conspectus: agents of transmission 21 



the following season. The soil around the infected plant may 

 serve, it is believed, for years as a source of infection to other 

 species (crown gall), or to other individuals of the same kind 

 (various leaf-spots). Occasionally, however, a parasite seems 

 to die out of certain soils {Bacterium solanacearum, Bacillus 

 tracheiphilus) . The pear blight organism probably dies as 

 quickly in soils as it does in a majority of the blighted branches. 

 Pear blight or cucurbit wilt by soil-infection is not known. 



Among extraneous agents, wind and water have been sus- 

 pected. I have never seen any clear indications of purely wind- 

 borne infection, not even when contiguity seemed to invite it, 

 but water and, of course, in driving rains, the wind, also, often 

 carries parasites and furnishes conditions favorable to infection 

 (citrus canker, angular leaf-spot of cotton, and bacterial canker 

 of the tomato due to Aplanobacter michiganense) . Home has 

 shown that the olive tubercle in California may be transmitted 

 from the surface of diseased branches to sound branches by rain 

 or dew (see Fig. 300). Honing, in the tobacco fields of Sumatra, 

 has traced infection several times to the watering of plants 

 from infected wells, and has cultivated the parasite from the 

 water. I have discovered experimentally that to obtain in 

 abundance several sorts of bacterial leaf-spots, e.g., those occur- 

 ring on bean, cotton, peach, plum, carnation, larkspur, sorghum, 

 geranium, etc., the leaves must be kept moist to the same extent 

 they would be in case of prolonged dews or frequent hght show- 

 ers. In nature such conditions are necessary to enable the 

 bacteria to penetrate the stomata and begin to grow. In case of 

 water-pores, however, the plant itself furnishes the water neces- 

 sary for infection, if the nights are cool enough, i.e., if the air 

 remains near enough to saturation to prevent for some hours the 

 evaporation of the excreted water from the leaf-serratures. 

 Every plant with functioning water-pores awaits its appropriate 

 bacterial parasite. The genus hnpatiens is a good example. I 

 have looked for one on it in vain but I am sure it must occur. 



Man and the domestic animals, especially through the agency 

 of the dung-heap, infallible repository of all sorts of discarded 

 refuse, undoubtedly help to spread certain bacterial diseases 

 of plants (potato rots, black rot of cabbage, etc.). 



