28 BULLETIN 879, U. S. DEPARTMENT OF AGRICULTURE. 
regard. The juices of both mosaic tobacco and cucumber plants 
may be diluted to 1 : 1,000 without affecting the power of infection, 
and dilutions of 1 : 10,000 will also produce the disease, giving, how- 
ever, a lower percentage of infected plants. 
The tobacco virus will pass through both the normal Berkefeld 
filters and those of the Chamberland type, while the cucumber virus 
passes only through the former. This difference is probably of minor 
importance, however, as Allard (4) has shown that the juices of 
tobacco mosaic are also rendered noninfectious if passed through a 
porous clay atmometer or through a f-inch layer of powdered talc, 
so that the behavior of the virus of both diseases is essentially the 
same as regards filtration. 
POSSIBLE NATURE OF THE CAUSAL FACTOR. 
Since no visible causal organism has been associated with the 
mosaic diseases, various theories have been advanced as to their 
nature and origin. 
Woods (32), Koning (21), and more recently Freiberg (13) and 
Chapman (7) have held that enzyms, particularly oxidases, peroxi- 
dases, and catalase, are in some way connected with the cause of 
tobacco mosaic. Allard (4), on the other hand, has claimed that the 
disease is due to a specific pathogenic agent, probably an ultra- 
microscopic organism. 
Both theories are based principally on work with the mosaic of 
tobacco, but they apply equally well to the corresponding disease 
of cucumbers. The evidence so far accumulated, however, seems to 
accord better with the theory advanced by Allard than with the 
enzymic hypothesis. 
The virus of cucumber mosaic, like that of tobacco, seems to pos- 
sess many of the characteristics of living matter. It loses its power 
of infection if heated above 70° C, is easily destroyed by chemicals, 
and will not withstand desiccation. In support of the enzymic 
theory of the nature of tobacco mosaic it is claimed that similar 
properties are possessed by enzyms, and this is undoubtedly true to 
a great extent. The virus of cucumber mosaic, however, does not 
have such marked enzymic qualities. Unlike that of tobacco, it 
loses the power of infection within 24 to 48 hours after the juices are 
expressed from the plant, regardless of the use of preservatives or the 
temperature at which it is kept, and will not withstand desiccation. 
It is quite conceivable, however, that an organism might be destroyed 
rapidly after removal from its natural environment in the plant tis- 
sues, especially as the juices of the cucumber undergo rapid chemical 
changes when expressed. 
Another point which seems to support the theory of an organism 
as the cause of mosaic is the ability of the juices of mosaic plants to 
produce infection in a dilution of 1 : 10,000. Two or three drops 
