SOME FACTORS IN RESEARCH 287 



tunity. (1) Nutrition of the fungi, (2) solution requirements of 

 green plants, (3) " mosaic" diseases, and (4) the role of colloids 

 in the water relation of the cell. I desist from outlining the prob- 

 lems or indicating their present status, except in two instances, 

 and these are the last two mentioned. The first is a problem on 

 the borderland of physiology and pathology. For years the ques- 

 tion of filterable "viruses" has been one of great importance in 

 animal pathology, and it has also received considerable attention 

 from the plant side, necessitated by the economic significance of 

 such diseases as the mosaic diseases of tobacco, peach yellows 

 and little peach. On the whole the " organism" theory is most 

 generally accepted in the case of the twenty odd human and 

 animal diseases listed as belonging to filterable viruses. There 

 is apparently no unanimity of feeling as to how organisms could 

 exist, with organization and extensive correlation, possessed of 

 the size which many of these have been assumed to have, as in- 

 dicated by filtration relations. In a very few cases only have 

 cultures presumably been secured; that is, in practically all 

 cases the organism has not been seen, even in mass. In the great 

 majority of cases no characteristic of an organism has been dem- 

 onstrated further than the capacity of the virus to reproduce 

 itself (an important character, certainly) when introduced into 

 a healthy host organism. Nevertheless, there is wide variation 

 in the degree of infectiousness or contagiousness of the different 

 diseases. On the plant side there is convincing evidence that in 

 the case of yellows it is essential that budding or some other tis- 

 sue transplanting method be employed in order to insure infec- 

 tion by known means. The mosaic diseases stand in direct 

 contrast, since here the juice of affected plants is highly infectious. 

 New York and other northern states expend annually a consider- 

 able sum of money on inspection work in the hope of eradicating 

 the yellows. I have spent parts of several summers in the peach 

 yellows district and I have been able to see no indication that the 

 disease is being " eradicated." Of course, there is much less yel- 

 lows than when inspection began, which is due in part to up- 

 rooting the affected trees. The disease runs its course in about 

 three years and the average man sees less yellow today because 



