8o6 BACTERIA : THEIR WORK 



and provide the latter with dead organic material upon which 

 they can live and carry on various processes of putrefaction and 

 decay. 



It must be noticed, however, that even in these complicated 

 cases it is quite possible that the bacteria may be the c/w'^ cause 

 of the destructive effects observable upon the plants, although 

 their entrance can only be made after injuries previously inflicted 

 by other agents. 



There is little doubt that a considerable number of common 

 ailments of plants are of this nature, but recently it has been 

 convincingly shown by Erwin T. Smith, H. L. Russell and others 

 that some species of bacteria can effect an entrance into the 

 tissues of plants through normal channels, and are able directly 

 to set up disease in the infected plant. 



14. 'Black Eot' of Cabbages. — One of the best examples of 

 bacterial plant diseases is that known as the 'Brown Rot' or 

 ' Black Rot ' of the cabbage. The disease is not uncommon in 

 this country and is sometimes met with upon turnips, swedes, 

 kale, cauliflower and other cruciferous plants. 



In the first stages of the disease the affected cabbages show 

 pale yellowish-green patches near the edges of the leaves or 

 around holes or places torn or eaten by insects. The patches 

 turn brown afterwards, and on holding the leaf up to the light 

 the veins in the diseased parts are a dark brown colour. The 

 leaf wilts and shrinks, becoming tough somewhat like parchment. 



Plants badly affected become stunted, lose their leaves, and 

 may die altogether. In such cases the dark colour of the 

 vascular tissues of the leaves is continued from the leaf into the 

 stem, and on cutting across the latter the wood of the vascular 

 cylinder is stained a brown or blackish tint. 



Infected cabbages form no heads or only very small ones, and 

 the roots of turnips develop very poorly. 



The disease is caused by a bacterium named Pseudomonas 

 campestris PammeL It is a yellow, rod-shaped, motile organism 



