1913.] PUBLIC DOCUMENT — No. 31. 45 



long been proved fallacious, no discussion of them is necessary 

 at this time. Among the later theories may be mentioned that 

 of Beijerinck (3), who was the first to prove that the disease 

 was effective oidv through the dividing colls, and held the view 

 that the disease was caused by a liquid or fluid contagium (a 

 definite, distinct substance having characteristic properties). 

 This theory has not been proved, however, and the causative 

 agent or agents are not held to be necessarily a single special- 

 ized product. 



In support of Beijerinck's theory it may be said that, upon 

 crushing the leaves or rublnng them between the fingers and 

 then successively touching the terminal bud of other plants in 

 the field which are not affected with mosaic, in some cases 

 mosaic will occur on the leaves, developing after the buds have 

 been touched. This, however, we are inclined to think is more 

 due to the fact that the tender buds have been touched with the 

 fingers, causing some bruising or abrasion of their cellular sub- 

 _ stance rather than to any specialized contagion transmitted 

 from the diseased plant. Wliere the juice of diseased plants 

 was sprayed on the terminal bud, how^ever, we were unable to 

 transmit the disease except in very few cases. Experiments 

 were conducted to determine the truth of this observation, and 

 the method of doing this has been to rub a diseased leaf between 

 the fingers, then to touch successively the terminal buds of a 

 number of other plants unaffected with the mosaic with the 

 hand which had been rubbed with the diseased leaf; then on 

 another 'series of plants the hand was rubbed with a leaf which 

 had no mosaic, and the j^lants successively touched, without 

 touching any plants which were affected with the disease. In 

 a great number of eases the disease appeared on the plants 

 which were touched with the hand rubbed with the diseased 

 tissue, Avhile those touched with the hand which had been 

 rubbed with a healthy leaf were nearly all normal, only a few 

 developing the disease. This method of infection has been 

 used bv other investigators. The infection, however, we do 

 not believe to be caused by a specialized virus, hut rather hy 

 a combination of enzymes, which view has been held by A. F. 

 Woods, and also proved by us at this laboratory. 



The direct cause may be found in the presence of free hydro- 



