74 MISSOURI BOTANICAL GARDEN BULLETIN 
Experiments conducted in the Missouri Botanical Garden 
have shown that by employing a series of filters of differing 
porosity it is possible to determine with porcelain filters, in 
relation to the sizes of the pores, the approximate diameter 
of this infective agency which has thus far been designated 
merely as a filterable virus. Employing various colloidal sub- 
stances, the approximate diameter of whose particles are 
known as a result of ultra-microscopic investigation, it is pos- 
sible to choose a colloid which has filtration relations similar 
to that of the infective agency of the mosaic disease. Some 
elaborate experiments in this direction have yielded the result 
that the sizes of the infective particles of the tobacco mosaic 
are about 30 ##, or 0.03 4, corresponding very closely to the 
sizes of the particles of hemoglobin in solution. 
Experiments will be exhibited in the experimental green- 
houses, showing the results of inoculation with this virus of 
the tobacco filtered through different bacteriological filters, 
the pore sizes of which have been standardized by employing 
colloidal solutions mentioned above. 
Again, experiments will be exhibited showing the compara- 
tively slight effect on the disease agency of dehydrating 
rapidly diseased tissues of the tobacco by employing absolute 
alcohol and acetone, then employing infusions of these dehy- 
drated tissues in the inoculation of the host. The disease 
‘““agency’’ possesses a remarkable power of resistance against 
such reagents, contrasting strongly with the average plant or 
animal cell. 
Other mosaic diseases of cultivated plants will be shown, 
such as those of sugar cane, columbine, sweet-potato, poke 
weed, etc., that may be prevalent in the greeenhouse or garden 
. at this season. Mosaic diseases are a source of great loss to 
growers of beans, cucumbers, potatoes, corn, peppers and other 
crops. 
In contrast with these mosaic diseases there will be exhibited 
a few types of variegation and chlorosis of ornamental plants, 
some of which have been demonstrated to be infectious 
through grafting and some of which have thus far given no 
indication of infectiousness. The infectious diesases of this 
type are believed to be caused also by filterable viruses. They 
differ chiefly then from the true ‘‘mosaic’’ diseases in that 
the transfer of infection must be by means of the insertion 
