308 BACTERIA IN RELATION TO PLANT DISEASES. 
require quite a good many hours for a dust-dry bacterium to becomesufficiently moistened so 
as to multiply and enter the sub-stomatic chamber. A sufficient reply is that: (1) the organ- 
ism does multiply considerably in this extruded fluid, as the writer demonstrated in vitro in 
1897, ‘“‘very little” nutrient material being sufficient, and (2) the hypothetical, dust-dry, 
wind-borne bacterium requiring a half day or more to moisten it, is probably not the one 
that usually enters the water-pores and induces the disease, but rather a fresh germ recently 
come from the interior of some affected leaf as an extrusion from some water-pore already 
diseased, or left in the vicinity of the water-pore by some wandering insect, which during 
its feedings on diseased leaves has first contaminated its own body and then various un- 
injured parts of the same plant and of other plants; such a bacterium would be ready to 
grow as soon as it found lodgment in a moist place. These mountains of difficulty therefore 
disappear as soon as the actual conditions are known. 
Water-pore infections take place only when the weather conditions are such that the 
extruded fluid from the plant remains 
over the water-pores for some hours 
in the form of drops. Moist weather 
with a day temperature of 20° C. 
appears to be very favorable for in- 
fection. Under these circumstances 
if any living rods of this organism 
happen to be lying in the vicinity, so 
as to be wetted, they are stimulated 
into growth and, being motile, they 
find their way readily into the substo- 
matic chamber. Proof of water-pore 
infections was furnished by the writer 
in August, 1897, and subsequently by 
Russell, by Hecke, and by Brenner.* 
Hecke made water-pore inoculations 
on 14 kohlrabi plants, of which only 2 
were entirely negative while 8 were 
very successful. The period of incu- 
bation, that is, the time from the 
entrance of the organism to the ap- 
pearance of the disease in the veins of 
the cabbage-leaf, is usually several 
weeks (11 to 20 days in kohlrabi, Hecke) this being the period required for the multiplication 
of the bacteria in the substomatic chamber and their passage through the intercellular spaces 
of the epithem into the vessels of the leaf. Generally, however, in artificial inoculations 
there is a slight darkening of the infected leaf-tooth as early as the sixth to tenth day. For 
illustrations see Vol. I, figs. 76, 77, 78, 79, 87, 115, 116, 117. These infections were obtained 
by atomizing upon the plants in inoculation cages (Vol. I, fig. 95) agar cultures diffused in 
water. Russell placed the bacteria in drops of water extruding from particular water-pores. 
Hecke tried both methods successfully. The writer’s first successes were by plunging leaves 
into water containing the bacteria and allowing them to remain for some hours. Brenner 
likewise obtained waterpore infections by this last method, and also by placing the bacteria 
*In fluid collected from water-pores Brenner found the organism multiplied twenty fold in the course of 10 days. 
Russell also collected several cubic.centimeters of fluid from the water-pores, inoculated it with Bacterium campestre 
and made poured plates. The second series of poured plates made 12 hours later ‘“‘showed many more colonies of the 
specific germ, thus indicating that the bacteria originally seeded were able to grow in the water.”’ 
{Fic. 106.—A small turnip root, showing center rotted out by Bacterium campestre. An old plant, but no normal 
expansion of root. From a field near Baltimore, Md., Sept., 1896. x circa 8. 
