306 BACTERIA IN RELATION TO PLANT DISEASES. 
The plants did not contract the disease even when the root-system was wounded. Russell 
also states that the disease does not find its way into the plant through the root-system. 
Stewart and Harding believe, however, that it may be communicated through the root- 
system, and this is not at all unlikely in early stages of growth when the tissues are soft. 
Later the woody stem offers an impassable barrier. Potter states that the disease has 
occurred repeatedly in Northumberland, England, in Swedish turnips, but that he observed 
it only on the root after it was well developed and always beginning in a local root-injury 
of some sort. 
Infections above ground take place readily through wounded surfaces and the organism 
which causes the disease may be disseminated by a variety of leaf-eating insects (fig.108), 
Fig. 104.* 
either by being introduced directly into wounds or more often perhaps by being left here 
and there on the uninjured margins of the leaves subsequently to find its way into the plant 
in the manner next to be described. Probably the leaves of plants are occasionally infected 
from the dust of the fields. The disease does not appear, however, to be one which is spread 
widely through the medium of the air. At least, as already recorded (Farmers’ Bulletin, 
January, 1898) the writer has seen fields nearly free from the disease with only a fence 
separating them from fields in which half the plants were badly diseased and had been for 
many weeks, while multitudes of new infections were taking place right and left. This 
field also contained a multitude of infected weeds (charlock). 
In hothouse experiments the writer succeeded in transmitting the disease by means 
of the larve of the cabbage butterfly (Plusia) and by slugs (Agriolimax). Brenner con- 
*Fic. 104.—Sections of kohlrabi, showing blackened vascular bundles due to Bacterium campestre. Photographed 
by the writer at Miami, Fla., March, 1904. The right and left were from different plants. See fig. 107. 
