacco plants, 

 the virus actually multiplies until it has consumed the 

 greater part of the leaf, transforming the substance of the 

 leaf largely into the virus material. From such diseased 

 leaves the tobacco mosaic virus can be obtained in nearly 

 pure form, ready for crystallization. 



Even so, the crystallization of this virus is no easy matter, 

 since crystals are not readily formed by any material of 

 this type (protein). The workers of The Rockefeller Insti- 

 tute for Medical Research at Princeton, New Jersey, ex- 

 perimented for many years on the crystallization of similar 

 material. In 1935 one of them, W. M. Stanley, equipped 

 with all the special experience gathered in this line attacked 

 the great problem of crystallizing a virus, and finally suc- 

 ceeded. He isolated from mosaic-diseased plants a crys- 

 talline substance possessing all the properties of the virus. 

 The disease producing power and the composition of the 

 crystals obtained from different batches of leaves were 

 identical. Nor did these properties change if the virus 

 material was dissolved and allowed to crystallize out; the 

 substance thus obtained after repeated crystallization was 

 exceedingly active; a minute amount, one hundred-millionth 

 part of a grain, dissolved in a few drops of water and then 

 injected into healthy leaves, sufficed to start a fresh out- 

 break of the disease which will rapidly consume the entire 

 leaf. Even if the crystallization of the virus substance was 

 repeated ten times, its activity in producing the disease was 

 not diminished. 



Definite evidence was thus collected by Stanley to show 

 that the crystals obtained were unfiling bul virus. If the 

 originally collected virus had contained anything else of 

 importance for its propagation, this would have disappeared 

 in the repeated crystallization, or at least it would have 

 been reduced to a negligible quantity; repeated crystalliza- 



