5io 



DISEASES OF CULTIVATED PLANTS 



Coal and peat, like other organic matter, are liable to 

 decomposition as soon as conditions are presented suitable 

 for the life of aerobic organisms. The carbon is then once 

 more liberated, in the form of CO 2 , to play its role in the life- 

 cycle. It is thus conceivable that the vast supplies of carbon 

 locked up in the world's coal-fields may become available for 

 plant nutrition without the intervention of direct combustion. 



Potter, M. C, 'Bacteria as Agents in the Oxidation of 

 Amorphous Carbon,' Proc. Roy. Soc.> Ser. B, 80, 239 (1908). 



Black rot of cabbage. A serious bacterial disease of 

 cabbages,, caused by Pseudomonas campestris (Smith), has 



FIG. 154. Pseudomonas campestris, causing bacteriosis in 

 cabbage; cabbage skin cut across, showing the discoloration 

 caused by the organism. 



long been known in the United States, where it sometimes 

 devastates the entire crop in a field in a very short period of 

 time. It has also been recorded from several European 

 countries, and was first observed in England in 1902. In 

 addition to cabbages, many other members of the Cruciferae 

 are also attacked, as brussels-sprouts, kale, rape, broccoli, 

 radish, turnips, both white and swedes, etc. The fibro- 

 vascular bundles are attacked and soon turn black, a character 

 by which this disease can be recognised ; the veins of a 

 diseased leaf become quite black and are very conspicuous, 



