Brown-rot of Swedes. 



3i5 



gations into the nature of the attack, which he completely 

 proved to be of a pathogenic character and due to a bacterium, 

 Pseudomonas campestris. Smith noted the failure of a whole 

 crop of white turnips in Baltimore, and heavy losses to market 

 gardeners in Racine (Wisconsin), through the destruction of 

 cabbages, both of which he showed to be due to the same 

 species of bacterium. 



One of the most noticeable features of the disease is the 

 brown or black staining of the tissues occupied by the bacteria. 

 The coloured drawing here reproduced represents exactly the 

 appearance of the roots as I have found them attacked in the 

 fields. In the initial stages the outermost wood-vessels are 

 the first to be invaded, and these can be clearly distinguished 

 by the naked eye standing out as dark brown spots near the 

 rind (see figure) ; often every one of these wood-bundles is thus 

 discoloured, and a ring of dark spots marks out their position at 

 the periphery of the root. The more internal bundles are also 

 indicated by dark spots extending radially. As the attack 

 proceeds the discolouration may be traced outwards to the rind 

 and inwards to the centre of the root ; this gradually extends 

 and develops until a large portion, and often the whole " flesh," 

 becomes a dark brown mass (see figure). Even in the advanced 

 stages, however, the tissues remain firm to the touch and are 

 not reduced to a soft, pulpy condition. 



The disease is of a markedly vascular character, its progress 

 being clearly traceable along the woody portion of the vascular 

 bundles, and, in the early stages, at least, the bacteria are 

 entirely confined to the cavities of the wood-vessels and inter- 

 cellular spaces. 



The various inorganic salts absorbed by the roots from the 

 soil are conveyed to the leaves by means of the wood-vessels. 

 These wood-vessels contain in addition soluble carbohydrates, 

 and hence their contents are of a nature to greatly favour the 

 invading organism. It will thus be readily understood why the 

 bacteria should proceed in this direction, and also the great 

 damage which ensues to the plant from the blocking up of the 

 wood-vessels and consequent stoppage of the supplies necessary 

 for the manufacture of its food material. 



I have succeeded in producing all stages of the disease upon 



