3 o8 



BACTERIA IN RELATION TO PLANT DISEASES. 



require quite a good manyhours for a dust-dry bacterium to become sufficiently moistened so 

 as to multiply and enter the sub-stomatic chamber. A sufficient reply is that : (i) 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 (i i 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." 



fFio. 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. 



Fig. 106t. 



