1923] 



DUGGAR & KARRER — NATURE OF MOSAIC DISEASE PARTICLES 201 



that we are dealing in the case of a colloidal solution with particles 

 and not with molecules. This particle size is to be compared with 

 an average short diameter of about 1000 [l\l for many pathogens. 



If we are dealing with an organism, that is, an organized 

 ultra-microscopic individual of approximately 30 [l\l in diameter, 

 its life relations must be very different from those of an organism 

 whose volume relations are to this as 37000 to 1 or about 1,000,000 

 to 26. This would be the relation between the average bacterial 

 plant pathogen and the mosaic virus. Assuming a complex 

 organization, many theoretical questions would arise forconsidera- 

 tion. Among these might be mentioned perhaps above all that 

 of the surface tension conditions in such a structure, also the 

 possibility of organization at all (membrane existence, etc.) as 

 now comprehended. 



The filtration work has been repeated with scrupulous care and 

 it has led to results similar to those above described, — invariably 

 pointing to an infectious particle with a size approximating that of 

 fresh 1 per cent hemoglobin. A question which then forces itself 

 upon the attention is: What is the peculiar nature of such a par- 

 ticle? To arrive at a tentative answer to this question, it would 

 be necessary to consider all known properties of the agency, to 

 analyze the data already -carefully worked out, to plan many 

 experiments of an entirely new type with a view to determining 

 the behavior of the body concerned, and to contrast the inception 

 and course of the mosaic disease or related phenomena of chlorosis 

 in other plants. As far as possible it would be essential to examine 



also any possible relations of the viruses of animal diseases that 

 may assist in one or more general interpretations. 



Under the most favorable growing conditions the period of 

 incubation of the tobacco mosaic is from 10 days to 2 weeks. 

 By period of incubation, in this connection, is meant the time 

 required for the development in the young leaves of the infected 

 plant of unmistakable symptoms of mottling. In this interval of 

 time the infective agency is widely distributed in the plant. 

 It is not confined to the leaves (young) capable of exhibiting 

 pronounced or favorable mosaicing, but may be found in older 

 leaves, roots, etc. It has in reality a phenomenal power of " mi- 

 gration " from cell to cell, — a power none the less pronounced 



