1917] 
FREIBERG—MOSAIC DISEASES 209 
bacco and cucumber plots were kept separately. The seed 
was treated in different ways. The commercial seed was 
sterilized with formaldehyde and a check was run against 
this with unsterilized seed. Seed was then collected from dis- 
eased tobacco and cucumber plants and applied in a sterile 
and non-sterile form to sterile and non-sterile vessels. 
The necessity of growing plants which are to be used in 
physiological experiments under sterile conditions, has been 
recognized for some time, but comparatively little stress has 
been laid upon the desirability of employing sterile cultures 
of host plants in pathologieal work. When dealing with 
problems such as are encountered in the ‘‘physiological’’ dis- 
eases, methods like the above become quite essential In 
order that contamination might be detected in the sterile cul- 
tures, about 5 to 10 сс. of ordinary potato agar was poured 
on top of the soil before planting the seed. The number of 
cultures totaled 352. The cultures were set up October 29 
and 30, 1916, and allowed to run until January 10, 1917. At 
this time many of the plants had died, none of them, however, 
having shown the slightest indication of mosaic. The nega- 
tive results obtained do not prove nor disprove anything, and 
this is particularly true, since the spontaneous occurrence of 
the disease was not observed anywhere in the greenhouse at 
that time of the year. The experiment will be repeated as 
soon as time permits. 
Another line of attack on this phase of the problem was the 
detection of metabolic activity in a medium containing an 
extract of diseased plants. The technique involved, however, 
was more complicated than was originally supposed, and 
while no results are available at this time, it is hoped that 
the experiments outlined will prove of great value. If the 
infective principle is of the nature of an enzyme, we should 
expect a definite chemical reaction to occur which should be 
governed by the laws dominating chemical reactions. The 
speed of the reaction might be influenced by acidity or alka- 
linity, but there should be no change in the nature of the end 
product. If, on the other hand, the infective principle is of 
the nature of an organism, we should expect a relatively 
